Evidence for single top-quark production in the s-channel in proton–proton collisions at √s = 8 TeV with the ATLAS detector using the Matrix Element Method

Wed 23 Aug 2017 09:40:04 KRAT

This Letter presents evidence for single top-quark production in the s -channel using proton–proton collisions at a centre-of-mass energy of 8 TeV8 TeV with the ATLAS detector at the CERN Large Hadron Collider. The analysis is performed on events containing one isolated electron or muon, large missing transverse momentum and exactly two b -tagged jets in the final state. The analysed data set corresponds to an integrated luminosity of 20.3 fb−120.3 fb−1. The signal is extracted using a maximum-likelihood fit of a discriminant which is based on the matrix element method and optimized in order to separate single-top-quark s-channel events from the main background contributions, which are top-quark pair production and W boson production in association with heavy-flavour jets. The measurement leads to an observed signal significance of 3.2 standard deviations and a measured cross-section of View the MathML sourceσs=4.8±0.8(stat.)−1.3+1.6(syst.) pb, which is consistent with the Standard Model expectation. The expected significance for the analysis is 3.9 standard deviations.

Muon reconstruction performance of the ATLAS detector in proton-proton collision data at √s = 13 TeV

Wed 23 Aug 2017 09:40:03 KRAT

This article documents the performance of the ATLAS muon identification and reconstruction using the LHC dataset recorded at √s=13 TeV in 2015. Using a large sample of J/ψ→μμ and Z→μμ decays from 3.2 fb−1 of pp collision data, measurements of the reconstruction efficiency, as well as of the momentum scale and resolution, are presented and compared to Monte Carlo simulations. The reconstruction efficiency is measured to be close to 99 % over most of the covered phase space (|η|<2.5 and 52.2, the pT resolution for muons from Z→μμ decays is 2.9 % while the precision of the momentum scale for low-pT muons from J/ψ→μμ decays is about 0.2 %.

Search for new phenomena in dijet mass and angular distributions from pp collisions at √s = 13 TeV with the ATLAS detector

Wed 23 Aug 2017 09:40:03 KRAT

This Letter describes a model-agnostic search for pairs of jets (dijets) produced by resonant and non-resonant phenomena beyond the Standard Model in 3.6 fb−13.6 fb−1 of proton–proton collisions with a centre-of-mass energy of View the MathML sources=13 TeV recorded by the ATLAS detector at the Large Hadron Collider. The distribution of the invariant mass of the two leading jets is examined for local excesses above a data-derived estimate of the smoothly falling prediction of the Standard Model. The data are also compared to a Monte Carlo simulation of Standard Model angular distributions derived from the rapidity of the two jets. No evidence of anomalous phenomena is observed in the data, which are used to exclude, at 95% CL, quantum black holes with threshold masses below 8.3 TeV, 8.1 TeV, or 5.1 TeV5.1 TeV in three different benchmark scenarios; resonance masses below 5.2 TeV for excited quarks, 2.6 TeV in a W′W′ model, a range of masses starting from mZ′=1.5 TeVmZ′=1.5 TeV and couplings from gq=0.2gq=0.2 in a Z′Z′ model; and contact interactions with a compositeness scale below 12.0 TeV and 17.5 TeV respectively for destructive and constructive interference between the new interaction and QCD processes. These results significantly extend the ATLAS limits obtained from 8 TeV data. Gaussian-shaped contributions to the mass distribution are also excluded if the effective cross-section exceeds values ranging from approximately 50–300 fb for masses below 2 TeV to 2–20 fb for masses above 4 TeV.

Measurement of the charge asymmetry in highly boosted top-quark pair production in √s = 8 TeV pp collision data collected by the ATLAS experiment

Wed 23 Aug 2017 09:40:02 KRAT

In the View the MathML sourcepp→tt¯ process the angular distributions of top and anti-top quarks are expected to present a subtle difference, which could be enhanced by processes not included in the Standard Model. This Letter presents a measurement of the charge asymmetry in events where the top-quark pair is produced with a large invariant mass. The analysis is performed on 20.3 fb−1 of pp collision data at View the MathML sources=8TeV collected by the ATLAS experiment at the LHC, using reconstruction techniques specifically designed for the decay topology of highly boosted top quarks. The charge asymmetry in a fiducial region with large invariant mass of the top-quark pair (View the MathML sourcemtt¯>0.75 TeV) and an absolute rapidity difference of the top and anti-top quark candidates within View the MathML source−2<|yt|−|yt¯|<2 is measured to be 4.2±3.2%4.2±3.2%, in agreement with the Standard Model prediction at next-to-leading order. A differential measurement in three View the MathML sourcett¯ mass bins is also presented.

Measurement of the transverse momentum and φη* distributions of Drell-Yan lepton pairs in proton-proton collisions at √s = 8 TeV with the ATLAS detector

Wed 23 Aug 2017 09:40:01 KRAT

Distributions of transverse momentum pℓℓT and the related angular variable ϕ∗η of DrellΓÇôYan lepton pairs are measured in 20.3┬áfb−1 of protonΓÇôproton collisions at √s=8┬áTeV with the ATLAS detector at the LHC. Measurements in electron-pair and muon-pair final states are corrected for detector effects and combined. Compared to previous measurements in protonΓÇôproton collisions at √s=7┬áTeV, these new measurements benefit from a larger data sample and improved control of systematic uncertainties. Measurements are performed in bins of lepton-pair mass above, around and below the Z-boson mass peak. The data are compared to predictions from perturbative and resummed QCD calculations. For values of ϕ∗η<1 the predictions from the Monte Carlo generator ResBos are generally consistent with the data within the theoretical uncertainties. However, at larger values of ϕ∗η this is not the case. Monte Carlo generators based on the parton-shower approach are unable to describe the data over the full range of pℓℓT while the fixed-order prediction of Dynnlo falls below the data at high values of pℓℓT. ResBos and the parton-shower Monte Carlo generators provide a much better description of the evolution of the ϕ∗η and pℓℓT distributions as a function of lepton-pair mass and rapidity than the basic shape of the data.

Measurement of the dependence of transverse energy production at large pseudorapidity on the hard-scattering kinematics of proton–proton collisions at √s = 2.76 TeV with ATLAS

Wed 23 Aug 2017 09:39:57 KRAT

The relationship between jet production in the central region and the underlying-event activity in a pseudorapidity-separated region is studied in 4.0 pb−1 of View the MathML sources=2.76 TeVpp collision data recorded with the ATLAS detector at the LHC. The underlying event is characterised through measurements of the average value of the sum of the transverse energy at large pseudorapidity downstream of one of the protons, which are reported here as a function of hard-scattering kinematic variables. The hard scattering is characterised by the average transverse momentum and pseudorapidity of the two highest transverse momentum jets in the event. The dijet kinematics are used to estimate, on an event-by-event basis, the scaled longitudinal momenta of the hard-scattered partons in the target and projectile beam-protons moving toward and away from the region measuring transverse energy, respectively. Transverse energy production at large pseudorapidity is observed to decrease with a linear dependence on the longitudinal momentum fraction in the target proton and to depend only weakly on that in the projectile proton. The results are compared to the predictions of various Monte Carlo event generators, which qualitatively reproduce the trends observed in data but generally underpredict the overall level of transverse energy at forward pseudorapidity.

Combination of searches for WW, WZ, and ZZ resonances in pp collisions at √s = 8 TeV with the ATLAS detector

Wed 23 Aug 2017 09:39:53 KRAT

The ATLAS experiment at the CERN Large Hadron Collider has performed searches for new, heavy bosons decaying to WW, WZ and ZZ final states in multiple decay channels using 20.3 fb−120.3 fb−1 of pp collision data at View the MathML sources=8 TeV. In the current study, the results of these searches are combined to provide a more stringent test of models predicting heavy resonances with couplings to vector bosons. Direct searches for a charged diboson resonance decaying to WZ in the ℓνℓ′ℓ′ℓνℓ′ℓ′ (ℓ=μ,eℓ=μ,e), View the MathML sourceℓℓqq¯, View the MathML sourceℓνqq¯ and fully hadronic final states are combined and upper limits on the rate of production times branching ratio to the WZ bosons are compared with predictions of an extended gauge model with a heavy W′W′ boson. In addition, direct searches for a neutral diboson resonance decaying to WW and ZZ in the View the MathML sourceℓℓqq¯, View the MathML sourceℓνqq¯, and fully hadronic final states are combined and upper limits on the rate of production times branching ratio to the WW and ZZ bosons are compared with predictions for a heavy, spin-2 graviton in an extended Randall–Sundrum model where the Standard Model fields are allowed to propagate in the bulk of the extra dimension.

Measurements of four-lepton production in pp collisions at √s = 8 TeV with the ATLAS detector

Wed 23 Aug 2017 09:39:51 KRAT

The four-lepton (4ℓ , ℓ=e,μℓ=e,μ) production cross section is measured in the mass range from 80 to 1000 GeV using 20.3 fb−1 of data in pp collisions at View the MathML sources=8 TeV collected with the ATLAS detector at the LHC. The 4ℓ events are produced in the decays of resonant Z and Higgs bosons and the non-resonant ZZ continuum originating from View the MathML sourceqq¯, gg, and qg initial states. A total of 476 signal candidate events are observed with a background expectation of 26.2±3.626.2±3.6 events, enabling the measurement of the integrated cross section and the differential cross section as a function of the invariant mass and transverse momentum of the four-lepton system.

Search for new phenomena in final states with large jet multiplicities and missing transverse momentum with ATLAS using √s = 13 TeV proton-proton collisions

Wed 23 Aug 2017 09:33:01 KRAT

Results are reported of a search for new phenomena, such as supersymmetric particle production, that could be observed in high-energy proton–proton collisions. Events with large numbers of jets, together with missing transverse momentum from unobserved particles, are selected. The data analysed were recorded by the ATLAS experiment during 2015 using the 13 TeV centre-of-mass proton–proton collisions at the Large Hadron Collider, and correspond to an integrated luminosity of 3.2 fb−1. The search selected events with various jet multiplicities from ≥7 to ≥10 jets, and with various b-jet multiplicity requirements to enhance sensitivity. No excess above Standard Model expectations is observed. The results are interpreted within two supersymmetry models, where gluino masses up to 1400 GeV are excluded at 95% confidence level, significantly extending previous limits.

Charged-particle distributions in √s = 13 TeV pp interactions measured with the ATLAS detector at the LHC

Wed 23 Aug 2017 09:32:56 KRAT

Charged-particle distributions are measured in proton–proton collisions at a centre-of-mass energy of 13 TeV, using a data sample of nearly 9 million events, corresponding to an integrated luminosity of 170 μb−1170 μb−1, recorded by the ATLAS detector during a special Large Hadron Collider fill. The charged-particle multiplicity, its dependence on transverse momentum and pseudorapidity and the dependence of the mean transverse momentum on the charged-particle multiplicity are presented. The measurements are performed with charged particles with transverse momentum greater than 500 MeV and absolute pseudorapidity less than 2.5, in events with at least one charged particle satisfying these kinematic requirements. Additional measurements in a reduced phase space with absolute pseudorapidity less than 0.8 are also presented, in order to compare with other experiments. The results are corrected for detector effects, presented as particle-level distributions and are compared to the predictions of various Monte Carlo event generators.

Constraints on non-Standard Model Higgs boson interactions in an effective Lagrangian using differential cross sections measured in the H→γγ decay channel at √s = 8 TeV with the ATLAS detector

Wed 23 Aug 2017 09:11:53 KRAT

The strength and tensor structure of the Higgs boson's interactions are investigated using an effective Lagrangian, which introduces additional CP-even and CP-odd interactions that lead to changes in the kinematic properties of the Higgs boson and associated jet spectra with respect to the Standard Model. The parameters of the effective Lagrangian are probed using a fit to five differential cross sections previously measured by the ATLAS experiment in the H→γγH→γγ decay channel with an integrated luminosity of 20.3 fb−1 at View the MathML sources=8 TeV. In order to perform a simultaneous fit to the five distributions, the statistical correlations between them are determined by re-analysing the H→γγH→γγ candidate events in the proton–proton collision data. No significant deviations from the Standard Model predictions are observed and limits on the effective Lagrangian parameters are derived. The statistical correlations are made publicly available to allow for future analysis of theories with non-Standard Model interactions.

Dijet production in √s = 7 TeV pp collisions with large rapidity gaps at the ATLAS experiment

Wed 23 Aug 2017 09:11:53 KRAT

A 6.8 nb−1 sample of pp collision data collected under low-luminosity conditions at View the MathML sources=7TeV by the ATLAS detector at the Large Hadron Collider is used to study diffractive dijet production. Events containing at least two jets with View the MathML sourcepT>20GeV are selected and analysed in terms of variables which discriminate between diffractive and non-diffractive processes. Cross sections are measured differentially in ΔηFΔηF, the size of the observable forward region of pseudorapidity which is devoid of hadronic activity, and in an estimator, View the MathML sourceξ˜, of the fractional momentum loss of the proton assuming single diffractive dissociation (pp→pXpp→pX). Model comparisons indicate a dominant non-diffractive contribution up to moderately large ΔηFΔηF and small View the MathML sourceξ˜, with a diffractive contribution which is significant at the highest ΔηFΔηF and the lowest View the MathML sourceξ˜. The rapidity-gap survival probability is estimated from comparisons of the data in this latter region with predictions based on diffractive parton distribution functions.

Search for single production of a vector-like quark via a heavy gluon in the 4b final state with the ATLAS detector in pp collisions at √s = 8 TeV

Wed 23 Aug 2017 09:11:52 KRAT

A search is performed for the process View the MathML sourcepp→G⁎→BHb¯/B¯Hb→Hbb¯→bb¯bb¯, predicted in composite Higgs scenarios, where G⁎G⁎ is a heavy colour octet vector resonance and BHBH a vector-like quark of charge −1/3−1/3. The data were obtained from pp collisions at a centre-of-mass energy of 8 TeV corresponding to an integrated luminosity of 19.5 fb−119.5 fb−1, recorded by the ATLAS detector at the LHC. The largest background, multijet production, is estimated using a data-driven method. No significant excess of events with respect to Standard Model predictions is observed, and upper limits on the production cross section times branching ratio are set. Comparisons to the predictions from a specific benchmark model are made, resulting in lower mass limits in the two-dimensional mass plane of mG⁎mG⁎ vs. mBHmBH.

Search for invisible decays of a Higgs boson using vector-boson fusion in pp collisions at √s = 8 TeV with the ATLAS detector

Wed 22 Mar 2017 17:35:04 KRAT

A search for a Higgs boson produced via vector-boson fusion and decaying into invisible particles is presented, using 20.3 fb−1 of proton-proton collision data at a centre-of-mass energy of 8 TeV recorded by the ATLAS detector at the LHC. For a Higgs boson with a mass of 125 GeV, assuming the Standard Model production cross section, an upper bound of 0.28 is set on the branching fraction of H → invisible at 95% confidence level, where the expected upper limit is 0.31. The results are interpreted in models of Higgs-portal dark matter where the branching fraction limit is converted into upper bounds on the dark-matter-nucleon scattering cross section as a function of the dark-matter particle mass, and compared to results from the direct dark-matter detection experiments.

Search for resonances in diphoton events at √s = 13 TeV with the ATLAS detector

Wed 22 Mar 2017 17:35:03 KRAT

Searches for new resonances decaying into two photons in the ATLAS experiment at the CERN Large Hadron Collider are described. The analysis is based on proton-proton collision data corresponding to an integrated luminosity of 3.2 fb−1 at s√=13 TeV recorded in 2015. Two searches are performed, one targeted at a spin-2 particle of mass larger than 500 GeV, using Randall-Sundrum graviton states as a benchmark model, and one optimized for a spin-0 particle of mass larger than 200 GeV. Varying both the mass and the decay width, the most significant deviation from the background-only hypothesis is observed at a diphoton invariant mass around 750 GeV with local significances of 3.8 and 3.9 standard deviations in the searches optimized for a spin-2 and spin-0 particle, respectively. The global significances are estimated to be 2.1 standard deviations for both analyses. The consistency between the data collected at 13 TeV and 8 TeV is also evaluated. Limits on the production cross section times branching ratio to two photons for the two resonance types are reported.

Measurement of the inclusive isolated prompt photon cross section in pp collisions at √s = 8 TeV with the ATLAS detector

Wed 22 Mar 2017 17:35:02 KRAT

A measurement of the cross section for the inclusive production of isolated prompt photons in proton-proton collisions at a centre-of-mass energy of s√=8 TeV is presented. The measurement covers the pseudorapidity ranges |ηγ | < 1.37 and 1.56 ≤ |ηγ | < 2.37 in the transverse energy range 25 < ETγ < 1500 GeV. The results are based on an integrated luminosity of 20.2 fb−1, recorded by the ATLAS detector at the LHC. Photon candidates are identified by combining information from the calorimeters and the inner tracker. The background is subtracted using a data-driven technique, based on the observed calorimeter shower-shape variables and the deposition of hadronic energy in a narrow cone around the photon candidate. The measured cross sections are compared with leading-order and next-to-leading order perturbative QCD calculations and are found to be in a good agreement over ten orders of magnitude.

Measurement of the CP-violating phase фs and the B0s meson decay width difference with B0s → J/ψф decays in ATLAS

Wed 05 Apr 2017 10:18:55 KRAT

A measurement of the Bs0 decay parameters in the Bs0 → J/ψϕ channel using an integrated luminosity of 14.3 fb−1 collected by the ATLAS detector from 8 TeV pp collisions at the LHC is presented. The measured parameters include the CP -violating phase ϕs, the decay width Γs and the width difference between the mass eigenstates ΔΓs. The values measured for the physical parameters are statistically combined with those from 4.9 fb−1 of 7 TeV data, leading to the following:

Measurement of the branching fraction for ψ(3770) → γχc0

Tue 22 Aug 2017 09:02:12 KRAT

By analyzing a data set of 2.92 fb−12.92 fb−1 of e+e−e+e− collision data taken at View the MathML sources=3.773 GeV and 106.41×106106.41×106ψ(3686)ψ(3686) decays taken at View the MathML sources=3.686 GeV with the BESIII detector at the BEPCII collider, we measure the branching fraction and the partial decay width for ψ(3770)→γχc0ψ(3770)→γχc0 to be B(ψ(3770)→γχc0)=(6.88±0.28±0.67)×10−3B(ψ(3770)→γχc0)=(6.88±0.28±0.67)×10−3 and Γ[ψ(3770)→γχc0]=(187±8±19) keVΓ[ψ(3770)→γχc0]=(187±8±19) keV, respectively. These are the most precise measurements to date.

Measurement of the e+e− → π+π− cross section between 600 and 900 MeV using initial state radiation

Tue 22 Aug 2017 09:02:08 KRAT

We extract the e+e−→π+π−e+e−→π+π− cross section in the energy range between 600 and 900 MeV, exploiting the method of initial state radiation. A data set with an integrated luminosity of 2.93 fb−12.93 fb−1 taken at a center-of-mass energy of 3.773 GeV with the BESIII detector at the BEPCII collider is used. The cross section is measured with a systematic uncertainty of 0.9%. We extract the pion form factor |Fπ|2|Fπ|2 as well as the contribution of the measured cross section to the leading-order hadronic vacuum polarization contribution to (g−2)μ(g−2)μ. We find this value to be View the MathML sourceaμππ,LO(600–900MeV)=(368.2±2.5stat±3.3sys)⋅10−10, which is between the corresponding values using the BaBar or KLOE data.

Measurement of the charged-particle multiplicity inside jets from √s = 8 TeV pp collisions with the ATLAS detector

Tue 13 Mar 2018 11:23:55 KRAT

The number of charged particles inside jets is a widely used discriminant for identifying the quark or gluon nature of the initiating parton and is sensitive to both the perturbative and non-perturbative components of fragmentation. This paper presents a measurement of the average number of charged particles with pT>500 MeV inside high-momentum jets in dijet events using 20.3 fb−1 of data recorded with the ATLAS detector in pp collisions at s√=8 TeV collisions at the LHC. The jets considered have transverse momenta from 50 GeV up to and beyond 1.5 TeV. The reconstructed charged-particle track multiplicity distribution is unfolded to remove distortions from detector effects and the resulting charged-particle multiplicity is compared to several models. Furthermore, quark and gluon jet fractions are used to extract the average charged-particle multiplicity for quark and gluon jets separately.

Measurements of the center-of-mass energies at BESIII via the di-muon process

Tue 13 Mar 2018 11:23:54 KRAT

From 2011 to 2014, the BESIII experiment collected about 5 fb−1 data at center-of-mass energies around 4 GeV for the studies of the charmonium-like and higher excited charmonium states. By analyzing the di-muon process e+e− → γISR/FSRμ+μ−, the center-of-mass energies of the data samples are measured with a precision of 0.8 MeV. The center-of-mass energy is found to be stable for most of the time during data taking.

Search for new phenomena with photon+jet events in proton-proton collisions at √s=13 TeV with the ATLAS detector

Tue 13 Jun 2017 13:35:30 KRAT

A search is performed for the production of high-mass resonances decaying into a photon and a jet in 3.2 fb−1 of proton-proton collisions at a centre-of-mass energy of s√=13 TeV collected by the ATLAS detector at the Large Hadron Collider. Selected events have an isolated photon and a jet, each with transverse momentum above 150 GeV. No significant deviation of the γ+jet invariant mass distribution from the background-only hypothesis is found. Limits are set at 95% confidence level on the cross sections of generic Gaussian-shaped signals and of a few benchmark phenomena beyond the Standard Model: excited quarks with vector-like couplings to the Standard Model particles, and non-thermal quantum black holes in two models of extra spatial dimensions. The minimum excluded visible cross sections for Gaussian-shaped resonances with width-to-mass ratios of 2% decrease from about 6 fb for a mass of 1.5 TeV to about 0.8 fb for a mass of 5 TeV. The minimum excluded visible cross sections for Gaussian-shaped resonances with width-to-mass ratios of 15% decrease from about 50 fb for a mass of 1.5 TeV to about 1.0 fb for a mass of 5 TeV. Excited quarks are excluded below masses of 4.4 TeV, and non-thermal quantum black holes are excluded below masses of 3.8 (6.2) TeV for Randall-Sundrum (Arkani-Hamed-Dimopoulous-Dvali) models with one (six) extra dimensions.

Measurement of the relative width difference of the B0-B¯0 system with the ATLAS detector

Tue 13 Jun 2017 13:35:20 KRAT

This paper presents the measurement of the relative width difference ΔΓd/Γd of the B0-B¯¯¯¯0 system using the data collected by the ATLAS experiment at the LHC in pp collisions at s√=7 TeV and s√=8 TeV and corresponding to an integrated luminosity of 25.2 fb−1. The value of ΔΓd/Γd is obtained by comparing the decay-time distributions of B0 → J/ψKS and B0 → J/ψK*0(892) decays. The result is ΔΓd/Γd = (−0.1±1.1 (stat.)± 0.9 (syst.)) × 10−2. Currently, this is the most precise single measurement of ΔΓd/Γd. It agrees with the Standard Model prediction and the measurements by other experiments.

Search for charged Higgs bosons in the H± → tb decay channel in pp collisions at √s = 8 TeV using the ATLAS detector

Tue 13 Jun 2017 13:35:19 KRAT

Charged Higgs bosons heavier than the top quark and decaying via H± → tb are searched for in proton-proton collisions measured with the ATLAS experiment at ( sqrt{s}=8 ) TeV corresponding to an integrated luminosity of 20.3fb−1. The production of a charged Higgs boson in association with a top quark, gb → tH±, is explored in the mass range 200 to 600 GeV using multi-jet final states with one electron or muon. In order to separate the signal from the Standard Model background, analysis techniques combining several kinematic variables are employed. An excess of events above the background-only hypothesis is observed across a wide mass range, amounting to up to 2.4 standard deviations. Upper limits are set on the gb → tH± production cross section times the branching fraction BR(H± → tb). Additionally, the complementary s-channel production, qq′ → H±, is investigated through a reinterpretation of W′ → tb searches in ATLAS. Final states with one electron or muon are relevant for H± masses from 0.4 to 2.0 TeV, whereas the all-hadronic final state covers the range 1.5 to 3.0 TeV. In these search channels, no significant excesses from the predictions of the Standard Model are observed, and upper limits are placed on the qq′ → H± production cross section times the branching fraction BR(H± → tb).

Search for the standard model Higgs boson produced in association with a vector boson and decaying into a tau pair in pp collisions at √s=8 TeV with the ATLAS detector

Tue 04 Dec 2018 12:47:18 KRAT

A search for the standard model Higgs boson produced in association with a vector boson with the decay H→ττ is presented. The data correspond to 20.3 fb−1 of integrated luminosity from proton-proton collisions at √s=8 TeV recorded by the ATLAS experiment at the LHC during 2012. The data agree with the background expectation, and 95% confidence-level upper limits are placed on the cross section of this process. The observed (expected) limit, expressed in terms of the signal strength μ

Observation of an anomalous line shape of the η′π+π−mass spectrum near the pp mass threshold in J/ψ→γη′π+π−

Tue 04 Apr 2017 10:58:58 KRAT

Using 1.09×109 J/ψ events collected by the BESIII experiment in 2012, we study the J/ψ→γη′π+π− process and observe a significant abrupt change in the slope of the η′π+π− invariant mass distribution at the proton-antiproton (p¯p) mass threshold. We use two models to characterize the η′π+π− line shape around 1.85 GeV/c2: one that explicitly incorporates the opening of a decay threshold in the mass spectrum (Flatté formula), and another that is the coherent sum of two resonant amplitudes. Both fits show almost equally good agreement with data, and suggest the existence of either a broad state around 1.85 GeV/c2 with strong couplings to the p¯p final states or a narrow state just below the p¯p mass threshold. Although we cannot distinguish between the fits, either one supports the existence of a p¯p moleculelike state or bound state with greater than 7σ significance.

Measurement of the production cross-section of a single top quark in association with a W boson at 8 TeV with the ATLAS experiment

Tue 02 May 2017 12:56:23 KRAT

The cross-section for the production of a single top quark in association with a W boson in proton-proton collisions at s√=8TeV is measured. The dataset corresponds to an integrated luminosity of 20.3 fb−1, collected by the ATLAS detector in 2012 at the Large Hadron Collider at CERN. Events containing two leptons and one central b-jet are selected. The W t signal is separated from the backgrounds using boosted decision trees, each of which combines a number of discriminating variables into one classifier. Production of W t events is observed with a significance of 7.7σ. The cross-section is extracted in a profile likelihood fit to the classifier output distributions. The W t cross-section, inclusive of decay modes, is measured to be 23.0 ± 1.3(stat.)− 3.5+ 3.2(syst.)±1.1(lumi.) pb. The measured cross-section is used to extract a value for the CKM matrix element |Vtb| of 1.01 ± 0.10 and a lower limit of 0.80 at the 95% confidence level. The cross-section for the production of a top quark and a W boson is also measured in a fiducial acceptance requiring two leptons with pT> 25 GeV and |η| < 2.5, one jet with pT> 20 GeV and |η| < 2.5, and ETmiss > 20 GeV, including both W t and top-quark pair events as signal. The measured value of the fiducial cross-section is 0.85 ± 0.01(stat.)− 0.07+ 0.07(syst.)±0.03(lumi.) pb.

Measurement of the correlation between the polar angles of leptons from top quark decays in the helicity basis at √s = 7 TeV using the ATLAS detector

Thu 22 Mar 2018 09:47:57 KRAT

A measurement of the correlations between the polar angles of leptons from the decay of pair-produced t and ¯t quarks in the helicity basis is reported, using proton-proton collision data collected by the ATLAS detector at the LHC. The dataset corresponds to an integrated luminosity of 4.6 fb−1 at a center-of-mass energy of √s=7 TeV collected during 2011. Candidate events are selected in the dilepton topology with large missing transverse momentum and at least two jets. The angles θ1 and θ2 between the charged leptons and the direction of motion of the parent quarks in the t¯t rest frame are sensitive to the spin information, and the distribution of cosθ1⋅cosθ2 is sensitive to the spin correlation between the t and ¯t quarks. The distribution is unfolded to parton level and compared to the next-to-leading order prediction. A good agreement is observed.

Beam-induced and cosmic-ray backgrounds observed in the ATLAS detector during the LHC 2012 proton-proton running period

Thu 15 Mar 2018 09:41:01 KRAT

This paper discusses various observations on beam-induced and cosmic-ray backgrounds in the ATLAS detector during the LHC 2012 proton-proton run. Building on published results based on 2011 data, the correlations between background and residual pressure of the beam vacuum are revisited. Ghost charge evolution over 2012 and its role for backgrounds are evaluated. New methods to monitor ghost charge with beam-gas rates are presented and observations of LHC abort gap population by ghost charge are discussed in detail. Fake jets from colliding bunches and from ghost charge are analysed with improved methods, showing that ghost charge in individual radio-frequency buckets of the LHC can be resolved. Some results of two short periods of dedicated cosmic-ray background data-taking are shown; in particular cosmic-ray muon induced fake jet rates are compared to Monte Carlo simulations and to the fake jet rates from beam background. A thorough analysis of a particular LHC fill, where abnormally high background was observed, is presented. Correlations between backgrounds and beam intensity losses in special fills with very high β* are studied.

A new method to distinguish hadronically decaying boosted Z bosons from W bosons using the ATLAS detector

Thu 15 Mar 2018 09:40:58 KRAT

The distribution of particles inside hadronic jets produced in the decay of boosted W and Z bosons can be used to discriminate such jets from the continuum background. Given that a jet has been identified as likely resulting from the hadronic decay of a boosted W or Z boson, this paper presents a technique for further differentiating Z bosons from W bosons. The variables used are jet mass, jet charge, and a b-tagging discriminant. A likelihood tagger is constructed from these variables and tested in the simulation of (W'

Search for dark matter produced in association with a Higgs boson decaying to two bottom quarks in pp collisions at √s = 8 TeV with the ATLAS detector

Thu 15 Mar 2018 09:40:54 KRAT

This article reports on a search for dark matter pair production in association with a Higgs boson decaying to a pair of bottom quarks, using data from 20.3 fb −1 of p p collisions at a center-of-mass energy of 8 TeV collected by the ATLAS detector at the LHC. The decay of the Higgs boson is reconstructed as a high-momentum b¯b system with either a pair of small-radius jets, or a single large-radius jet with substructure. The observed data are found to be consistent with the expected Standard Model backgrounds. Model-independent upper limits are placed on the visible cross sections for events with a Higgs boson decaying into b¯b and large missing transverse momentum with thresholds ranging from 150 to 400 GeV. Results are interpreted using a simplified model with a Z′ gauge boson decaying into different Higgs bosons predicted in a two-Higgs-doublet model, of which the heavy pseudoscalar Higgs decays into a pair of dark matter particles. Exclusion limits are also presented for the mass scales of various effective field theory operators that describe the interaction between dark matter particles and the Higgs boson.

Search for metastable heavy charged particles with large ionization energy loss in pp collisions at √s = 13 TeV using the ATLAS experiment

Sat 10 Mar 2018 12:34:47 KRAT

This paper presents a search for massive charged long-lived particles produced in pp collisions at s√= 13 TeV at the LHC using the ATLAS experiment. The dataset used corresponds to an integrated luminosity of 3.2 fb−1. Many extensions of the Standard Model predict the existence of massive charged long-lived particles, such as R-hadrons. These massive particles are expected to be produced with a velocity significantly below the speed of light, and therefore to have a specific ionization higher than any Standard Model particle of unit charge at high momenta. The Pixel subsystem of the ATLAS detector is used to measure the ionization energy loss of reconstructed charged particles and to search for such highly ionizing particles. The search presented here has much greater sensitivity than a similar search performed using the ATLAS detector in the s√= 8 TeV dataset, thanks to the increase in expected signal cross-section due to the higher center-of-mass energy of collisions, to an upgraded detector with a new silicon layer close to the interaction point, and to analysis improvements. No significant deviation from Standard Model background expectations is observed, and lifetime-dependent upper limits on R-hadron production cross-sections and masses are set. Gluino R-hadrons with lifetimes above 0.4 ns and decaying to qq¯ plus a 100 GeV neutralino are excluded at the 95% confidence level, with lower mass limit ranging between 740 GeV and 1590 GeV. In the case of stable R-hadrons the lower mass limit at the 95% confidence level is 1570 GeV.

Measurement of the centrality dependence of the charged-particle pseudorapidity distribution in proton-lead collisions at √sNN = 5.02 TeV with the ATLAS detector

Mon 19 Feb 2018 10:25:49 KRAT

The centrality dependence of the mean charged-particle multiplicity as a function of pseudorapidity is measured in approximately 1 μμ b −1−1 of proton–lead collisions at a nucleon–nucleon centre-of-mass energy of sNN−−−√=5.02sNN=5.02 TeVTeV using the ATLAS detector at the Large Hadron Collider. Charged particles with absolute pseudorapidity less than 2.7 are reconstructed using the ATLAS pixel detector. The collision centrality is characterised by the total transverse energy measured in the Pb-going direction of the forward calorimeter. The charged-particle pseudorapidity distributions are found to vary strongly with centrality, with an increasing asymmetry between the proton-going and Pb-going directions as the collisions become more central. Three different estimations of the number of nucleons participating in the collision have been carried out using the Glauber model as well as two Glauber–Gribov inspired extensions to the Glauber model. Charged-particle multiplicities per participant pair are found to vary differently for these three models, highlighting the importance of including colour fluctuations in nucleon–nucleon collisions in the modelling of the initial state of collisions.

Erratum to: Study of the spin and parity of the Higgs boson in diboson decays with the ATLAS detector

Mon 16 Dec 2019 15:01:23 KRAT

Erratum to: Eur. Phys. J. C (2015) 75:476 DOI 10.1140/epjc/s10052-015-3685-1

Search for new phenomena in events with at least three photons collected in pp collisions at √s = 8 TeV with the ATLAS detector

Mon 03 Dec 2018 11:48:39 KRAT

Results of a search for new phenomena in events with at least three photons are reported. Data from proton–proton collisions at a centre-of-mass energy of 8 TeV, corresponding to an integrated luminosity of 20.3 fb−1, were collected with the ATLAS detector at the LHC. The observed data are well described by the Standard Model. Limits at the 95 % confidence level on new phenomena are presented based on the rate of events in an inclusive signal region and a restricted signal region targeting the rare decay Z→3γ, as well as di-photon and tri-photon resonance searches. For a Standard Model Higgs boson decaying to four photons via a pair of intermediate pseudoscalar particles (a), limits are found to be σ× BR (h→aa)× BR (a→γγ)2<10−3σSM for 10 GeV 125 GeV, and for a Z′ decaying to three photons via Z′→a+γ→3γ. Additionally, the observed limit on the branching ratio of the Z boson decay to three photons is found to be BR(Z→3γ)<2.2×10−6, a result five times stronger than the previous result from LEP.

Search for a high-mass Higgs boson decaying to a W boson pair in pp collisions at √s = 8 TeV with the ATLAS detector

Mon 03 Dec 2018 11:34:41 KRAT

A search for a high-mass Higgs boson H is performed in the H → WW → ℓνℓν and H → WW → ℓνqq decay channels using pp collision data corresponding to an integrated luminosity of 20.3 fb−1 collected at ( sqrt{s}=8 ) TeV by the ATLAS detector at the Large Hadron Collider. No evidence of a high-mass Higgs boson is found. Limits on σH × BR(H → WW) as a function of the Higgs boson mass mH are determined in three different scenarios: one in which the heavy Higgs boson has a narrow width compared to the experimental resolution, one for a width increasing with the boson mass and modeled by the complex-pole scheme following the same behavior as in the Standard Model, and one for intermediate widths. The upper range of the search is mH = 1500 GeV for the narrow-width scenario and mH = 1000 GeV for the other two scenarios. The lower edge of the search range is 200–300 GeV and depends on the analysis channel and search scenario. For each signal interpretation, individual and combined limits from the two WW decay channels are presented. At mH = 1500 GeV, the highest-mass point tested, σH × BR(H → WW) for a narrow-width Higgs boson is constrained to be less than 22 fb and 6.6 fb at 95% CL for the gluon fusion and vector-boson fusion production modes, respectively.

Search for Higgs and Z boson decays to фγ with the ATLAS detector

Mon 03 Dec 2018 11:27:51 KRAT

A search for the decays of the Higgs and Z bosons to a ϕ meson and a photon is performed with a pp collision data sample corresponding to an integrated luminosity of 2.7 fb−1 collected at s√=13 TeV with the ATLAS detector at the LHC. No significant excess of events is observed above the background, and 95% confidence level upper limits on the branching fractions of the Higgs and Z boson decays to ϕγ of 1.4×10−3 and 8.3×10−6, respectively, are obtained.

Search for strong gravity in multijet final states produced in pp collisions at √s = 13 TeV using the ATLAS detector at the LHC

Mon 03 Dec 2018 11:20:57 KRAT

A search is conducted for new physics in multijet final states using 3.6 inverse femtobarns of data from proton-proton collisions at s√=13 TeV taken at the CERN Large Hadron Collider with the ATLAS detector. Events are selected containing at least three jets with scalar sum of jet transverse momenta (HT) greater than 1 TeV. No excess is seen at large HT and limits are presented on new physics: models which produce final states containing at least three jets and having cross sections larger than 1.6 fb with HT> 5.8 TeV are excluded. Limits are also given in terms of new physics models of strong gravity that hypothesize additional space-time dimensions.

Identification of high transverse momentum top quarks in pp collisions at √s = 8 TeV with the ATLAS detector

Mon 03 Dec 2018 11:20:50 KRAT

This paper presents studies of the performance of several jet-substructure techniques, which are used to identify hadronically decaying top quarks with high transverse momentum contained in large-radius jets. The efficiency of identifying top quarks is measured using a sample of top-quark pairs and the rate of wrongly identifying jets from other quarks or gluons as top quarks is measured using multijet events collected with the ATLAS experiment in 20.3 fb−1 of 8 TeV proton-proton collisions at the Large Hadron Collider. Predictions from Monte Carlo simulations are found to provide an accurate description of the performance. The techniques are compared in terms of signal efficiency and background rejection using simulations, covering a larger range in jet transverse momenta than accessible in the dataset. Additionally, a novel technique is developed that is optimized to reconstruct top quarks in events with many jets.

Search for the production of single vector-like and excited quarks in the Wt final state in pp collisions at √s = 8 TeV with the ATLAS detector

Mon 03 Dec 2018 11:20:48 KRAT

A search for vector-like quarks and excited quarks in events containing a top quark and a W boson in the final state is reported here. The search is based on 20.3 fb−1 of proton-proton collision data taken at the LHC at a centre-of-mass energy of 8 TeV recorded by the ATLAS detector. Events with one or two leptons, and one, two or three jets are selected with the additional requirement that at least one jet contains a b-quark. Single-lepton events are also required to contain at least one large-radius jet from the hadronic decay of a high-pTW boson or a top quark. No significant excess over the expected background is observed and upper limits on the cross-section times branching ratio for different vector-like quark and excited-quark model masses are derived. For the excited-quark production and decay to Wt with unit couplings, quarks with masses below 1500 GeV are excluded and coupling-dependent limits are set.

Search for anomalous couplings in the W tb vertex from the measurement of double differential angular decay rates of single top quarks produced in the t-channel with the ATLAS detector

Mon 03 Dec 2018 11:13:46 KRAT

The electroweak production and subsequent decay of single top quarks is determined by the properties of the Wtb vertex. This vertex can be described by the complex parameters of an effective Lagrangian. An analysis of angular distributions of the decay products of single top quarks produced in the t -channel constrains these parameters simultaneously. The analysis described in this paper uses 4.6 fb−1 of proton-proton collision data at s√=7 TeV collected with the ATLAS detector at the LHC. Two parameters are measured simultaneously in this analysis. The fraction f1 of decays containing transversely polarised W bosons is measured to be 0.37 ± 0.07 (stat.⊕syst.). The phase δ− between amplitudes for transversely and longitudinally polarised W bosons recoiling against left-handed b-quarks is measured to be −0.014π ± 0.036π (stat.⊕syst.). The correlation in the measurement of these parameters is 0.15. These values result in two-dimensional limits at the 95% confidence level on the ratio of the complex coupling parameters gR and VL, yielding Re[gR/VL] ∈ [−0.36, 0.10] and Im[gR/VL] ∈ [−0.17, 0.23] with a correlation of 0.11. The results are in good agreement with the predictions of the Standard Model.

Search for the Standard Model Higgs boson decaying into bb¯ produced in association with top quarks decaying hadronically in pp collisions at √s = 8 TeV with the ATLAS detector

Mon 03 Dec 2018 11:13:45 KRAT

A search for Higgs boson production in association with a pair of top quarks (tt¯H) is performed, where the Higgs boson decays to bb¯¯, and both top quarks decay hadronically. The data used correspond to an integrated luminosity of 20.3 fb−1 of pp collisions at s√=8 TeV collected with the ATLAS detector at the Large Hadron Collider. The search selects events with at least six energetic jets and uses a boosted decision tree algorithm to discriminate between signal and Standard Model background. The dominant multijet background is estimated using a dedicated data-driven technique. For a Higgs boson mass of 125 GeV, an upper limit of 6.4 (5.4) times the Standard Model cross section is observed (expected) at 95% confidence level. The best-fit value for the signal strength is μ = 1.6 ± 2.6 times the Standard Model expectation for mH = 125 GeV. Combining all tt¯H searches carried out by ATLAS at s√=8 and 7 TeV, an observed (expected) upper limit of 3.1 (1.4) times the Standard Model expectation is obtained at 95% confidence level, with a signal strength μ = 1.7 ± 0.8.

Search for squarks and gluinos in final states with jets and missing transverse momentum at √s = 13 TeV with the ATLAS detector

Mon 03 Dec 2018 11:13:42 KRAT

A search for squarks and gluinos in final states containing hadronic jets, missing transverse momentum but no electrons or muons is presented. The data were recorded in 2015 by the ATLAS experiment in (sqrt{s}=13~{mathrm{TeV}}) proton–proton collisions at the Large Hadron Collider. No excess above the Standard Model background expectation was observed in 3.2 (mathrm{fb}^{-1}) of analyzed data. Results are interpreted within simplified models that assume R-parity is conserved and the neutralino is the lightest supersymmetric particle. An exclusion limit at the 95 % confidence level on the mass of the gluino is set at 1.51 ({mathrm{TeV}}) for a simplified model incorporating only a gluino octet and the lightest neutralino, assuming the lightest neutralino is massless. For a simplified model involving the strong production of mass-degenerate first- and second-generation squarks, squark masses below 1.03 ({mathrm{TeV}}) are excluded for a massless lightest neutralino. These limits substantially extend the region of supersymmetric parameter space excluded by previous measurements with the ATLAS detector.

Search for new phenomena in events with a photon and missing transverse momentum in pp collisions at √s = 13 TeV with the ATLAS detector

Mon 03 Dec 2018 11:13:39 KRAT

Results of a search for new phenomena in events with an energetic photon and large missing transverse momentum with the ATLAS experiment at the Large Hadron Collider are reported. The data were collected in proton-proton collisions at a centre-of-mass energy of 13 TeV and correspond to an integrated luminosity of 3.2 fb−1. The observed data are in agreement with the Standard Model expectations. Exclusion limits are presented in models of new phenomena including pair production of dark matter candidates or large extra spatial dimensions. In a simplified model of dark matter and an axial-vector mediator, the search excludes mediator masses below 710 GeV for dark matter candidate masses below 150 GeV. In an effective theory of dark matter production, values of the suppression scale M∗ up to 570 GeV are excluded and the effect of truncation for various coupling values is reported. For the ADD large extra spatial dimension model the search places more stringent limits than earlier searches in the same event topology, excluding MD up to about 2.3 (2.8) TeV for two (six) additional spatial dimensions; the limits are reduced by 20-40% depending on the number of additional spatial dimensions when applying a truncation procedure.

Measurement of the ZZ production cross section in pp collisions at √s = 13 TeV with the ATLAS detector

Mon 03 Dec 2018 09:50:12 KRAT

The ZZ production cross section in proton-proton collisions at 13 TeV center-of-mass energy is measured using 3.2 fb−1 of data recorded with the ATLAS detector at the Large Hadron Collider. The considered Z boson candidates decay to an electron or muon pair of mass 66–116 GeV. The cross section is measured in a fiducial phase space reflecting the detector acceptance. It is also extrapolated to a total phase space for Z bosons in the same mass range and of all decay modes, giving 16.7+2.2−2.0(stat)+0.9−0.7(syst)+1.0−0.7(lumi) pb. The results agree with standard model predictions.

Observation of long-range elliptic azimuthal anisotropies in √s = 13 TeV and 2.76 TeV pp collisions with the ATLAS detector

Mon 03 Dec 2018 09:50:10 KRAT

ATLAS has measured two-particle correlations as a function of the relative azimuthal angle, Δϕ, and pseudorapidity, Δη, in s√=13 and 2.76 TeV pp collisions at the LHC using charged particles measured in the pseudorapidity interval |η|<2.5. The correlation functions evaluated in different intervals of measured charged-particle multiplicity show a multiplicity-dependent enhancement at Δϕ∼0 that extends over a wide range of Δη, which has been referred to as the “ridge.” Per-trigger-particle yields, Y(Δϕ), are measured over 2<|Δη|<5. For both collision energies, the Y(Δϕ) distribution in all multiplicity intervals is found to be consistent with a linear combination of the per-trigger-particle yields measured in collisions with less than 20 reconstructed tracks, and a constant combinatoric contribution modulated by cos(2Δϕ). The fitted Fourier coefficient, v2,2, exhibits factorization, suggesting that the ridge results from per-event cos(2ϕ) modulation of the single-particle distribution with Fourier coefficients v2. The v2 values are presented as a function of multiplicity and transverse momentum. They are found to be approximately constant as a function of multiplicity and to have a pT dependence similar to that measured in p+Pb and Pb+Pb collisions. The v2 values in the 13 and 2.76 TeV data are consistent within uncertainties. These results suggest that the ridge in pp collisions arises from the same or similar underlying physics as observed in p+Pb collisions, and that the dynamics responsible for the ridge has no strong s√ dependence.

Observation of the singly Cabibbo-suppressed decay D+ → ωπ+ and evidence for D0 → ωπ0

Mon 03 Dec 2018 09:50:07 KRAT

Based on 2.93 fb−1 e+e− collision data taken at center-of-mass energy of 3.773 GeV by the BESIII detector, we report searches for the singly Cabibbo-suppressed decays D+→ωπ+ and D0→ωπ0. A double tag technique is used to measure the absolute branching fractions B(D+→ωπ+)=(2.79±0.57±0.16)×10−4 and B(D0→ωπ0)=(1.17±0.34±0.07)×10−4, with statistical significances of 5.5σ and 4.1σ, where the first and second uncertainties are statistical and systematic, respectively.

Measurements of absolute hadronic branching fractions of the Λc+ baryon

Mon 03 Dec 2018 09:49:55 KRAT

We report the first measurement of absolute hadronic branching fractions of Λ+c baryon at the Λ+cΛ¯−c production threshold, in the 30 years since the Λ+c discovery. In total, 12 Cabibbo-favored Λ+c hadronic decay modes are analyzed with a double-tag technique, based on a sample of 567 pb−1 of e+e− collisions at s√=4.599 GeV recorded with the BESIII detector. A global least-squares fitter is utilized to improve the measured precision. Among the measurements for twelve Λ+c decay modes, the branching fraction for Λ+c→pK−π+ is determined to be (5.84±0.27±0.23)%, where the first uncertainty is statistical and the second is systematic. In addition, the measurements of the branching fractions of the other 11 Cabibbo-favored hadronic decay modes are significantly improved.

Measurement of event-shape observables in Z → l+l− events in pp collisions at √s = 7 TeV with the ATLAS detector at the LHC

Mon 03 Dec 2018 09:49:54 KRAT

Event-shape observables measured using charged particles in inclusive Z-boson events are presented, using the electron and muon decay modes of the Z bosons. The measurements are based on an integrated luminosity of (1.1~mathrm{fb}^{-1}) of proton–proton collisions recorded by the ATLAS detector at the LHC at a centre-of-mass energy (sqrt{s}=7) ({mathrm{TeV}}). Charged-particle distributions, excluding the lepton–antilepton pair from the Z-boson decay, are measured in different ranges of transverse momentum of the Z boson. Distributions include multiplicity, scalar sum of transverse momenta, beam thrust, transverse thrust, spherocity, and ({mathcal {F}})-parameter, which are in particular sensitive to properties of the underlying event at small values of the Z-boson transverse momentum. The measured observables are compared with predictions from Pythia 8, Sherpa, and Herwig 7. Typically, all three Monte Carlo generators provide predictions that are in better agreement with the data at high Z-boson transverse momenta than at low Z-boson transverse momenta, and for the observables that are less sensitive to the number of charged particles in the event.

Observation of hc Radiative Decay hc → γη' and Evidence for hc → γη

Mon 03 Dec 2018 09:49:51 KRAT

A search for radiative decays of the P-wave spin singlet charmonium resonance hc is performed based on 4.48×108 ψ′ events collected with the BESIII detector operating at the BEPCII storage ring. Events of the reaction channels hc→γη′ and γη are observed with a statistical significance of 8.4σ and 4.0σ, respectively, for the first time. The branching fractions of hc→γη′ and hc→γη are measured to be B(hc→γη′)=(1.52±0.27±0.29)×10−3 and B(hc→γη)=(4.7±1.5±1.4)×10−4, respectively, where the first errors are statistical and the second are systematic uncertainties.

Measurement of azimuthal asymmetries in inclusive charged dipion production in e+e- annihilations at √s = 3.65 GeV

Mon 03 Dec 2018 09:49:41 KRAT

We present a measurement of the azimuthal asymmetries of two charged pions in the inclusive process e+e−→ππX, based on a data set of 62 pb−1 at the center-of-mass energy of 3.65 GeV collected with the BESIII detector. These asymmetries can be attributed to the Collins fragmentation function. We observe a nonzero asymmetry, which increases with increasing pion momentum. As our energy scale is close to that of the existing semi-inclusive deep inelastic scattering experimental data, the measured asymmetries are important inputs for the global analysis of extracting the quark transversity distribution inside the nucleon and are valuable to explore the energy evolution of the spin-dependent fragmentation function.

A search for top squarks with R-parity-violating decays to all-hadronic final states with the ATLAS detector in √s = 8 TeV proton-proton collisions

Mon 03 Dec 2018 09:35:48 KRAT

A search for the pair production of top squarks, each with R-parity-violating decays into two Standard Model quarks, is performed using 17.4 fb−1 of s√=8 TeV proton-proton collision data recorded by the ATLAS experiment at the LHC. Each top squark is assumed to decay to a b- and an s-quark, leading to four quarks in the final state. Background discrimination is achieved with the use of b-tagging and selections on the mass and substructure of large-radius jets, providing sensitivity to top squark masses as low as 100 GeV. No evidence of an excess beyond the Standard Model background prediction is observed and top squarks decaying to b¯¯s¯¯¯ are excluded for top squark masses in the range 100 ≤mt¯≤ 315 GeV at 95% confidence level.

A search for prompt lepton-jets in pp collisions at √s = 8 TeV with the ATLAS detector

Mon 03 Dec 2018 09:35:41 KRAT

A search is presented for a new, light boson with a mass of about 1 GeV and decaying promptly to jets of collimated electrons and/or muons (lepton-jets). The analysis is performed with 20.3 fb−1 of data collected by the ATLAS detector at the Large Hadron Collider in proton-proton collisions at a centre-of-mass energy of 8 TeV. Events are required to contain at least two lepton-jets. This study finds no statistically significant deviation from predictions of the Standard Model and places 95% confidence-level upper limits on the contribution of new phenomena beyond the SM, incuding SUSY-portal and Higgs-portal models, on the number of events with lepton-jets.

Search for the electroweak production of supersymmetric particles in √s = 8 TeV pp collisions with the ATLAS detector

Mon 02 Apr 2018 10:59:21 KRAT

The ATLAS experiment has performed extensive searches for the electroweak production of charginos, neutralinos, and staus. This article summarizes and extends the search for electroweak supersymmetry with new analyses targeting scenarios not covered by previously published searches. New searches use vector-boson fusion production, initial-state radiation jets, and low-momentum lepton final states, as well as multivariate analysis techniques to improve the sensitivity to scenarios with small mass splittings and low-production cross sections. Results are based on 20 fb−1 of proton-proton collision data at √s=8 TeV recorded with the ATLAS experiment at the Large Hadron Collider. No significant excess beyond Standard Model expectations is observed. The new and existing searches are combined and interpreted in terms of 95% confidence-level exclusion limits in simplified models, where a single production process and decay mode is assumed, as well as within phenomenological supersymmetric models.

Measurement of the differential cross-section of highly boosted top quarks as a function of their transverse momentum in √s = 8 TeV proton-proton collisions using the ATLAS detector

Mon 02 Apr 2018 10:59:08 KRAT

The differential cross-section for pair production of top quarks with high transverse momentum is measured in

Performance of b-jet identification in the ATLAS experiment

Mon 02 Apr 2018 10:24:11 KRAT

The identification of jets containing b hadrons is important for the physics programme of the ATLAS experiment at the Large Hadron Collider. Several algorithms to identify jets containing b hadrons are described, ranging from those based on the reconstruction of an inclusive secondary vertex or the presence of tracks with large impact parameters to combined tagging algorithms making use of multi-variate discriminants. An independent b-tagging algorithm based on the reconstruction of muons inside jets as well as the b-tagging algorithm used in the online trigger are also presented. The b-jet tagging efficiency, the c-jet tagging efficiency and the mistag rate for light flavour jets in data have been measured with a number of complementary methods. The calibration results are presented as scale factors defined as the ratio of the efficiency (or mistag rate) in data to that in simulation. In the case of b jets, where more than one calibration method exists, the results from the various analyses have been combined taking into account the statistical correlation as well as the correlation of the sources of systematic uncertainty.

Search for magnetic monopoles and stable particles with high electric charges in 8 TeV pp collisions with the ATLAS detector

Mon 02 Apr 2018 10:24:06 KRAT

A search for highly ionizing particles produced in proton-proton collisions at 8 TeV center-of-mass energy is performed by the ATLAS Collaboration at the CERN Large Hadron Collider. The data set used corresponds to an integrated luminosity of 7.0 fb−1. A customized trigger significantly increases the sensitivity, permitting a search for such particles with charges and energies beyond what was previously accessible. No events were found in the signal region, leading to production cross section upper limits in the mass range 200–2500 GeV for magnetic monopoles with magnetic charge in the range 0.5gD<|g|<2.0gD, where gD is the Dirac charge, and for stable particles with electric charge in the range 10<|z|<60. Model-dependent limits are presented in given pair-production scenarios, and model-independent limits are presented in fiducial regions of particle energy and pseudorapidity.

Centrality, rapidity, and transverse momentum dependence of isolated prompt photon production in lead-lead collisions at √sNN = 2.76 TeV measured with the ATLAS detector

Mon 02 Apr 2018 10:23:59 KRAT

Prompt photon production in √sNN=2.76-TeV Pb+Pb collisions has been measured by the ATLAS experiment at the Large Hadron Collider using data collected in 2011 with an integrated luminosity of 0.14 nb−1. Inclusive photon yields, scaled by the mean nuclear thickness function, are presented as a function of collision centrality and transverse momentum in two pseudorapidity intervals, |η|<1.37 and1.52≤|η|<2.37. The scaled yields in the two pseudorapidity intervals, as well as the ratios of the forward yields to those at midrapidity, are compared to the expectations from next-to-leading-order perturbative QCD (pQCD) calculations. The measured cross sections agree well with the predictions for proton-proton collisions within statistical and systematic uncertainties. Both the yields and the ratios are also compared to two other pQCD calculations, one which uses the isospin content appropriate to colliding lead nuclei and another which includes nuclear modifications to the nucleon parton distribution functions.

Measurement of the charge asymmetry in top-quark pair production in the lepton-plus-jets final state in pp collision data at √s = 8 TeV with the ATLAS detector

Fri 29 Sep 2017 16:42:41 KRAT

This paper reports inclusive and differential measurements of the tˉt charge asymmetry AC in 20.3 fb−1 of √s=8 TeVpp collisions recorded by the ATLAS experiment at the Large Hadron Collider at CERN. Three differential measurements are performed as a function of the invariant mass, transverse momentum and longitudinal boost of the tˉt system. The tˉt pairs are selected in the single-lepton channels (e or μ) with at least four jets, and a likelihood fit is used to reconstruct the tˉt event kinematics. A Bayesian unfolding procedure is performed to infer the asymmetry at parton level from the observed data distribution. The inclusive tˉt charge asymmetry is measured to be AC=0.009±0.005 (stat. + syst.). The inclusive and differential measurements are compatible with the values predicted by the Standard Model.

Search for heavy long-lived charged R-hadrons with the ATLAS detector in 3.2 fb−1 of proton-proton collision data at √s = 13 TeV

Fri 29 Sep 2017 16:42:23 KRAT

A search for heavy long-lived charged R-hadrons is reported using a data sample corresponding to 3.2 fb−1 of proton–proton collisions at View the MathML sources=13 TeV collected by the ATLAS experiment at the Large Hadron Collider at CERN. The search is based on observables related to large ionisation losses and slow propagation velocities, which are signatures of heavy charged particles travelling significantly slower than the speed of light. No significant deviations from the expected background are observed. Upper limits at 95% confidence level are provided on the production cross section of long-lived R-hadrons in the mass range from 600 GeV to 2000 GeV and gluino, bottom and top squark masses are excluded up to 1580 GeV, 805 GeV and 890 GeV, respectively.

Search for TeV-scale gravity signatures in high-mass final states with leptons and jets with the ATLAS detector at √s=13TeV

Fri 29 Sep 2017 16:42:22 KRAT

A search for physics beyond the Standard Model, in final states with at least one high transverse momentum charged lepton (electron or muon) and two additional high transverse momentum leptons or jets, is performed using 3.2 fb−1 of proton–proton collision data recorded by the ATLAS detector at the Large Hadron Collider in 2015 at View the MathML sources=13 TeV. The upper end of the distribution of the scalar sum of the transverse momenta of leptons and jets is sensitive to the production of high-mass objects. No excess of events beyond Standard Model predictions is observed. Exclusion limits are set for models of microscopic black holes with two to six extra dimensions.

Search for an additional, heavy Higgs boson in the H → ZZ decay channel at √ s = 8 TeV in pp collision data with the ATLAS detector

Fri 29 Sep 2017 16:42:04 KRAT

A search is presented for a high-mass Higgs boson in the H→ZZ→ℓ+ℓ−ℓ+ℓ−, H→ZZ→ℓ+ℓ−νˉν, H→ZZ→ℓ+ℓ−qˉq, and H→ZZ→νˉνqˉq decay modes using the ATLAS detector at the CERN Large Hadron Collider. The search uses proton–proton collision data at a centre-of-mass energy of 8 TeV corresponding to an integrated luminosity of 20.3 fb−1. The results of the search are interpreted in the scenario of a heavy Higgs boson with a width that is small compared with the experimental mass resolution. The Higgs boson mass range considered extends up to 1 TeV for all four decay modes and down to as low as 140 GeV, depending on the decay mode. No significant excess of events over the Standard Model prediction is found. A simultaneous fit to the four decay modes yields upper limits on the production cross-section of a heavy Higgs boson times the branching ratio to Z boson pairs. 95 % confidence level upper limits range from 0.53 pb at mH=195 GeV to 0.008 pb at mH=950 GeV for the gluon-fusion production mode and from 0.31 pb at mH=195 GeV to 0.009 pb at mH=950 GeV for the vector-boson-fusion production mode. The results are also interpreted in the context of Type-I and Type-II two-Higgs-doublet models.

Search for flavour-changing neutral current top-quark decays to qZ in pp collision data collected with the ATLAS detector at √ s = 8 TeV

Fri 29 Sep 2017 16:42:03 KRAT

A search for the flavour-changing neutral-current decay t→qZ is presented. Data collected by the ATLAS detector during 2012 from proton–proton collisions at the Large Hadron Collider at a centre-of-mass energy of s√=8 TeV, corresponding to an integrated luminosity of 20.3 fb−1, are analysed. Top-quark pair-production events with one top quark decaying through the t→qZ (q=u,c) channel and the other through the dominant Standard Model mode t→bW are considered as signal. Only the decays of the Z boson to charged leptons and leptonic W boson decays are used. No evidence for a signal is found and an observed (expected) upper limit on the t→qZ branching ratio of 7×10−4 (8×10−4) is set at the 95 % confidence level

Reconstruction of hadronic decay products of tau leptons with the ATLAS experiment

Fri 29 Sep 2017 14:08:14 KRAT

This paper presents a new method of reconstructing the individual charged and neutral hadrons in tau decays with the ATLAS detector. The reconstructed hadrons are used to classify the decay mode and to calculate the visible four-momentum of reconstructed tau candidates, significantly improving the resolution with respect to the calibration in the existing tau reconstruction. The performance of the reconstruction algorithm is optimised and evaluated using simulation and validated using samples of Z→ττ and Z(→μμ)+jets events selected from proton–proton collisions at a centre-of-mass energy √s=8TeV, corresponding to an integrated luminosity of 5 fb−1.

Identification of boosted, hadronically decaying W bosons and comparisons with ATLAS data taken at √s = 8 TeV

Fri 29 Sep 2017 14:08:05 KRAT

This paper reports a detailed study of techniques for identifying boosted, hadronically decaying W bosons using 20.3 fb−1 of proton–proton collision data collected by the ATLAS detector at the LHC at a centre-of-mass energy √s=8TeV. A range of techniques for optimising the signal jet mass resolution are combined with various jet substructure variables. The results of these studies in Monte Carlo simulations show that a simple pairwise combination of groomed jet mass and one substructure variable can provide a 50 % efficiency for identifying W bosons with transverse momenta larger than 200 GeV while maintaining multijet background efficiencies of 2–4 % for jets with the same transverse momentum. These signal and background efficiencies are confirmed in data for a selection of tagging techniques.

Search for single top-quark production via flavour-changing neutral currents at 8 TeV with the ATLAS detector

Fri 29 Sep 2017 14:01:16 KRAT

A search for single top-quark production via flavour-changing neutral current processes from gluon plus up- or charm-quark initial states in proton–proton collisions at the LHC is presented. Data collected with the ATLAS detector in 2012 at a centre-of-mass energy of 8 TeV and corresponding to an integrated luminosity of 20.3 fb−1 are used. Candidate events for a top quark decaying into a lepton, a neutrino and a jet are selected and classified into signal- and background-like candidates using a neural network. No signal is observed and an upper limit on the production cross-section multiplied by the t→Wb branching fraction is set. The observed 95 % CL limit is σqg→t×B(t→Wb)<3.4pb and the expected 95 % CL limit is σqg→t×B(t→Wb)<2.9pb. The observed limit can be interpreted as upper limits on the coupling constants of the flavour-changing neutral current interactions divided by the scale of new physics κugt/Λ<5.8×10−3TeV−1 and κcgt/Λ<13×10−3TeV and on the branching fractions B(t→ug)<4.0×10−5 and B(t→cg)<20×10−5.

Measurements of the Higgs boson production and decay rates and coupling strengths using pp collision data at √s = 7 and 8 TeV in the ATLAS experiment

Fri 29 Sep 2017 14:01:15 KRAT

Combined analyses of the Higgs boson production and decay rates as well as its coupling strengths to vector bosons and fermions are presented. The combinations include the results of the analyses of the H→γγ,ZZ∗,WW∗,Zγ,bˉb,ττ and μμ decay modes, and the constraints on the associated production with a pair of top quarks and on the off-shell coupling strengths of the Higgs boson. The results are based on the LHC proton-proton collision datasets, with integrated luminosities of up to 4.7 fb−1 at √s=7 TeV and 20.3 fb−1 at √s=8 TeV, recorded by the ATLAS detector in 2011 and 2012. Combining all production modes and decay channels, the measured signal yield, normalised to the Standard Model expectation, is 1.18+0.15−0.14. The observed Higgs boson production and decay rates are interpreted in a leading-order coupling framework, exploring a wide range of benchmark coupling models both with and without assumptions on the Higgs boson width and on the Standard Model particle content in loop processes. The data are found to be compatible with the Standard Model expectations for a Higgs boson at a mass of 125.36 GeV for all models considered.

Measurement of the differential cross-sections of prompt and non-prompt production of J/ψ and ψ(2S) in pp collisions at √s = 7 and 8 TeV with the ATLAS detector

Fri 29 Sep 2017 14:01:13 KRAT

The production rates of prompt and non-prompt J/ψ and ψ(2S) mesons in their dimuon decay modes are measured using 2.1 and 11.4 fb−1 of data collected with the ATLAS experiment at the Large Hadron Collider, in proton–proton collisions at √s=7 and 8 respectively. Production cross-sections for prompt as well as non-prompt sources, ratios of ψ(2S) to J/ψ production, and the fractions of non-prompt production for J/ψ and ψ(2S) are measured as a function of meson transverse momentum and rapidity. The measurements are compared to theoretical predictions.

Search for direct top squark pair production in final states with two tau leptons in pp collisions at √ s = 8 TeV with the ATLAS detector

Fri 29 Sep 2017 14:01:13 KRAT

A search for direct pair production of the supersymmetric partner of the top quark, decaying via a scalar tau to a nearly massless gravitino, has been performed using 20 fb−1 of proton–proton collision data at √s=8 TeV. The data were collected by the ATLAS experiment at the LHC in 2012. Top squark candidates are searched for in events with either two hadronically decaying tau leptons, one hadronically decaying tau and one light lepton, or two light leptons. No significant excess over the Standard Model expectation is found. Exclusion limits at 95 % confidence level are set as a function of the top squark and scalar tau masses. Depending on the scalar tau mass, ranging from the 87 GeV LEP limit to the top squark mass, lower limits between 490 and 650 GeV are placed on the top squark mass within the model considered.

Study of the B + c → J /ψ D + s and B + c → J /ψ D ∗+ s decays with the ATLAS detector

Fri 29 Sep 2017 14:01:13 KRAT

The decays Bc+→J/ψDs+ and Bc+→J/ψDs∗+ are studied with the ATLAS detector at the LHC using a dataset corresponding to integrated luminosities of 4.9 and 20.6 fb -1 of pp collisions collected at centre-of-mass energies s=7 TeV and 8 TeV, respectively. Signal candidates are identified through J/ψ→μ+μ- and Ds(∗)+→ϕπ+(γ/π0) decays. With a two-dimensional likelihood fit involving the Bc+ reconstructed invariant mass and an angle between the μ+ and Ds+ candidate momenta in the muon pair rest frame, the yields of Bc+→J/ψDs+ and Bc+→J/ψDs∗+ , and the transverse polarisation fraction in Bc+→J/ψDs∗+ decay are measured. The transverse polarisation fraction is determined to be Γ±±(Bc+→J/ψDs∗+)/Γ(Bc+→J/ψDs∗+)=0.38±0.23±0.07 , and the derived ratio of the branching fractions of the two modes is BBc+→J/ψDs∗+/BBc+→J/ψDs+=2.8-0.8+1.2±0.3 , where the first error is statistical and the second is systematic. Finally, a sample of Bc+→J/ψπ+ decays is used to derive the ratios of branching fractions BBc+→J/ψDs+/BBc+→J/ψπ+=3.8±1.1±0.4±0.2 and BBc+→J/ψDs∗+/BBc+→J/ψπ+=10.4±3.1±1.5±0.6 , where the third error corresponds to the uncertainty of the branching fraction of Ds+→ϕ(K+K-)π+ decay. The available theoretical predictions are generally consistent with the measurement.

Searches for scalar leptoquarks in pp collisions at √s = 8 TeV with the ATLAS detector

Fri 29 Sep 2017 14:01:08 KRAT

Searches for pair-produced scalar leptoquarks are performed using 20 fb−1 of proton–proton collision data provided by the LHC and recorded by the ATLAS detector at √s=8 TeV. Events with two electrons (muons) and two or more jets in the final state are used to search for first (second)-generation leptoquarks. The results from two previously published ATLAS analyses are interpreted in terms of third-generation leptoquarks decaying to bντˉb¯ντ and tντˉt¯ντ final states. No statistically significant excess above the Standard Model expectation is observed in any channel and scalar leptoquarks are excluded at 95 % CL with masses up to mLQ1< 1050 GeV for first-generation leptoquarks, mLQ2< 1000 GeV for second-generation leptoquarks, mLQ3< 625 GeV for third-generation leptoquarks in the bντˉb¯ντ channel, and 200

Measurements of fiducial cross-sections for t t ¯ production with one or two additional b-jets in pp collisions at √s = 8 TeV using the ATLAS detector

Fri 29 Sep 2017 14:01:04 KRAT

Fiducial cross-sections for tˉt production with one or two additional b-jets are reported, using an integrated luminosity of 20.3 fb−1 of proton–proton collisions at a centre-of-mass energy of 8 TeV at the Large Hadron Collider, collected with the ATLAS detector. The cross-section times branching ratio for tˉt events with at least one additional b-jet is measured to be 950 ± 70 (stat.) +240−190 (syst.) fb in the lepton-plus-jets channel and 50 ± 10 (stat.) +15−10 (syst.) fb in the eμ channel. The cross-section times branching ratio for events with at least two additional b-jets is measured to be 19.3 ± 3.5 (stat.) ± 5.7 (syst.) fb in the dilepton channel (eμ, μμ, and ee) using a method based on tight selection criteria, and 13.5 ± 3.3 (stat.) ± 3.6 (syst.) fb using a looser selection that allows the background normalisation to be extracted from data. The latter method also measures a value of 1.30 ± 0.33 (stat.) ± 0.28 (syst.)% for the ratio of tˉt production with two additional b-jets to tˉt production with any two additional jets. All measurements are in good agreement with recent theory predictions.

Charged-particle distributions in pp interactions at √s = 8 TeV measured with the ATLAS detector

Fri 29 Sep 2017 14:01:02 KRAT

This paper presents measurements of distributions of charged particles which are produced in proton–proton collisions at a centre-of-mass energy of √s=8TeV and recorded by the ATLAS detector at the LHC. A special dataset recorded in 2012 with a small number of interactions per beam crossing (below 0.004) and corresponding to an integrated luminosity of 160 μb−1 was used. A minimum-bias trigger was utilised to select a data sample of more than 9 million collision events. The multiplicity, pseudorapidity, and transverse momentum distributions of charged particles are shown in different regions of kinematics and charged-particle multiplicity, including measurements of final states at high multiplicity. The results are corrected for detector effects and are compared to the predictions of various Monte Carlo event generator models which simulate the full hadronic final state.

Probing lepton flavour violation via neutrinoless τ → 3μ decays with the ATLAS detector

Fri 29 Sep 2017 14:01:01 KRAT

This article presents the sensitivity of the ATLAS experiment to the lepton-flavour-violating decays of τ→3μ. A method utilising the production of τ leptons via W→τν decays is used. This method is applied to the sample of 20.3 fb−1 of pp collision data at a centre-of-mass energy of 8 TeV collected by the ATLAS experiment at the LHC in 2012. No event is observed passing the selection criteria, and the observed (expected) upper limit on the τ lepton branching fraction into three muons, Br(τ→3μ), is 3.76×10−7 (3.94×10−7) at 90 % confidence level.

Search for scalar leptoquarks in pp collisions at √s = 13 TeV with the ATLAS experiment

Fri 07 Dec 2018 11:36:41 KRAT

An inclusive search for a new-physics signature of lepton-jet resonances has been performed by the ATLAS experiment. Scalar leptoquarks, pair-produced in pp collisions at s√ = 13 TeV at the large hadron collider, have been considered. An integrated luminosity of 3.2 fb(−)(1), corresponding to the full 2015 dataset was used. First (second) generation leptoquarks were sought in events with two electrons (muons) and two or more jets. The observed event yield in each channel is consistent with Standard Model background expectations. The observed (expected) lower limits on the leptoquark mass at 95% confidence level are 1100 and 1050 GeV (1160 and 1040 GeV) for first and second generation leptoquarks, respectively, assuming a branching ratio into a charged lepton and a quark of 100%. Upper limits on the aforementioned branching ratio are also given as a function of leptoquark mass. Compared with the results of earlier ATLAS searches, the sensitivity is increased for leptoquark masses above 860 GeV, and the observed exclusion limits confirm and extend the published results.