Subjects: Physics >> Nuclear Physics submitted time 2016-08-29
Abstract： The production of W±Z events in proton--proton collisions at a centre-of-mass energy of 13 TeV is measured with the ATLAS detector at the LHC. The collected data correspond to an integrated luminosity of 3.2 fb−1. The W±Z candidates are reconstructed using leptonic decays of the gauge bosons into electrons or muons. The measured inclusive cross section in the detector fiducial region for leptonic decay modes is σfid.W±Z→ℓ′νℓℓ=63.2±3.2 (stat.) ±2.6 (sys.) ±1.5 (lumi.) fb. In comparison, the next-to-leading-order Standard Model prediction is 53.4+3.6−2.8 fb. The extrapolation of the measurement from the fiducial to the total phase space yields σtot.W±Z=50.6±2.6 (stat.) ±2.0 (sys.) ±0.9 (th.) ±1.2 (lumi.) pb, in agreement with a recent next-to-next-to-leading-order calculation of 48.2+1.1−1.0 pb. The cross section as a function of jet multiplicity is also measured, together with the charge-dependent W+Z and W−Z cross sections and their ratio.
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Awaiting Review
Subjects: Physics >> Nuclear Physics submitted time 2016-08-29
Abstract： A search for W′ bosons in events with one lepton (electron or muon) and missing transverse momentum is presented. The search uses 3.2 fb−1 of pp collision data collected at s√=13 TeV by the ATLAS experiment at the LHC in 2015. The transverse mass distribution is examined and no significant excess of events above the level expected from Standard Model processes is observed. Upper limits on the W′boson cross-section times branching ratio to leptons are set as a function of the W′ mass. Assuming a W′ boson as predicted by the Sequential Standard Model, W′ masses below 4.07 TeV are excluded at the 95% confidence level. This extends the limit set using LHC data at s√=8 TeV by around 800 GeV.
Peer Review Status:Awaiting Review
Subjects: Physics >> Nuclear Physics submitted time 2016-08-29
Abstract： The results of a search for the stop, the supersymmetric partner of the top quark, in final states with one isolated electron or muon, jets, and missing transverse momentum are reported. The search uses the 2015 LHC pp collision data at a center-of-mass energy of s√=13 TeV recorded by the ATLAS detector and corresponding to an integrated luminosity of 3.2 fb−1. The analysis targets two types of signal models: gluino-mediated pair production of stops with a nearly mass-degenerate stop and neutralino; and direct pair production of stops, decaying to the top quark and the lightest neutralino. The experimental signature in both signal scenarios is similar to that of a top quark pair produced in association with large missing transverse momentum. No significant excess over the Standard Model background prediction is observed, and exclusion limits on gluino and stop masses are set at 95% confidence level. The results extend the LHC Run-1 exclusion limit on the gluino mass up to 1460 GeV in the gluino-mediated scenario in the high gluino and low stop mass region, and add an excluded stop mass region from 745 to 780 GeV for the direct stop model with a massless lightest neutralino. The results are also reinterpreted to set exclusion limits in a model of vector-like top quarks.
Peer Review Status:Awaiting Review
Subjects: Physics >> Nuclear Physics submitted time 2016-08-29
Abstract： 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.
Peer Review Status:Awaiting Review
Subjects: Physics >> Nuclear Physics submitted time 2016-08-29
Abstract： This paper describes a measurement of the inclusive top quark pair production cross-section (σtt¯) with a data sample of 3.2 fb−1 of proton--proton collisions at a centre-of-mass energy of s√=13 TeV, collected in 2015 by the ATLAS detector at the LHC. This measurement uses events with an opposite-charge electron--muon pair in the final state. Jets containing b-quarks are tagged using an algorithm based on track impact parameters and reconstructed secondary vertices. The numbers of events with exactly one and exactly two b-tagged jets are counted and used to determine simultaneously σtt¯ and the efficiency to reconstruct and b-tag a jet from a top quark decay, thereby minimising the associated systematic uncertainties. The cross-section is measured to be: σtt¯= 818 ± 8 (stat) ± 27 (syst) ± 19 (lumi) ± 12 (beam) pb, where the four uncertainties arise from data statistics, experimental and theoretical systematic effects, the integrated luminosity and the LHC beam energy, giving a total relative uncertainty of 4.4%. The result is consistent with theoretical QCD calculations at next-to-next-to-leading order. A fiducial measurement corresponding to the experimental acceptance of the leptons is also presented.
Peer Review Status:Awaiting Review
Subjects: Physics >> Nuclear Physics submitted time 2016-08-29
Abstract： This Letter presents a measurement of the inelastic proton-proton cross section using 60 μb−1 of pp collisions at a center-of-mass energy s√ of 13 TeV with the ATLAS detector at the LHC. Inelastic interactions are selected using plastic scintillators in the forward region of the detector. A cross section of 68.2±1.5 mb is measured in the fiducial region ξ=M2X/s>10−6, where MX is the largest invariant mass of the two hadronic systems separated by the largest rapidity gap in the event. In this ξ range the scintillators are highly efficient. For diffractive events this corresponds to cases where at least one proton dissociates to a system with MX>13 GeV. The measured cross section is compared with a range of theoretical predictions. When extrapolated to the full phase space, a cross-section of 79.3±2.9 mb is measured, consistent with the inelastic cross section increasing with center-of-mass energy.
Peer Review Status:Awaiting Review
Subjects: Physics >> Nuclear Physics submitted time 2016-08-29
Abstract： 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 s√=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.
Peer Review Status:Awaiting Review
Subjects: Physics >> Nuclear Physics submitted time 2016-08-29
Abstract： A search with the ATLAS detector is presented for the Standard Model Higgs boson produced by vector-boson fusion and decaying to a pair of bottom quarks, using 20.2 fb−1 of LHC proton--proton collision data at s√ = 8 TeV. The signal is searched for as a resonance in the invariant mass distribution of a pair of jets containing b-hadrons in vector-boson-fusion candidate events. The yield is measured to be −0.8±2.3 times the Standard Model cross-section for a Higgs boson mass of 125 GeV. The upper limit on the cross-section times the branching ratio is found to be 4.4 times the Standard Model cross-section at the 95% confidence level, consistent with the expected limit value of 5.4 (5.7) in the background-only (Standard Model production) hypothesis.
Peer Review Status:Awaiting Review
Subjects: Physics >> Nuclear Physics submitted time 2016-08-29
Abstract： The top quark mass is measured in the tt¯→ dilepton channel (lepton =e,μ) using ATLAS data recorded in the year 2012 at the LHC. The data were taken at a proton--proton centre-of-mass energy of s√= 8 TeV and correspond to an integrated luminosity of about 20.2 fb−1. Exploiting the template method, and using the distribution of invariant masses of lepton--b-jet pairs, the top quark mass is measured to be mtop= 172.99 ± 0.41 (stat) ± 0.74 (syst) GeV, with a total uncertainty of 0.84 GeV. Finally, a combination with previous ATLAS mtop measurements from s√=7 TeV data in the tt¯→ dilepton and tt¯→ lepton+jets channels results in mtop= 172.84 ± 0.34 (stat) ± 0.61 (syst) GeV, with a total uncertainty of 0.70 GeV.
Peer Review Status:Awaiting Review
Subjects: Physics >> Nuclear Physics submitted time 2016-08-29
Abstract： The algorithms used by the ATLAS Collaboration to reconstruct and identify prompt photons are described. Measurements of the photon identification efficiencies are reported, using 4.9 fb−1 of pp collision data collected at the LHC at s√=7 TeV and 20.3 fb−1 at s√=8 TeV. The efficiencies are measured separately for converted and unconverted photons, in four different pseudo rapidity regions, for transverse momenta between 10 GeV and 1.5 TeV. The results from the combination of three data-driven techniques are compared to the predictions from a simulation of the detector response, after correcting the electromagnetic shower momenta in the simulation for the average differences observed with respect to data. Data-to-simulation efficiency ratios used as correction factors in physics measurements are determined to account for the small residual efficiency differences. These factors are measured with uncertainties between 0.5% and 10% in 7 TeV data and between 0.5% and 3% in 8 TeV data, depending on the photon transverse momentum and pseudorapidity.
Peer Review Status:Awaiting Review
Subjects: Physics >> Nuclear Physics submitted time 2016-08-29
Abstract： Measurements of distributions of charged particles produced in proton--proton collisions with a centre-of-mass energy of 13 TeV are presented. The particles are required to have a transverse momentum greater than 100 MeV and an absolute pseudorapidity less than 2.5. The charged-particle multiplicity, its dependence on transverse momentum and pseudorapidity and the dependence of the mean transverse momentum on multiplicity are measured in events containing at least two charged particles satisfying the above kinematic criteria. The data were recorded by the ATLAS detector at the LHC and correspond to an integrated luminosity of 170 μb−1. The results are corrected for detector effects and compared to the predictions from several Monte Carlo event generators.
Peer Review Status:Awaiting Review
Subjects: Physics >> Nuclear Physics submitted time 2016-08-26
Abstract： 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√=8TeV 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.
Peer Review Status:Awaiting Review
Subjects: Physics >> Nuclear Physics submitted time 2016-08-26
Abstract： 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 yield in each channel is consistent with Standard Model background expectations. The observed (expected) leptoquark mass limits at 95% confidence level are 1100 GeV and 1050 GeV (1160 GeV 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 the higher range of accessible leptoquark masses, and the observed exclusion limits confirm and extend the published results.
Peer Review Status:Awaiting Review
Subjects: Physics >> Nuclear Physics submitted time 2016-08-25
Abstract： 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 s√= 13 TeV proton--proton collisions at the Large Hadron Collider. No excess above the Standard Model background expectation was observed in 3.2 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 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 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.
Peer Review Status:Awaiting Review
Subjects: Physics >> Nuclear Physics submitted time 2016-08-25
Abstract： Results of a search for new phenomena in final states with an energetic jet and large missing transverse momentum are reported. The search uses proton--proton collision data corresponding to an integrated luminosity of 3.2 fb−1 at s√=13 TeV collected in 2015 with the ATLAS detector at the Large Hadron Collider. Events are required to have at least one jet with a transverse momentum above 250 GeV and no leptons. Several signal regions are considered with increasing missing-transverse-momentum requirements between EmissT>250 GeV and EmissT>700 GeV. Good agreement is observed between the number of events in data and Standard Model predictions. The results are translated into exclusion limits in models with large extra spatial dimensions, pair production of weakly interacting dark-matter candidates, and the production of supersymmetric particles in several compressed scenarios.
Peer Review Status:Awaiting Review
Subjects: Physics >> Nuclear Physics submitted time 2016-08-24
Abstract： A study of the decays B0→μ+μ− and B0s→μ+μ− has been performed using data corresponding to an integrated luminosity of 25 fb−1 of 7 TeV and 8 TeV proton--proton collisions collected with the ATLAS detector during the LHC Run 1. For B0, an upper limit on the branching fraction is set at B(B0→μ+μ−)<4.2×10−10 at 95% confidence level. For B0s, the branching fraction B(B0s→μ+μ−)=(0.9+1.1−0.8)×10−9 is measured. The results are consistent with the Standard Model expectation with a p-value of 4.8%, corresponding to 2.0 standard deviations.
Peer Review Status:Awaiting Review
Subjects: Physics >> Nuclear Physics submitted time 2016-08-24
Abstract： 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 of up to 710 GeV for dark matter candidate masses up to 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.
Peer Review Status:Awaiting Review
Subjects: Physics >> Nuclear Physics submitted time 2016-08-24
Abstract： Charged Higgs bosons produced in association with a single top quark and decaying via H±→τν are searched for with the ATLAS experiment at the LHC, using proton-proton collision data at s√=13TeV corresponding to an integrated luminosity of 3.2 fb−1. The final state is characterised by the presence of a hadronic τ decay and missing transverse momentum, as well as a hadronically decaying top quark, resulting in the absence of high-transverse-momentum electrons and muons. The data are found to be consistent with the expected background from Standard Model processes. A statistical analysis leads to 95% confidence-level upper limits on the production cross section times branching fraction, σ(pp→[b]tH±)×BR(H±→τν), between 1.9 pb and 15 fb, for charged Higgs boson masses ranging from 200 to 2000 GeV. The exclusion limits for this search surpass those obtained with the proton-proton collision data recorded at s√=8 TeV.
Peer Review Status:Awaiting Review
Subjects: Physics >> Nuclear Physics submitted time 2016-08-23
Abstract： Searches for high-mass resonances in the dijet invariant mass spectrum with one or two jets identified as b-jets are performed using an integrated luminosity of 3.2 fb−1 of proton--proton collisions with a centre-of-mass energy of s√=13 TeV recorded by the ATLAS detector at the Large Hadron Collider. No evidence of anomalous phenomena is observed in the data, which are used to exclude, at 95% credibility level, excited b∗ quarks with masses from 1.1 TeV to 2.1 TeV and leptophobic Z′ bosons with masses from 1.1 TeV to 1.5 TeV. Contributions of a Gaussian signal shape with effective cross sections ranging from approximately 0.4 to 0.001 pb are also excluded in the mass range 1.5-5.0 TeV.
Peer Review Status:Awaiting Review
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