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At least 991 records · Page 55
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DOE OSTI2018

Operation and performance of the ATLAS Tile Calorimeter in Run 1

The Tile Calorimeter is the hadron calorimeter covering the central region of the ATLAS experiment at the Large Hadron Collider. Approximately 10,000 photomultipliers collect light from scintillating tiles acting as the active material sandwiched between slabs of steel absorber. This paper gives an overview of the calorimeter’s performance during the years 2008–2012 using cosmic-ray muon events and proton–proton collision data at centre-of-mass energies of 7 and 8 TeV with a total integrated luminosity of nearly 30 fb –1 . The signal reconstruction methods, calibration systems as well as the detector operation status are presented. The energy and time calibration methods performed excellently, resulting in good stability of the calorimeter response under varying conditions during the LHC Run 1. Lastly, the Tile Calorimeter response to isolated muons and hadrons as well as to jets from proton–proton collisions is presented. The results demonstrate excellent performance in accord with specifications mentioned in the Technical Design Report.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2019

Study of the rare decays of B s 0 and B 0 mesons into muon pairs using data collected during 2015 and 2016 with the ATLAS detector

A study of the decays B s 0 → μ + μ – and B 0 → μ + μ – has been performed using 26.3 fb –1 of 13 TeV LHC proton-proton collision data collected with the ATLAS detector in 2015 and 2016. Since the detector resolution in μ + μ – invariant mass is comparable to the B s 0 -B 0 mass difference, a single fit determines the signal yields for both decay modes. This results in a measurement of the branching fraction B(B s 0 → μ + μ – ) = (3.2 +1.1 –1.0 ) × 10 –9 and an upper limit B(B 0 → μ + μ – ) < 4.3×10 –10 at 95% confidence level. The result is combined with the Run 1 ATLAS result, yielding B(B s 0 → μ + μ – ) = (2.8 +0.8 –0.7 ) × 10 –9 and B(B 0 → μ + μ – ) < 2.1 × 10 –10 at 95% confidence level. The combined result is consistent with the Standard Model prediction within 2.4 standard deviations in the B(B 0 → μ + μ – ) – B(B 0 s → μ + μ – ) plane.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2019

Search for top-quark decays t → Hq with 36 fb ₋1 of pp collision data at $\sqrt{s}$= 13 TeV with the ATLAS detector

A search for flavour-changing neutral current decays of a top quark into an up-type quark ( q = u, c ) and the Standard Model Higgs boson, t → Hq , is introduced. The search is based on a dataset of pp collisions at $\sqrt{s}$= 13 TeV recorded in 2015 and 2016 with the ATLAS detector at the CERN Large Hadron Collider and corresponding to an integrated luminosity of 36.1 fb ₋1 . Two complementary analyses are performed to search for top-quark pair events in which one top quark decays into Wb and the other top quark decays into Hq, and target the H→$b\bar{b}$ and H → τ + τ ₋ decay modes, respectively. The high multiplicity of b-quark jets, or the presence of hadronically decaying τ-leptons, is exploited in the two analyses respectively. Multivariate techniques are used to separate the signal from the background, which is dominated by top-quark pair production. No significant excess of events above the background expectation is found, and 95% CL upper limits on the t → Hq branching ratios are derived. The combination of these searches with ATLAS searches in diphoton and multilepton final states yields observed (expected) 95% CL upper limits on the t → Hc and t → Hu branching ratios of 1.1 × 10 ₋3 (8.3 × 10 ₋4 ) and 1.2 × 10 ₋3 (8.3 × 10 ₋4 ), respectively. The corresponding combined observed (expected) upper limits on the |λ tcH | and |λ tuH | couplings are 0.064 (0.055) and 0.066 (0.055), respectively.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2016

Measurement of the relative width difference of the B 0 - B$\bar{B}$ 0 system with the ATLAS detector

This paper presents the measurement of the relative width difference ΔΓ d /Γ d of the B 0 - B$\bar{B}$ 0 system using the data collected by the ATLAS experiment at the LHC in pp collisions at √s=7TeV and √s=8 TeV and corresponding to an integrated luminosity of 25.2 fb -1 . The value of ΔΓ d /Γ d is obtained by comparing the decay-time distributions of B° → J/ψK S and B° → J/ψK*°(892) decays. The result is ΔΓ d /Γ d = (-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.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2016

Measurements of the Higgs boson production and decay rates and constraints on its couplings from a combined ATLAS and CMS analysis of the LHC pp collision data at $ \sqrt{s}=7 $ and 8 TeV

Combined ATLAS and CMS measurements of the Higgs boson production and decay rates, as well as constraints on its couplings to vector bosons and fermions, are presented. The combination is based on the analysis of five production processes, namely gluon fusion, vector boson fusion, and associated production with a W or a Z boson or a pair of top quarks, and of the six decay modes H → ZZ, W W , γγ, ττ, bb, and μμ. All results are reported assuming a value of 125.09 GeV for the Higgs boson mass, the result of the combined measurement by the ATLAS and CMS experiments. The analysis uses the CERN LHC proton-proton collision data recorded by the ATLAS and CMS experiments in 2011 and 2012, corresponding to integrated luminosities per experiment of approximately 5 fb$^{−1}$ at $ \sqrt{s}=7 $ TeV and 20 fb$^{−1}$ at $ \sqrt{s}=8 $ TeV. The Higgs boson production and decay rates measured by the two experiments are combined within the context of three generic parameterisations: two based on cross sections and branching fractions, and one on ratios of coupling modifiers. Several interpretations of the measurements with more model-dependent parameterisations are also given. The combined signal yield relative to the Standard Model prediction is measured to be 1.09 ± 0.11. The combined measurements lead to observed significances for the vector boson fusion production process and for the H → ττ decay of 5.4 and 5.5 standard deviations, respectively. The data are consistent with the Standard Model predictions for all parameterisations considered.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2015

Search for flavour-changing neutral current top quark decays t → Hq in pp collisions at √s = 8 TeV with the ATLAS detector

A search for flavour-changing neutral current decays of a top quark to an uptype quark (q = u, c) and the Standard Model Higgs boson, where the Higgs boson decays to bb¯, is presented. The analysis searches for top quark pair events in which one top quark decays to Wb, with the W boson decaying leptonically, and the other top quark decays to Hq. The search is based on pp collisions at √s = 8 TeV recorded in 2012 with the ATLAS detector at the CERN Large Hadron Collider and uses an integrated luminosity of 20.3 fb -1 . Data are analysed in the lepton-plus-jets final state, characterised by an isolated electron or muon and at least four jets. The search exploits the high multiplicity of b-quark jets characteristic of signal events, and employs a likelihood discriminant that uses the kinematic differences between the signal and the background, which is dominated by tt¯→ WbWb decays. No significant excess of events above the background expectation is found, and observed (expected) 95% CL upper limits of 0.56% (0.42%) and 0.61% (0.64%) are derived for the t → Hc and t → Hu branching ratios respectively. The combination of this search with other ATLAS searches in the H → γγ and H → WW*, ττ decay modes significantly improves the sensitivity, yielding observed (expected) 95% CL upper limits on the t → Hc and t → Hu branching ratios of 0.46% (0.25%) and 0.45% (0.29%) respectively. The corresponding combined observed (expected) upper limits on the |λ tcH | and |λ tuH | couplings are 0.13 (0.10) and 0.13 (0.10) respectively. As a result, these are the most restrictive direct bounds on tqH interactions measured so far.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2019

Electron and photon energy calibration with the ATLAS detector using 2015–2016 LHC proton-proton collision data

This paper presents the electron and photon energy calibration obtained with the ATLAS detector using about 36 fb –1 of LHC proton-proton collision data recorded at √s = 13 TeV in 2015 and 2016. The different calibration steps applied to the data and the optimization of the reconstruction of electron and photon energies are discussed. The absolute energy scale is set using a large sample of Z boson decays into electron-positron pairs. The systematic uncertainty in the energy scale calibration varies between 0.03% to 0.2% in most of the detector acceptance for electrons with transverse momentum close to 45 GeV . For electrons with transverse momentum of 10 GeV the typical uncertainty is 0.3% to 0.8% and it varies between 0.25% and 1% for photons with transverse momentum around 60 GeV . Validations of the energy calibration with J/ψ → e + e – decays and radiative Z boson decays are also presented.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2019

Measurements of inclusive and differential fiducial cross-sections of $t\bar{t}\gamma $ production in leptonic final states at $\sqrt{s}=13~\text {TeV}$ TeV in ATLAS

Inclusive and differential cross-sections for the production of a top-quark pair in association with a photon are measured with proton-proton collision data corresponding to an integrated luminosity of 36.1 fb –1 , collected by the ATLAS detector at the LHC in 2015 and 2016 at a centre-of-mass energy of 13 TeV. The measurements are performed in single-lepton and dilepton final states in a fiducial volume. Events with exactly one photon, one or two leptons, a channel-dependent minimum number of jets, and at least one b-jet are selected. Neural network algorithms are used to separate the signal from the backgrounds. The fiducial cross-sections are measured to be 521 ± 9(stat.) ± 41(sys.) fb and 69 ± 3(stat.) ± 4(sys.) fb for the single-lepton and dilepton channels, respectively. The differential cross-sections are measured as a function of photon transverse momentum, photon absolute pseudorapidity, and angular distance between the photon and its closest lepton in both channels, as well as azimuthal opening angle and absolute pseudorapidity difference between the two leptons in the dilepton channel. All measurements are in agreement with the theoretical predictions.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2018

Measurement of inclusive jet and dijet cross-sections in proton-proton collisions at $$ \sqrt{s}=13 $$ TeV with the ATLAS detector

Inclusive jet and dijet cross-sections are measured in proton-proton collisions at a centre-of-mass energy of 13 TeV. The measurement uses a dataset with an integrated luminosity of 3.2 fb -1 recorded in 2015 with the ATLAS detector at the Large Hadron Collider. Jets are identified using the anti- k t algorithm with a radius parameter value of R = 0 . 4. The inclusive jet cross-sections are measured double-differentially as a function of the jet transverse momentum, covering the range from 100 GeV to 3.5 TeV, and the absolute jet rapidity up to | y | = 3. The double-differential dijet production cross-sections are presented as a function of the dijet mass, covering the range from 300 GeV to 9 TeV, and the half absolute rapidity separation between the two leading jets within | y | < 3, y * , up to y * = 3. Next-to-leading-order, and next-to-next-to-leading-order for the inclusive jet measurement, perturbative QCD calculations corrected for non-perturbative and electroweak effects are compared to the measured cross-sections.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2018

Search for flavour-changing neutral current top-quark decays t → qZ in proton-proton collisions at $\sqrt{s}=13 $ TeV with the ATLAS detector

A search for flavour-changing neutral-current processes in top-quark decays is presented. Data collected with the ATLAS detector from proton-proton collisions at the Large Hadron Collider at a centre-of-mass energy of $\sqrt{s}=13 $ TeV, corresponding to an integrated luminosity of 36.1 fb-1, are analysed. The search is performed using top-quark pair events, with one top quark decaying through the t → qZ (q = u, c) flavour-changing neutral-current channel, and the other through the dominant Standard Model mode t → bW. Only Z boson decays into charged leptons and leptonic W boson decays are considered as signal. Consequently, the final-state topology is characterized by the presence of three isolated charged leptons (electrons or muons), at least two jets, one of the jets originating from a b-quark, and missing transverse momentum from the undetected neutrino. Here, the data are consistent with Standard Model background contributions, and at 95% confidence level the search sets observed (expected) upper limits of 1.7 × 10 -4 (2.4 × 10 -4 ) on the t → uZ branching ratio and 2.4 × 10 -4 (3.2 × 10 -4 ) on the t → cZ branching ratio, constituting the most stringent limits to date.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2015

Search for lepton-flavour-violating H → μτ decays of the Higgs boson with the ATLAS detector

A direct search for lepton-flavour-violating H → μτ decays of the recently discovered Higgs boson with the ATLAS detector at the LHC is presented. The analysis is performed in the H → μτ had channel, where τhad is a hadronically decaying τ -lepton. The search is based on the data sample of proton-proton collisions collected by the ATLAS experiment corresponding to an integrated luminosity of 20.3 fb -1 at a centre-of-mass energy of √s=8 TeV. No statistically significant excess of data over the predicted background is observed. The observed (expected) 95% confidence-level upper limit on the branching fraction, Br(H → μτ ), is 1.85% (1.24%).

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2016

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

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.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2016

Search for pair production of gluinos decaying via stop and sbottom in events with b -jets and large missing transverse momentum in p p collisions at s = 13 TeV with the ATLAS detector

A search for supersymmetry involving the pair production of gluinos decaying via third-generation squarks to the lightest neutralino ( χ$~0\atop{1}$ ) is reported. It uses an LHC proton-proton data set at a center-of-mass energy √ s = 13 TeV with an integrated luminosity of 3.2 fb -1 collected with the ATLAS detector in 2015. The signal is searched for in events containing several energetic jets, of which at least three must be identified as b jets, large missing transverse momentum, and, potentially, isolated electrons or muons. Large-radius jets with a high mass are also used to identify highly boosted top quarks. No excess is found above the predicted background. For χ$~0\atop{1}$ masses below approximately 700 GeV, gluino masses of less than 1.78 TeV and 1.76 TeV are excluded at the 95% C.L. in simplified models of the pair production of gluinos decaying via sbottom and stop, respectively. These results significantly extend the exclusion limits obtained with the √ s = 8 TeV data set.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗