The algorithms used by the ATLAS Collaboration during Run 2 of the Large Hadron Collider to identify jets containing b-hadrons are presented. The performance of the algorithms is evaluated in the ...simulation and the efficiency with which these algorithms identify jets containing b-hadrons is measured in collision data. The measurement uses a likelihood-based method in a sample highly enriched in Formula omitted events. The topology of the Formula omitted decays is exploited to simultaneously measure both the jet flavour composition of the sample and the efficiency in a transverse momentum range from 20 to 600 GeV. The efficiency measurement is subsequently compared with that predicted by the simulation. The data used in this measurement, corresponding to a total integrated luminosity of 80.5 Formula omitted, were collected in proton-proton collisions during the years 2015-2017 at a centre-of-mass energy Formula omitted 13 TeV. By simultaneously extracting both the efficiency and jet flavour composition, this measurement significantly improves the precision compared to previous results, with uncertainties ranging from 1 to 8% depending on the jet transverse momentum.
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DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
A measurement of Formula omitted boson production in Pb+Pb collisions at Formula omitted is reported using data recorded by the ATLAS experiment at the LHC in 2015, corresponding to a total ...integrated luminosity of Formula omitted. The Formula omitted bosons are reconstructed in the electron or muon leptonic decay channels. Production yields of leptonically decaying Formula omitted bosons, normalised by the total number of minimum-bias events and the nuclear thickness function, are measured within a fiducial region defined by the detector acceptance and the main kinematic requirements. These normalised yields are measured separately for Formula omitted and Formula omitted bosons, and are presented as a function of the absolute value of pseudorapidity of the charged lepton and of the collision centrality. The lepton charge asymmetry is also measured as a function of the absolute value of lepton pseudorapidity. In addition, nuclear modification factors are calculated using the Formula omitted boson production cross-sections measured in pp collisions. The results are compared with predictions based on next-to-leading-order calculations with CT14 parton distribution functions as well as with predictions obtained with the EPPS16 and nCTEQ15 nuclear parton distribution functions. No dependence of normalised production yields on centrality and a good agreement with predictions are observed for mid-central and central collisions. For peripheral collisions, the data agree with predictions within 1.7 (0.9) standard deviations for Formula omitted ( Formula omitted) bosons.
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DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
The production cross-sections for Formula omitted and Z bosons are measured using ATLAS data corresponding to an integrated luminosity of 4.0 pb Formula omitted collected at a centre-of-mass energy ...Formula omitted TeV. The decay channels Formula omitted and Formula omitted are used, where Formula omitted can be an electron or a muon. The cross-sections are presented for a fiducial region defined by the detector acceptance and are also extrapolated to the full phase space for the total inclusive production cross-section. The combined (average) total inclusive cross-sections for the electron and muon channels are: sigmaW+right arrowlνtot=2312±26(stat.)±27(syst.)±72(lumi.)±30(extr.)pb,sigmaW-right arrowlνtot=1399±21(stat.)±17(syst.)±43(lumi.)±21(extr.)pb,sigmaZright arrowlltot=323.4±9.8(stat.)±5.0(syst.)±10.0(lumi.)±5.5(extr.)pb.Measured ratios and asymmetries constructed using these cross-sections are also presented. These observables benefit from full or partial cancellation of many systematic uncertainties that are correlated between the different measurements.
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DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
A measurement of fiducial and differential cross-sections for Formula omitted production in proton-proton collisions at Formula omitted TeV with the ATLAS experiment at the Large Hadron Collider ...using data corresponding to an integrated luminosity of 36.1 Formula omitted is presented. Events with one electron and one muon are selected, corresponding to the decay of the diboson system as Formula omitted. To suppress top-quark background, events containing jets with a transverse momentum exceeding 35 GeV are not included in the measurement phase space. The fiducial cross-section, six differential distributions and the cross-section as a function of the jet-veto transverse momentum threshold are measured and compared with several theoretical predictions. Constraints on anomalous electroweak gauge boson self-interactions are also presented in the framework of a dimension-six effective field theory.
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DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
This paper describes a study of techniques for identifying Higgs bosons at high transverse momenta decaying into bottom-quark pairs, Formula omitted, for proton-proton collision data collected by the ...ATLAS detector at the Large Hadron Collider at a centre-of-mass energy Formula omitted Formula omitted. These decays are reconstructed from calorimeter jets found with the anti- Formula omitted Formula omitted jet algorithm. To tag Higgs bosons, a combination of requirements is used: b-tagging of Formula omitted track-jets matched to the large-R calorimeter jet, and requirements on the jet mass and other jet substructure variables. The Higgs boson tagging efficiency and corresponding multijet and hadronic top-quark background rejections are evaluated using Monte Carlo simulation. Several benchmark tagging selections are defined for different signal efficiency targets. The modelling of the relevant input distributions used to tag Higgs bosons is studied in 36 fb Formula omitted of data collected in 2015 and 2016 using Formula omitted and Formula omitted event selections in data. Both processes are found to be well modelled within the statistical and systematic uncertainties.
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A search for excited electrons produced in pp collisions at Formula omitted = 13 Formula omitted via a contact interaction Formula omitted is presented. The search uses 36.1 fb Formula omitted of ...data collected in 2015 and 2016 by the ATLAS experiment at the Large Hadron Collider. Decays of the excited electron into an electron and a pair of quarks ( Formula omitted) are targeted in final states with two electrons and two hadronic jets, and decays via a gauge interaction into a neutrino and a Formula omitted boson ( Formula omitted) are probed in final states with an electron, missing transverse momentum, and a large-radius jet consistent with a hadronically decaying Formula omitted boson. No significant excess is observed over the expected backgrounds. Upper limits are calculated for the Formula omitted and Formula omitted production cross sections as a function of the excited electron mass Formula omitted at 95% confidence level. The limits are translated into lower bounds on the compositeness scale parameter Formula omitted of the model as a function of Formula omitted. For Formula omitted Formula omitted, the lower bound for Formula omitted is 11 Formula omitted. In the special case of Formula omitted, the values of Formula omitted Formula omitted are excluded. The presented limits on Formula omitted are more stringent than those obtained in previous searches.
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This paper presents measurements of the Formula omitted and Formula omitted cross-sections and the associated charge asymmetry as a function of the absolute pseudorapidity of the decay muon. The data ...were collected in proton-proton collisions at a centre-of-mass energy of 8 Formula omitted with the ATLAS experiment at the LHC and correspond to a total integrated luminosity of Formula omitted. The precision of the cross-section measurements varies between 0.8 and 1.5% as a function of the pseudorapidity, excluding the 1.9% uncertainty on the integrated luminosity. The charge asymmetry is measured with an uncertainty between 0.002 and 0.003. The results are compared with predictions based on next-to-next-to-leading-order calculations with various parton distribution functions and have the sensitivity to discriminate between them.
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Searches for scalar leptoquarks pair-produced in proton-proton collisions at Formula omitted TeV at the Large Hadron Collider are performed by the ATLAS experiment. A data set corresponding to an ...integrated luminosity of 36.1 fb Formula omitted is used. Final states containing two electrons or two muons and two or more jets are studied, as are states with one electron or muon, missing transverse momentum and two or more jets. No statistically significant excess above the Standard Model expectation is observed. The observed and expected lower limits on the leptoquark mass at 95% confidence level extend up to 1.29 TeV and 1.23 TeV for first- and second-generation leptoquarks, respectively, as postulated in the minimal Buchmüller-Rückl-Wyler model, assuming a branching ratio into a charged lepton and a quark of 50%. In addition, measurements of particle-level fiducial and differential cross sections are presented for the Formula omitted, Formula omitted and Formula omitted processes in several regions related to the search control regions. Predictions from a range of generators are compared with the measurements, and good agreement is seen for many of the observables. However, the predictions for the Formula omitted measurements in observables sensitive to jet energies disagree with the data.
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DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
This paper presents measurements of charged-particle distributions sensitive to the properties of the underlying event in events containing a Formula omitted boson decaying into a muon pair. The data ...were obtained using the ATLAS detector at the LHC in proton-proton collisions at a centre-of-mass energy of 13 Formula omitted with an integrated luminosity of Formula omitted. Distributions of the charged-particle multiplicity and of the charged-particle transverse momentum are measured in regions of the azimuth defined relative to the Formula omitted boson direction. The measured distributions are compared with the predictions of various Monte Carlo generators which implement different underyling event models. The Monte Carlo model predictions qualitatively describe the data well, but with some significant discrepancies.
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DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
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