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 tt events. The topology of the t → W b 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 fb −1 , were collected in proton-proton collisions during the years 2015-2017 at a centre-of-mass energy √ s = 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.
The result of a search for the pair production of the lightest supersymmetric partner of the bottom quark (b 1 ) using 139 fb −1 of proton-proton data collected at √ s = 13 TeV by the ATLAS detector is reported. In the supersymmetric scenarios considered both of the bottom-squarks decay into a b-quark and the second-lightest neutralino,b 1 → b +χ 0 2 . Eachχ 0 2 is assumed to subsequently decay with 100% branching ratio into a Higgs boson (h) like the one in the Standard Model and the lightest neutralino:χ 0 2 → h +χ 0 1 . Theχ 0 1 is assumed to be the lightest supersymmetric particle (LSP) and is stable. Two signal mass configurations are targeted: the first has a constant LSP mass of 60 GeV; and the second has a constant mass difference between theχ 0 2 andχ 0 1 of 130 GeV. The final states considered contain no charged leptons, three or more b-jets, and large missing transverse momentum. No significant excess of events over the Standard Model background expectation is observed in any of the signal regions considered. Limits at the 95% confidence level are placed in the supersymmetric models considered, and bottom-squarks with mass up to 1.5 TeV are excluded.
Combinations of single-top-quark production cross-section measurements and | f LV V t b | determinations at √ s = 7 and 8 TeV with the ATLAS and CMS experimentsThe ATLAS and CMS Collaborations This paper presents the combinations of single-top-quark production cross-section measurements by the ATLAS and CMS Collaborations, using data from LHC proton-proton collisions at √ s = 7 and 8 TeV corresponding to integrated luminosities of 1.17 to 5.1 fb −1 at √ s = 7 TeV, and 12.2 to 20.3 fb −1 at √ s = 8 TeV. These combinations are performed per centre-of-mass energy and for each production mode: t-channel, tW, and s-channel. The combined t-channel cross-sections are 67.5 ± 5.7 pb and 87.7 ± 5.8 pb at √ s = 7 and 8 TeV respectively. The combined tW cross-sections are 16.3 ± 4.1 pb and 23.1 ± 3.6 pb at √ s = 7 and 8 TeV respectively. For the s-channel cross-section, the combination yields 4.9 ± 1.4 pb at √ s = 8 TeV. The square of the magnitude of the CKM matrix element V tb multiplied by a form factor f LV is determined for each production mode and centre-of-mass energy, using the ratio of the measured cross-section to its theoretical prediction. It is assumed that the top-quark-related CKM matrix elements obey the relation |V t d |, |V ts | |V tb |. All the | f LV V tb | 2 determinations, extracted from individual ratios at √ s = 7 and 8 TeV, are combined, resulting in | f LV V tb | = 1.02 ± 0.04 (meas.) ± 0.02 (theo.). All combined measurements are consistent with their corresponding Standard Model predictions.
This paper describes a study of techniques for identifying Higgs bosons at high transverse momenta decaying into bottom-quark pairs, H → bb, for proton-proton collision data collected by the ATLAS detector at the Large Hadron Collider at a centre-of-mass energy √ s = 13 TeV. These decays are reconstructed from calorimeter jets found with the anti-k t R = 1.0 jet algorithm. To tag Higgs bosons, a combination of requirements is used: b-tagging of R = 0.2 track-jets matched to the largeR 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 −1 of data collected in 2015 and 2016 using g → bb and Z (→ bb)γ event selections in data. Both processes are found to be well modelled within the statistical and systematic uncertainties.
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