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This article documents the performance of the ATLAS muon identification and reconstruction using the LHC dataset recorded at  TeV in 2015. Using a large sample of and decays from 3.2 fb 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 over most of the covered phase space ( and  GeV). The isolation efficiency varies between 93 and depending on the selection applied and on the momentum of the muon. Both efficiencies are well reproduced in simulation. In the central region of the detector, the momentum resolution is measured to be () for muons from () decays, and the momentum scale is known with an uncertainty of . In the region , the resolution for muons from decays is while the precision of the momentum scale for low- muons from decays is about .

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A direct search for the standard model Higgs boson decaying to a pair of charm quarks is presented. Associated production of the Higgs and Z bosons, in the decay mode ZH→ℓ^{+}ℓ^{-}cc[over ¯] is studied. A data set with an integrated luminosity of 36.1  fb^{-1} of pp collisions at sqrt[s]=13TeV recorded by the ATLAS experiment at the LHC is used. The H→cc[over ¯] signature is identified using charm-tagging algorithms. The observed (expected) upper limit on σ(pp→ZH)×B(H→cc[over ¯]) is 2.7 (3.9_{-1.1}^{+2.1}) pb at the 95% confidence level for a Higgs boson mass of 125 GeV, while the standard model value is 26 fb.

A search for the decay of the Standard Model Higgs boson into a bb pair when produced in association with a W or Z boson is performed with the ATLAS detector. The analysed data, corresponding to an integrated luminosity of 36.1 fb −1 , were collected in proton-proton collisions in Run 2 of the Large Hadron Collider at a centre-of-mass energy of 13 TeV. Final states containing zero, one and two charged leptons (electrons or muons) are considered, targeting the decays Z → νν, W → ν and Z → . For a Higgs boson mass of 125 GeV, an excess of events over the expected background from other Standard Model processes is found with an observed significance of 3.5 standard deviations, compared to an expectation of 3.0 standard deviations. This excess provides evidence for the Higgs boson decay into b-quarks and for its production in association with a vector boson. The combination of this result with that of the Run 1 analysis yields a ratio of the measured signal events to the Standard Model expectation equal to 0. [7]. The properties of the discovered particle have been measured using the Run 1 dataset, collected at centre-of-mass energies of 7 TeV and 8 TeV, and were found to be compatible with those predicted by the Standard Model (SM) within uncertainties, typically of the order of ±20% [8-11]. The Run 2 dataset at an energy of 13 TeV provides an opportunity to increase the precision of such measurements, and to challenge theory predictions further. While analyses of Higgs bosons decaying into vector bosons are entering an era of detailed differential measurements, direct evidence for the coupling of the Higgs boson to fermions was established only via the observation of the decay into τ -leptons through the combination of ATLAS and CMS Run 1 results [11], and, more recently, through the combination of CMS Run 1 and Run 2 results [12]. Although the gluon-gluon fusion production mode provides indirect evidence for the coupling of the Higgs boson to top quarks, there is currently no direct observation of the coupling of the Higgs boson to quarks. The decay of the SM Higgs boson into pairs of b-quarks is expected to have a branching ratio of 58% for m H = 125 GeV [13], the largest among all decay modes. Accessing H → bb decays is therefore crucial for constraining, under fairly general assumptions [14,15], the overall Higgs boson decay width. At the LHC, the very large backgrounds arising from multi-jet production make an inclusive search extremely challenging. The most sensitive production modes for probing H → bb decays are those where the Higgs boson is produced in association with a W or Z boson [16]; their leptonic decay modes lead to clean signatures that can be efficiently triggered on, while rejecting most of the multi-jet backgrounds.Searches for a Higgs boson in the bb decay mode were conducted at the Tevatron by the CDF and D0 Collaborations. They reported an excess of events in V H associated production (where V is used to denote W or Z) in the mass range of 120 GeV to 135 GeV, with a global signifi...