The simulation software for the ATLAS Experiment at the Large Hadron Collider is being used for largescale production of events on the LHC Computing Grid. This simulation requires many components, from the generators that simulate particle collisions, through packages simulating the response of the various detectors and triggers. All of these components come together under the AT-LAS simulation infrastructure. In this paper, that infrastructure is discussed, including that supporting the detector description, interfacing the event generation, and combining the GEANT4 simulation of the response of the individual detectors. Also described are the tools allowing the software validation, performance testing, and the validation of the simulated output against known physics processes.

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Results of a search for H → τ τ decays are presented, based on the full set of proton-proton collision data recorded by the ATLAS experiment at the LHC during 2011 and 2012. The data correspond to integrated luminosities of 4.5 fb −1 and 20.3 fb −1 at centre-of-mass energies of √ s = 7 TeV and √ s = 8 TeV respectively. All combinations of leptonic (τ → νν with = e, µ) and hadronic (τ → hadrons ν) tau decays are considered. An excess of events over the expected background from other Standard Model processes is found with an observed (expected) significance of 4.5 (3.4) standard deviations. This excess provides evidence for the direct coupling of the recently discovered Higgs boson to fermions. The measured signal strength, normalised to the Standard Model expectation, of µ = 1.43 +0.43−0.37 is consistent with the predicted Yukawa coupling strength in the Standard Model. The ATLAS collaboration 58 Keywords: Hadron-Hadron ScatteringThe investigation of the origin of electroweak symmetry breaking and, related to this, the experimental confirmation of the Brout-Englert-Higgs mechanism [1-6] is one of the prime goals of the physics programme at the Large Hadron Collider (LHC) [7]. With the discovery of a Higgs boson with a mass of approximately 125 GeV by the ATLAS [8] and CMS [9] collaborations, an important milestone has been reached. More precise measurements of the properties of the discovered particle [10,11] as well as tests of the spin-parity quantum numbers [12][13][14] continue to be consistent with the predictions for the Standard Model (SM) Higgs boson.These measurements rely predominantly on studies of the bosonic decay modes, H → γγ, H → ZZ * and H → W W * . To establish the mass generation mechanism for fermions as implemented in the SM, it is of prime importance to demonstrate the direct coupling of the Higgs boson to fermions and the proportionality of its strength to mass [15]. The most promising candidate decay modes are the decays into tau leptons, H → τ τ , and bottom quarks (b-quarks), H → bb. Due to the high background, the search for decays to bb is restricted to Higgs bosons produced in modes which have a more distinct signature but a lower cross-section, such as H production with an associated vector boson. The smaller rate of these processes in the presence of still large background makes their detection challenging. More favourable signal-to-background conditions are expected for H → τ τ decays. Recently, the CMS Collaboration published evidence for H → τ τ decays at a significance in terms of standard deviations of 3.2σ [16], and an excess corresponding to a significance of 2.1σ in the search for H → bb decays [17]. The combination of channels provides evidence for fermionic couplings with a significance of 3.8σ [18]. The yield of events in the search for H → bb decays observed by the ATLAS Collaboration has a signal significance of 1.4σ [19]. The Tevatron experiments have observed an excess corresponding to 2.8σ in the H → bb search [20].In this paper, the results of a search for...

Measurements of inclusive jet production are performed in pp and Pb+Pb collisions at √(s)NN=2.76  TeV with the ATLAS detector at the LHC, corresponding to integrated luminosities of 4.0 and 0.14  nb(-1), respectively. The jets are identified with the anti-k(t) algorithm with R=0.4, and the spectra are measured over the kinematic range of jet transverse momentum 32

High-precision measurements by the ATLAS Collaboration are presented of inclusive , and () Drell–Yan production cross sections at the LHC. The data were collected in proton–proton collisions at with an integrated luminosity of . Differential and cross sections are measured in a lepton pseudorapidity range . Differential cross sections are measured as a function of the absolute dilepton rapidity, for , for three intervals of dilepton mass, , extending from 46 to . The integrated and differential electron- and muon-channel cross sections are combined and compared to theoretical predictions using recent sets of parton distribution functions. The data, together with the final inclusive scattering cross-section data from H1 and ZEUS, are interpreted in a next-to-next-to-leading-order QCD analysis, and a new set of parton distribution functions, ATLAS-epWZ16, is obtained. The ratio of strange-to-light sea-quark densities in the proton is determined more accurately than in previous determinations based on collider data only, and is established to be close to unity in the sensitivity range of the data. A new measurement of the CKM matrix element is also provided.