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
The χ(b)(nP) quarkonium states are produced in proton-proton collisions at the Large Hadron Collider at sqrt[s] = 7 TeV and recorded by the ATLAS detector. Using a data sample corresponding to an integrated luminosity of 4.4 fb(-1), these states are reconstructed through their radiative decays to Υ(1S,2S) with Υ → μ+ μ-. In addition to the mass peaks corresponding to the decay modes χ(b)(1P,2P) → Υ(1S)γ, a new structure centered at a mass of 10.530 ± 0.005(stat) ± 0.009(syst) GeV is also observed, in both the Υ(1S)γ and Υ(2S)γ decay modes. This structure is interpreted as the χ(b)(3P) system.
A QCD analysis is reported of ATLAS data on inclusive W(±) and Z boson production in pp collisions at the LHC, jointly with ep deep-inelastic scattering data from HERA. The ATLAS data exhibit sensitivity to the light quark sea composition and magnitude at Bjorken x∼0.01. Specifically, the data support the hypothesis of a symmetric composition of the light quark sea at low x. The ratio of the strange-to-down sea quark distributions is determined to be 1.00(-0.28)(+0.25) at absolute four-momentum transfer squared Q(2)=1.9 GeV(2) and x=0.023.
A measurement of the total pp cross section at the LHC at √ s = 7 TeV is presented. In a special run with high-β beam optics, an integrated luminosity of 80 µb −1 was accumulated in order to measure the differential elastic cross section as a function of the Mandelstam momentum transfer variable t. The measurement is performed with the ALFA sub-detector of ATLAS. Using a fit to the differential elastic cross section in the |t| range from 0.01 GeV 2 to 0.1 GeV 2 to extrapolate to |t| → 0, the total cross section, σ tot (pp → X), is measured via the optical theorem to be:where the first error is statistical, the second accounts for all experimental systematic uncertainties and the last is related to uncertainties in the extrapolation to |t| → 0. In addition, the slope of the elastic cross section at small |t| is determined to be B = 19.73 ± 0.14 (stat.) ± 0.26 (syst.) GeV −2 .
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