Studies of the spin and parity quantum numbers of the Higgs boson are presented, based on proton–proton collision data collected by the ATLAS experiment at the LHC. The Standard Model spin–parity JP=0+JP=0+ hypothesis is compared with alternative hypotheses using the Higgs boson decays H→γγH→γγ, H→ZZ⁎→4ℓH→ZZ⁎→4ℓ and H→WW⁎→ℓνℓνH→WW⁎→ℓνℓν, as well as the combination of these channels. The analysed dataset corresponds to an integrated luminosity of 20.7 fb−1 collected at a centre-of-mass energy of √s=8TeV. For the H→ZZ⁎→4ℓH→ZZ⁎→4ℓ decay mode the dataset corresponding to an integrated luminosity of 4.6 fb−1 collected at √s=7TeV is included. The data are compatible with the Standard Model JP=0+JP=0+ quantum numbers for the Higgs boson, whereas all alternative hypotheses studied in this Letter, namely some specific JP=0−,1+,1−,2+JP=0−,1+,1−,2+ models, are excluded at confidence levels above 97.8%. This exclusion holds independently of the assumptions on the coupling strengths to the Standard Model particles and in the case of the JP=2+JP=2+ model, of the relative fractions of gluon-fusion and quark–antiquark production of the spin-2 particle. The data thus provide evidence for the spin-0 nature of the Higgs boson, with positive parity being strongly preferre

The luminosity calibration for the ATLAS detector at the LHC during pp collisions at in 2010 and 2011 is presented. Evaluation of the luminosity scale is performed using several luminosity-sensitive detectors, and comparisons are made of the long-term stability and accuracy of this calibration applied to the pp collisions at . A luminosity uncertainty of is obtained for the 47 pb−1 of data delivered to ATLAS in 2010, and an uncertainty of is obtained for the 5.5 fb−1 delivered in 2011.

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.

The ATLAS experiment has observed 1995 Z boson candidates in data corresponding to 0:15 nb À1 of integrated luminosity obtained in the 2011 LHC Pb þ Pb run at ffiffiffiffiffiffiffiffi s NN p ¼ 2:76 TeV. The Z bosons are reconstructed via dielectron and dimuon decay channels, with a background contamination of less than 3%. Results from the two channels are consistent and are combined. Within the statistical and systematic uncertainties, the per-event Z boson yield is proportional to the number of binary collisions estimated by the Glauber model. The elliptic anisotropy of the azimuthal distribution of the Z boson with respect to the event plane is found to be consistent with zero.

Abstract:The results of a search for pair production of supersymmetric partners of the Standard Model third-generation quarks are reported. This search uses 20.1 fb −1 of pp collisions at √ s = 8 TeV collected by the ATLAS experiment at the Large Hadron Collider. The lightest bottom and top squarks (b 1 andt 1 respectively) are searched for in a final state with large missing transverse momentum and two jets identified as originating from b-quarks. No excess of events above the expected level of Standard Model background is found. The results are used to set upper limits on the visible cross section for processes beyond the Standard Model. Exclusion limits at the 95% confidence level on the masses of the third-generation squarks are derived in phenomenological supersymmetric R-parityconserving models in which either the bottom or the top squark is the lightest squark. Thẽ b 1 is assumed to decay viab 1 → bχ 0 1 and thet 1 viat 1 → bχ ± 1 , with undetectable products of the subsequent decay of theχ ± 1 due to the small mass splitting between theχ ± 1 and theχ 0 1 . Keywords: Hadron-Hadron Scattering, SupersymmetryOpen Access, Copyright CERN, for the benefit of the ATLAS collaboration The ATLAS collaboration 24 IntroductionSupersymmetry (SUSY) [1][2][3][4][5][6][7][8][9] provides an extension of the Standard Model (SM) that solves the hierarchy problem [10][11][12][13] by introducing supersymmetric partners of the known bosons and fermions. In the framework of the R-parity-conserving minimal supersymmetric extension of the SM (MSSM) [14][15][16][17][18], SUSY particles are produced in pairs and the lightest supersymmetric particle (LSP) is stable, providing a possible candidate for dark matter. In a large variety of models, the LSP is the lightest neutralino (χ 0 1 ). The coloured superpartners of quarks and gluons, the squarks (q) and the gluinos (g), if not too heavy, would be produced in strong interaction processes at the Large Hadron Collider (LHC) [19] and decay via cascades ending with the LSP. The undetected LSP would result in missing transverse momentum while the rest of the cascade would yield final states with multiple jets and possibly leptons.A study of the expected SUSY particle spectrum derived from naturalness considerations [20, 21] suggests that the supersymmetric partners of the third-generation SM quarks are the lightest coloured supersymmetric particles. This may lead to the lightest bottom squark (sbottom,b 1 ) and top squark (stop,t 1 ) mass eigenstates being significantly lighter than the other squarks and the gluinos. As a consequence,b 1 andt 1 could be pair-produced with relatively large cross sections at the LHC.-1 - JHEP10(2013)189Two possible sets of SUSY mass spectra are considered in this paper. In the first set of scenarios, the lightest sbottom is the only coloured sparticle contributing to the production processes and it only decays viab 1 → bχ 0 1 . In the second set, the lightest stop is the only coloured sparticle allowed in the production processes and it decays exclusively...