A search for the Standard Model Higgs boson in proton–proton collisions with the ATLAS detector at the LHC is presented. The datasets used correspond to integrated luminosities of approximately 4.8 fb−1 collected at √s=7 TeV in 2011 and 5.8 fb−1 at √s=8 TeV in 2012. Individual searches in the channels H→ZZ(⁎)→4ℓ, H→γγ and H→WW(⁎)→eνμν in the 8 TeV data are combined with previously published results of searches for H→ZZ(⁎), WW(⁎), bb and τ+τ− in the 7 TeV data and results from improved analyses of the H→ZZ(⁎)→4ℓ and H→γγ channels in the 7 TeV data. Clear evidence for the production of a neutral boson with a measured mass of 126.0±0.4(stat)±0.4(sys) GeV is presented. This observation, which has a significance of 5.9 standard deviations, corresponding to a background fluctuation probability of 1.7×10−9, is compatible with the production and decay of the Standard Model Higgs boson
By using the ATLAS detector, observations have been made of a centrality-dependent dijet asymmetry in the collisions of lead ions at the Large Hadron Collider. In a sample of lead-lead events with a per-nucleon center of mass energy of 2.76 TeV, selected with a minimum bias trigger, jets are reconstructed in fine-grained, longitudinally segmented electromagnetic and hadronic calorimeters. The transverse energies of dijets in opposite hemispheres are observed to become systematically more unbalanced with increasing event centrality leading to a large number of events which contain highly asymmetric dijets. This is the first observation of an enhancement of events with such large dijet asymmetries, not observed in proton-proton collisions, which may point to an interpretation in terms of strong jet energy loss in a hot, dense medium.
Detailed measurements of the electron performance of the ATLAS detector at the LHC are reported, using decays of the Z, W and J /ψ particles. Data collected in 2010 at √ s = 7 TeV are used, corresponding to an integrated luminosity of almost 40 pb −1 . The inter-alignment of the inner detector and the electromagnetic calorimeter, the determination of the electron energy scale and resolution, and the performance in terms of response uniformity and linearity are discussed. The electron identification, reconstruction and trigger efficiencies, as well as the charge misidentification probability, are also presented.
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.
A search for new phenomena in events with a high-energy jet and large missing transverse momentum is performed using data from proton-proton collisions at √ s = 7 TeV with the ATLAS experiment at the Large Hadron Collider. Four kinematic regions are explored using a dataset corresponding to an integrated luminosity of 4.7 fb −1 . No excess of events beyond expectations from Standard Model processes is observed, and limits are set on large extra dimensions and the pair production of dark matter particles.
Keywords: Hadron-Hadron ScatteringArXiv ePrint: 1210.4491Open Access, Copyright CERN, for the benefit of the ATLAS collaboration 6 Results and interpretation 17 6.1 Large extra dimensions 18 6.2 WIMP pair production 22
Summary 27The ATLAS collaboration 35
IntroductionEvent topologies with a single jet with large transverse energy and large missing transverse momentum, referred to as monojets in the following, are important final states for searches for new phenomena beyond the Standard Model (SM) at a hadron collider. The primary SM process that results in a true monojet final state is Z-boson production in association with a jet, where the Z boson decays to two neutrinos. A further important reducible contribution to this final state consists of events that include a W boson and a jet, where the charged lepton from the W -boson decay is not reconstructed. Phenomenological scenarios beyond the Standard Model (BSM) that result in a monojet final state include supersymmetry [1][2][3][4][5][6][7][8][9][10][11] and large extra dimensions (LED) [12]. A model-independent treatment of the production of dark matter (DM) particles at the Large Hadron Collider (LHC) has been proposed recently, where DM particles are pair-produced in association with a jet [13][14][15]. In the following, a search for an excess of monojet events over SM expectations is performed. The results are interpreted in a framework of LED and DM particle pair production. They are based on a dataset of 4.7 fb −1 of proton-proton -1 -JHEP04 (2013) Models of large extra spatial dimensions have been proposed to remove the hierarchy problem [22][23][24][25] by addressing the weakness of gravity relative to all other forces. One popular model of LED that is often used to interpret the results of monojet searches at particle colliders is that of Arkani-Hamed, Dimopoulos, Dvali (ADD) [12]. In this model, gravity propagates in the (4 + n)-dimensional bulk of space-time, while the SM fields are confined to four dimensions. The large observed difference between the characteristic mass scale of gravity (the Planck mass) and the electroweak scale (as characterised by the W -boson mass) is the result of the four-dimensional interpretation of the Planck scale, M Pl = 1.2 × 10 19 GeV, which is related to the fundamental (4 + n)-dimensional Planck scale (M D ) by M P l 2 = 8π M D 2+n R n , where n and R are the number and size of the extra dimensions, respectively. An appropriate choice of R for a given n results in a value of M D close to the electroweak s...
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