Proton-proton collisions at √ s = 7 TeV and heavy ion collisions at √ s NN = 2.76 TeV were produced by the LHC and recorded using the ATLAS experiment's trigger system in 2010. The LHC is designed with a maximum bunch crossing rate of 40 MHz and the ATLAS trigger system is designed to record approximately 200 of these per second. The trigger system selects events by rapidly identifying signatures of muon, electron, photon, tau lepton, jet, and B meson candidates, as well as using global event signatures, such as missing transverse energy. An overview of the ATLAS trigger system, the evolution of the system during 2010 and the performance of the trigger system components and selections based on the 2010 collision data are shown. A brief outline of plans for the trigger system in 2011 is presented.
The ATLAS detector at the Large Hadron Collider is used to search for high-mass resonances decaying to dielectron or dimuon final states. Results are presented from an analysis of proton-proton (pp) collisions at a center-of-mass energy of 8 TeV corresponding to an integrated luminosity of 20.3 fb −1 in the dimuon channel. A narrow resonance with Standard Model Z couplings to fermions is excluded at 95% confidence level for masses less than 2.79 TeV in the dielectron channel, 2.53 TeV in the dimuon channel, and 2.90 TeV in the two channels combined. Limits on other model interpretations are also presented, including a grand-unification model based on the E 6 gauge group, Z Ã bosons, minimal Z 0 models, a spin-2 graviton excitation from Randall-Sundrum models, quantum black holes, and a minimal walking technicolor model with a composite Higgs boson.
This paper presents measurements of the polarization of W bosons in top quark decays, derived from tt events with missing transverse momentum, one charged lepton and at least four jets, or two charged leptons and at least two jets. Data from pp collisions at a centre-of-mass energy of 7 TeV were collected with the ATLAS experiment at the LHC and correspond to an integrated luminosity of 1.04 fb −1 . The measured fractions of longitudinally, left-and right-handed polarization are F 0 = 0.67 ± 0.07, F L = 0.32 ± 0.04 and F R = 0.01 ± 0.05, in agreement with the Standard Model predictions. As the polarization of the W bosons in top quark decays is sensitive to the W tb vertex Lorentz structure and couplings, the measurements were used to set limits on anomalous contributions to the W tb couplings. Keywords: Hadron-Hadron ScatteringOpen Access, Copyright CERN, for the benefit of the ATLAS collaboration One particular test is the study of the W tb vertex Lorentz structure and couplings, which can be probed by measuring the polarization of W bosons produced in top quark decays. At the LHC, top quarks are produced mainly in pairs via the strong interaction and are predicted to decay via the electroweak interaction into a W boson and a bottom quark with a nearly 100% branching fraction. Events with tt pairs can thus be classified according to the decay of the two produced W bosons. Each boson can decay either into -1 - JHEP06(2012)088a quark-antiquark pair or into a charged lepton and a neutrino. The single-lepton and dilepton topologies, both considered in the analyses presented in this paper, have one and two isolated charged leptons in the final state. Only electrons and muons, including those from τ decays, are considered here.The W tb vertex is defined by the electroweak interaction and has a (V − A) structure where V and A are the vector and axial-vector contributions to the vertex. Since the W bosons are produced as real particles in top quark decays, their polarization can be longitudinal, left-handed or right-handed. The fractions of events with a particular polarization, F 0 , F L and F R , are referred to as helicity fractions. They are predicted in next-to-nextto-leading-order (NNLO) QCD calculations to be F 0 = 0.687 ± 0.005, F L = 0.311 ± 0.005, F R = 0.0017 ± 0.0001 [4]. These fractions can be extracted from measurements of the angular distribution of the decay products of the top quark. The angle θ * is defined as the angle between the momentum direction of the charged lepton from the decay of the W boson and the reversed momentum direction of the b-quark from the decay of the top quark, both boosted into the W boson rest frame [5]. The angular distribution is:All previous measurements of the helicity fractions, performed by the CDF and DØ Collaborations [6][7][8] at the Tevatron, are in agreement with Standard Model predictions. Information about the polarization of the W bosons can also be obtained through complementary observables, such as the angular asymmetries, A + and A − , defined as:wi...
Using 1:8 fb À1 of pp collisions at a center-of-mass energy of 7 TeV recorded by the ATLAS detector at the Large Hadron Collider, we present measurements of the production cross sections of Çð1S; 2S; 3SÞ mesons. Ç mesons are reconstructed using the dimuon decay mode. Total production cross sections for p T < 70 GeV and in the rapidity interval jy Ç j <
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