Search for New Phenomena in tt Events with Large Missing Transverse Momentum in Proton-Proton Collisions at √s = 7 TeV with the ATLAS Detector

A search for new phenomena in tt̅ events with large missing transverse momentum in proton-proton collisions at a center-of-mass energy of 7 TeV is presented. The measurement is based on 1.04  fb-1 of data collected with the ATLAS detector at the LHC. Contributions to this final state may arise from a number of standard model extensions. The results are interpreted in terms of a model where new top-quark partners are pair produced and each decay to an on-shell top (or antitop) quark and a long-lived undetected neutral particle. The data are found to be consistent with standard model expectations. A limit at 95% confidence level is set excluding a cross section times branching ratio of 1.1 pb for a top-partner mass of 420 GeV and a neutral particle mass less than 10 GeV. In a model of exotic fourth generation quarks, top-partner masses are excluded up to 420 GeV and neutral particle masses up to 140 GeV

Search for the Standard Model Higgs Boson in the Diphoton Decay Channel with 4.9 fb^(-1) of pp Collision Data at √s = 7 TeV with ATLAS

A search for the standard model Higgs boson is performed in the diphoton decay channel. The data used correspond to an integrated luminosity of 4.9  fb^(-1) collected with the ATLAS detector at the Large Hadron Collider in proton-proton collisions at a center-of-mass energy of √s = 7  TeV. In the diphoton mass range 110–150 GeV, the largest excess with respect to the background-only hypothesis is observed at 126.5 GeV, with a local significance of 2.8 standard deviations. Taking the look-elsewhere effect into account in the range 110–150 GeV, this significance becomes 1.5 standard deviations. The standard model Higgs boson is excluded at 95% confidence level in the mass ranges of 113–115 GeV and 134.5–136 GeV

Measurement of the W →τν_τ cross section in pp collisions at √s = 7 TeV with the ATLAS experiment

The cross section for the production of W bosons with subsequent decay W→τν_τ is measured with the ATLAS detector at the LHC. The analysis is based on a data sample that was recorded in 2010 at a proton–proton center-of-mass energy of √s = 7TeV and corresponds to an integrated luminosity of 34 pb^(−1). The cross section is measured in a region of high detector acceptance and then extrapolated to the full phase space. The product of the total W production cross section and the W→τν_τ branching ratio is measured to be σ^(tot) _(W→τντ) = 11.1±0.3 (stat)±1.7 (syst)±0.4 (lumi) nb

Measurement of the W^±Z production cross section and limits on anomalous triple gauge couplings in proton–proton collisions at √s = 7 TeV with the ATLAS detector

This Letter presents a measurement of W^± Z production in 1.02 fb^(−1) of pp collision data at √s = 7 TeV collected by the ATLAS experiment in 2011. Doubly leptonic decay events are selected with electrons, muons and missing transverse momentum in the final state. In total 71 candidates are observed, with a background expectation of 12.1 ± 1.4(stat.)^(+4.1)_(−2.0)(syst.) events. The total cross section for W^± Z production for Z/γ^∗ masses within the range 66 GeV to 116 GeV is determined to be σ^(tot)_(WZ) = 20.5^(+3.1)_(−2.8)(stat.)^(+1.4)_(−1.3)(syst.)^(+0.9)_(−0.8)(lumi.) pb, which is consistent with the Standard Model expectation of 17.3^(+1.3) _(0.8) pb. Limits on anomalous triple gauge boson couplings are extracted

Measurement of the top quark-pair production cross section with ATLAS in pp collisions at √s=7 TeV

A measurement of the production cross-section for top quark pairs (tt) in pp collisions at √s = 7 TeV is presented using data recorded with the ATLAS detector at the Large Hadron Collider. Events are selected in two different topologies: single lepton (electron e or muon μ) with large missing transverse energy and at least four jets, and dilepton (ee, μμ or eμ) with large missing transverse energy and at least two jets. In a data sample of 2.9 pb^(−1), 37 candidate events are observed in the single-lepton topology and 9 events in the dilepton topology. The corresponding expected backgrounds from non-tt Standard Model processes are estimated using data-driven methods and determined to be 12.2±3.9 events and 2.5±0.6 events, respectively. The kinematic properties of the selected events are consistent with SM tt production. The inclusive top quark pair production cross-section is measured to be σ _tt=14531(stat)^(+42)_(−27)(syst) pb. The measurement agrees with perturbative QCD calculations

Search for decays of stopped, long-lived particles from 7 TeV pp collisions with the ATLAS detector

New metastable massive particles with electric and colour charge are features of many theories beyond the Standard Model. A search is performed for long-lived gluino-based R-hadrons with the ATLAS detector at the LHC using a data sample corresponding to an integrated luminosity of 31 pb^(−1). We search for evidence of particles that have come to rest in the ATLAS detector and decay at some later time during the periods in the LHC bunch structure without proton–proton collisions. No significant deviations from the expected backgrounds are observed, and a cross-section limit is set. It can be interpreted as excluding gluino-based R-hadrons with masses less than 341 GeV at the 95 % C.L., for lifetimes from 10^(−5) to 103 seconds and a neutralino mass of 100 GeV