Search for the Standard Model Higgs boson produced in association with a vector boson and decaying to a b-quark pair with the ATLAS detector

AbstractThis Letter presents the results of a direct search with the ATLAS detector at the LHC for a Standard Model Higgs boson of mass 110⩽mH⩽130 GeV produced in association with a W or Z boson and decaying to bb¯. Three decay channels are considered: ZH→ℓ+ℓ−bb¯, WH→ℓνbb¯ and ZH→νν¯bb¯, where ℓ corresponds to an electron or a muon. No evidence for Higgs boson production is observed in a dataset of 7 TeV pp collisions corresponding to 4.7 fb−1 of integrated luminosity collected by ATLAS in 2011. Exclusion limits on Higgs boson production, at the 95% confidence level, of 2.5 to 5.5 times the Standard Model cross section are obtained in the mass range 110–130 GeV. The expected exclusion limits range between 2.5 and 4.9 for the same mass interval

Search for extra dimensions using diphoton events in 7 TeV proton–proton collisions with the ATLAS detector

AbstractUsing data recorded in 2011 with the ATLAS detector at the Large Hadron Collider, a search for evidence of extra spatial dimensions has been performed through an analysis of the diphoton final state. The analysis uses data corresponding to an integrated luminosity of 2.12 fb−1 of s=7 TeV proton–proton collisions. The diphoton invariant mass (mγγ) spectrum is observed to be in good agreement with the expected Standard Model background. In the large extra dimension scenario of Arkani-Hamed, Dimopoulos and Dvali, the results provide 95% CL lower limits on the fundamental Planck scale between 2.27 and 3.53 TeV, depending on the number of extra dimensions and the theoretical formalism used. The results also set 95% CL lower limits on the lightest Randall–Sundrum graviton mass of between 0.79 and 1.85 TeV, for values of the dimensionless coupling k/M¯Pl varying from 0.01 to 0.1. Combining with previously published ATLAS results from the dielectron and dimuon final states, the 95% CL lower limit on the Randall–Sundrum graviton mass for k/M¯Pl=0.01 (0.1) is 0.80 (1.95) TeV

Measurement of the top quark pair production cross-section with ATLAS in the single lepton channel

A measurement of the production cross-section for top quark pairs (View the MathML source) in pp collisions at View the MathML source is presented using data recorded with the ATLAS detector at the Large Hadron Collider. Events are selected in the single lepton topology by requiring an electron or muon, large missing transverse momentum and at least three jets. With a data sample of 35 pb−1, two different multivariate methods, one of which uses b -quark jet identification while the other does not, use kinematic variables to obtain cross-section measurements of View the MathML source and View the MathML source respectively. The two measurements are in agreement with each other and with QCD calculations. The first measurement has a better a priori sensitivity and constitutes the main result of this Letter

Search for scalar top quark pair production in natural gauge mediated supersymmetry models with the ATLAS detector in pp collisions at s=7 TeV

AbstractThe results of a search for pair production of the lighter scalar partners of top quarks (t˜1) in 2.05 fb−1 of pp collisions at s=7 TeV using the ATLAS experiment at the LHC are reported. Scalar top quarks are searched for in events with two same flavour opposite-sign leptons (e,μ) with invariant mass consistent with the Z boson mass, large missing transverse momentum and jets in the final state. At least one of the jets is identified as originating from a b-quark. No excess over Standard Model expectations is found. The results are interpreted in the framework of R-parity conserving, gauge mediated Supersymmetry breaking ‘natural’ scenarios, where the neutralino (χ˜10) is the next-to-lightest supersymmetric particle. Scalar top quark masses up to 310 GeV are excluded for 115 GeV<mχ˜10<230 GeV at 95% confidence level, reaching an exclusion of mt˜1<330 GeV for mχ˜10=190 GeV. Scalar top quark masses below 240 GeV are excluded for all values of mχ˜10>mZ

Searches for supersymmetry with the ATLAS detector using final states with two leptons and missing transverse momentum in root s=7 TeV proton-proton collisions

Results of three searches are presented for the production of supersymmetric particles decaying into final states with missing transverse momentum and exactly two isolated leptons, e or mu. The analysis uses a data sample collected during the first half of 2011 that corresponds to a total integrated luminosity of 1 fb(-1) of root s = 7 TeV proton-proton collisions recorded with the ATLAS detector at the Large Hadron Collider. Opposite-sign and same-sign dilepton events are separately studied, with no deviations from the Standard Model expectation observed. Additionally, in opposite-sign events, a search is made for an excess of same-flavour over different-flavour lepton pairs. Effective production cross sections in excess of 9.9 fb for opposite-sign events containing supersymmetric particles with missing transverse momentum greater than 250 GeV are excluded at 95% CL For same-sign events containing supersymmetric particles with missing transverse momentum greater than 100 GeV, effective production cross sections in excess of 14.8 fb are excluded at 95% CL The latter limit is interpreted in a simplified electroweak gaugino production model excluding chargino masses up to 200 GeV, under the assumption that slepton decay is dominant. (C) 2012 CERN. Published by Elsevier B.V. All rights reserved