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Alexander, G.

Publications and source records attributed to Alexander, G..

At least 595 records · Page 33
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DOE OSTI2012

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 ($t\bar{t}$) in pp collisions at $\sqrt{s} = 7TeV 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 σ $t\bar{t}$ = 187±11 (stat.) $+18\atop{-17}$ (syst.) ± 6 (lumi.) pb and σ $t\bar{t}$ =173 ± 17 (stat.) $+18\atop{-16}$ (syst.) ± 6 (lumi.) pb respectively. In conclusion, 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.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2012

Measurement of the polarisation of W bosons produced with large transverse momentum in pp collisions at $\sqrt{s} = 7\ \mbox{TeV}$ with the ATLAS experiment

This paper describes an analysis of the angular distribution of W→eν and W→μν decays, using data from pp collisions at √s =7 TeV recorded with the ATLAS detector at the LHC in 2010, corresponding to an integrated luminosity of about 35 pb -1 . Using the decay lepton transverse momentum and the missing transverse momentum, the W decay angular distribution projected onto the transverse plane is obtained and analysed in terms of helicity fractions f 0 ,f L and f R over two ranges of W transverse momentum (p$W\atop{T}$): 35 < p$W\atop{T}$ < 50 GeV and p$W\atop{T}$ > 50 GeV. Good agreement is found with theoretical predictions. For p W T > 50 GeV, the values of f 0 and f L -f R , averaged over charge and lepton flavour, are measured to be: f 0 =0.127±0.030±0.108 and f L -f R =0.252±0.017±0.030, where the first uncertainties are statistical, and the second include all systematic effects.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2012

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 . Here, 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 10 3 seconds and a neutralino mass of 100 GeV.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2012

Search for diphoton events with large missing transverse momentum in 1 fb⁻¹ of 7 TeV proton–proton collision data with the ATLAS detector

A search for diphoton events with large missing transverse momentum has been performed using 1.07 fb⁻¹ of proton–proton collision data at √s = 7 TeV recorded with the ATLAS detector. No excess of events was observed above the Standard Model prediction and 95% Confidence Level (CL) upper limits are set on the production cross section for new physics. The limits depend on each model parameter space and vary as follows: σ<(22–129) fb in the context of a generalised model of gauge-mediated supersymmetry breaking (GGM) with a bino-like lightest neutralino, σ<(27–91) fb in the context of a minimal model of gauge-mediated supersymmetry breaking (SPS8), and σ<(15–27) fb in the context of a specific model with one universal extra dimension (UED). A 95% CL lower limit of 805 GeV , for bino masses above 50 GeV, is set on the GGM gluino mass. Lower limits of 145 TeV and 1.23 TeV are set on the SPS8 breaking scale Λ and on the UED compactification scale 1/R , respectively. These limits provide the most stringent tests of these models to date.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2012

Search for strong gravity signatures in same-sign dimuon final states using the ATLAS detector at the LHC

A search for microscopic black holes has been performed in a same-sign dimuon final state using 1.3 fb{sup -1} of proton-proton collision data collected with the ATLAS detector at a centre of mass energy of 7 TeV at the CERN Large Hadron Collider. The data are found to be consistent with the expectation from the Standard Model and the results are used to derive exclusion contours in the context of a low scale gravity model.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗