Abstract:

Using the ATLAS detector, a centrality-dependent suppression has been observed in the yield of J/Ψ mesons produced in the collisions of lead ions at the Large Hadron Collider. In a sample of minimum-bias lead-lead collisions at a nucleon-nucleon centre of mass energy sNN=2.76 TeV, corresponding to an integrated luminosity of about 6.7 μb-1, J/Ψ mesons are reconstructed via their decays to μ+μ- pairs. The measured J/Ψ yield, normalized to the number of binary nucleon-nucleon collisions, is found to significantly decrease from peripheral to central collisions. The centrality dependence is found to be qualitatively similar to the trends observed at previous, lower energy experiments. The same sample is used to reconstruct Z bosons in the μ+μ- final state, and a total of 38 candidates are selected in the mass window of 66 to 116 GeV. The relative Z yields as a function of centrality are also presented, although no conclusion can be inferred about their scaling with the number of binary collisions, because of limited statistics. This analysis provides the first results on J/Ψ and Z production in lead-lead collisions at the LHC. © 2011 CERN.

Abstract:

A measurement of the cross-section for Υ(1S)→μ+μ- production in proton-proton collisions at centre of mass energy of 7 TeV is presented. The cross-section is measured as a function of the Υ(1S) transverse momentum in two bins of rapidity, |yΥ(1S)|<1.2 and 1.2<|yΥ(1S)|<2.4. The measurement requires that both muons have transverse momentum pTμ>4 GeV and pseudorapidity |ημ|<2.5 in order to reduce theoretical uncertainties on the acceptance, which depend on the poorly known polarisation. The results are based on an integrated luminosity of 1.13 pb-1, collected with the ATLAS detector at the Large Hadron Collider. The cross-section measurement is compared to theoretical predictions: it agrees to within a factor of two with a prediction based on the NRQCD model including colour-singlet and colour-octet matrix elements as implemented in Pythia while it disagrees by up to a factor of ten with the next-to-leading order prediction based on the colour-singlet model. © 2011 CERN.

Abstract:

Making use of 36 pb-1 of proton-proton collision data at √s = 7 TeV, the ATLAS Collaboration has performed a search for diphoton events with large missing transverse energy. Observing no excess of events above the Standard Model prediction, a 95% Confidence Level (CL) upper limit is set on the cross section for new physics of σ < 0.38-0.65 pb in the context of a generalised model of gauge-mediated supersymmetry breaking (GGM) with a bino-like lightest neutralino, and of σ < 0.18-0.23 pb in the context of a specific model with one universal extra dimension (UED). A 95% CL lower limit of 560 GeV, for bino masses above 50 GeV, is set on the GGM gluino mass, while a lower limit of 1/R > 961 GeV is set on the UED compactification radius R. These limits provide the most stringent tests of these models to date.

Abstract:

A search for long-lived charged particles reaching the muon spectrometer is performed using a data sample of 37 pb-1 from pp collisions at √s=7 TeV collected by the ATLAS detector at the LHC in 2010. No excess is observed above the estimated background. Stable τ̃ sleptons are excluded at 95% CL up to a mass of 136 GeV, in GMSB models with N5=3, mmessenger=250 TeV, sign(μ)=1 and tanβ=5. Electroweak production of sleptons is excluded up to a mass of 110 GeV. Gluino R-hadrons in a generic interaction model are excluded up to masses of 530 GeV to 544 GeV depending on the fraction of R-hadrons produced as g̃-balls. © 2011 CERN.

Abstract:

A search for neutral Higgs bosons decaying to pairs of τ leptons with the ATLAS detector at the LHC is presented. The analysis is based on proton-proton collisions at a center-of-mass energy of 7 TeV, recorded in 2010 and corresponding to an integrated luminosity of 36 pb-1. After signal selection, 276 events are observed in this data sample. The observed number of events is consistent with the total expected background of 269±36 events. Exclusion limits at the 95% confidence level are derived for the production cross section of a generic Higgs boson φ as a function of the Higgs boson mass and for A/H/h production in the Minimal Supersymmetric Standard Model (MSSM) as a function of the parameters mA and tanβ. © 2011 CERN.