The production of events with multiple high transverse momentum particles including charged leptons and jets is measured, using 1.04 fb-1 of proton-proton collision data recorded by the ATLAS ...detector during the first half of 2011 at sqrt(s) = 7 TeV. No excess beyond Standard Model expectations is observed, and upper limits on the fiducial cross sections for non-Standard Model production of these final states are set. Using models for string ball and black hole production and decay, exclusion contours are determined as a function of mass threshold and the fundamental gravity scale.
A search for a heavy Standard Model Higgs boson decaying via H->ZZ->llqq, where l=e or mu, is presented. The search uses a data set of pp collisions at sqrt(s) = 7 TeV, corresponding to an integrated ...luminosity of 4.7 fb^-1 collected in 2011 by the ATLAS detector at the CERN LHC. No significant excess of events above the estimated background is found. Upper limits at 95% confidence level on the production cross section of a Higgs boson with a mass in the range between 200 and 600 GeV are derived. A Standard Model Higgs boson with a mass in the range 300 <= mH <= 322 GeV or 353 <= mH <= 410 GeV is excluded at 95% CL. The corresponding expected exclusion range is 351 <= mH <= 404 GeV at 95% CL.
A blind analysis searching for the decay Bs0 -> mumu has been performed using proton-proton collisions at a centre-of-mass energy of 7 TeV recorded with the ATLAS detector at the LHC. With an ...integrated luminosity of 2.4 fb^(-1) no excess of events over the background expectation is found and an upper limit is set on the branching fraction BR(Bs0 -> mu mu) < 2.2 (1.9) x10^(-8) at 95% (90%) confidence level.
Results on the search for flavor-changing neutral-current single top-quark production are reported. Data collected with the ATLAS detector at a center-of-mass energy of sqrt(s) = 7 TeV and ...corresponding to an integrated luminosity of 2.05 fb^-1 are used. Candidate events with a leptonic top-quark decay signature are classified into signal- and background-like events using a neural network. No signal is observed in the output distribution and a Bayesian upper limit on the production cross-section is placed. The observed 95% C.L. upper limit on the cross-section multiplied by the t -> Wb branching fraction is measured to be sigma(qg -> t) x B(t -> Wb) < 3.9 pb. This upper limit can be converted into an upper limit of the coupling strength k_ugt/Lambda < 6.9 x 10^-3 TeV^-1 and k_cgt/Lambda < 1.6 x 10^-2 TeV^-1 and on the branching fractions B(t -> ug) < 5.7 x 10^-5 and B(t -> cg) < 2.7 x 10^-4.
Results are presented of a search for new particles decaying to large numbers of jets in association with missing transverse momentum, using 4.7 fb^-1 of pp collision data at sqrt(s) = 7 TeV ...collected by the ATLAS experiment at the Large Hadron Collider in 2011. The event selection requires missing transverse momentum, no isolated electrons or muons, and from >=6 to >=9 jets. No evidence is found for physics beyond the Standard Model. The results are interpreted in the context of a MSUGRA/CMSSM supersymmetric model, where, for large universal scalar mass m_0, gluino masses smaller than 840 GeV are excluded at the 95% confidence level, extending previously published limits. Within a simplified model containing only a gluino octet and a neutralino, gluino masses smaller than 870 GeV are similarly excluded for neutralino masses below 100 GeV.
A search for the Standard Model Higgs boson is performed in the diphoton decay channel. The data used corresponds 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 sqrt(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.
This Letter presents a search for the Standard Model Higgs boson in the decay channel H->ZZ(*)->l+l-l'+l'-, where l,l'=e or mu, using proton-proton collisions at sqrt(s)=7 TeV recorded with the ATLAS ...detector and corresponding to an integrated luminosity of 4.8 fb^-1. The four-lepton mass distribution is compared with Standard Model background expectations to derive upper limits on the cross section of a Standard Model Higgs boson with a mass between 110 GeV and 600 GeV which exclude the mass ranges 134-156 GeV, 182-233 GeV, 256-265 GeV and 266-415 GeV at the 95% confidence level. The largest upward deviations from the background-only hypothesis are observed for Higgs boson masses of 125, 244 and 500 GeV with local significances of 2.1, 2.2 and 2.1 standard deviations, respectively. Once the look-elsewhere effect is considered, none of these excesses are significant.
In this paper, a measurement of tau polarization in W->taunu decays is presented. It is measured from the energies of the decay products in hadronic tau decays with a single final state charged ...particle. The data, corresponding to an integrated luminosity of 24 pb^-1, were collected by the ATLAS experiment at the Large Hadron Collider in 2010. The measured value of the tau polarization is Ptau = -1.06 +/- 0.04 (stat) + 0.05 (syst) - 0.07 (syst), in agreement with the Standard Model prediction, and is consistent with a physically allowed 95% CL interval -1,-0.91. Measurements of tau polarization have not previously been made at hadron colliders.
A search for top quark pair resonances in final states containing at least one electron or muon has been performed with the ATLAS experiment at the CERN Large Hadron Collider. The search uses a data ...sample corresponding to an integrated luminosity of 2.05 fb^-1, which was recorded in 2011 at a proton-proton centre-of-mass energy of 7 TeV. No evidence for a resonance is found and limits are set on the production cross-section times branching ratio to ttbar for narrow and wide resonances. For narrow Z' bosons, the observed 95% credibility level limits range from 9.3 pb to 0.95 pb for masses in the range of m_Z' = 500 GeV to m_Z' = 1300 GeV. The corresponding excluded mass region for a leptophobic topcolour Z' boson (Kaluza-Klein gluon excitation in the Randall-Sundrum model) is m_Z' < 880 GeV (m_{g_KK} < 1130 GeV).
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