Search for Magnetic Monopoles in s√=7 TeV pp Collisions with the ATLAS Detector

This Letter presents a search for magnetic monopoles with the ATLAS detector at the CERN Large Hadron Collider using an integrated luminosity of 2.0  fb−1 of pp collisions recorded at a center-of-mass energy of s√=7  TeV. No event is found in the signal region, leading to an upper limit on the production cross section at 95% confidence level of 1.6/ϵ  fb for Dirac magnetic monopoles with the minimum unit magnetic charge and with mass between 200 GeV and 1500 GeV, where ϵ is the monopole reconstruction efficiency. The efficiency ϵ is high and uniform in the fiducial region given by pseudorapidity |η|<1.37 and transverse kinetic energy 600–700<Ekinsinθ<1400  GeV. The minimum value of 700 GeV is for monopoles of mass 200 GeV, whereas the minimum value of 600 GeV is applicable for higher mass monopoles. Therefore, the upper limit on the production cross section at 95% confidence level is 2 fb in this fiducial region. Assuming the kinematic distributions from Drell-Yan pair production of spin-1/2 Dirac magnetic monopoles, the efficiency is in the range 1%–10%, leading to an upper limit on the cross section at 95% confidence level that varies from 145 fb to 16 fb for monopoles with mass between 200 GeV and 1200 GeV. This limit is weaker than the fiducial limit because most of these monopoles lie outside the fiducial region

Search for a Light Higgs Boson Decaying to Long-Lived Weakly Interacting Particles in Proton-Proton Collisions at √s = 7 TeV with the ATLAS Detector

A search for the decay of a light Higgs boson (120–140 GeV) to a pair of weakly interacting, long-lived particles in 1.94  fb^(-1) of proton-proton collisions at √s=7  TeV recorded in 2011 by the ATLAS detector is presented. The search strategy requires that both long-lived particles decay inside the muon spectrometer. No excess of events is observed above the expected background and limits on the Higgs boson production times branching ratio to weakly interacting, long-lived particles are derived as a function of the particle proper decay length

Observation of Spin Correlation in tt Events from pp Collisions at √s=7 TeV Using the ATLAS Detector

A measurement of spin correlation in tt production is reported using data collected with the ATLAS detector at the LHC, corresponding to an integrated luminosity of 2.1  fb^(-1). Candidate events are selected in the dilepton topology with large missing transverse energy and at least two jets. The difference in azimuthal angle between the two charged leptons in the laboratory frame is used to extract the correlation between the top and antitop quark spins. In the helicity basis the measured degree of correlation corresponds to A_(helicity)=0.40_(-0.08)+^(0.09), in agreement with the next-to-leading-order standard model prediction. The hypothesis of zero spin correlation is excluded at 5.1 standard deviations

Search for tb Resonances in Proton-Proton Collisions at s√=7 TeV with the ATLAS Detector

This Letter presents a search for tb resonances in 1.04  fb−1 of LHC proton-proton collision data collected by the ATLAS detector at a center-of-mass energy of 7 TeV. Events with a lepton, missing transverse momentum, and two jets are selected and the invariant mass of the corresponding final state is reconstructed. The search exploits the shape of the tb invariant mass distribution compared to the expected standard model backgrounds. The model of a right-handed W′R with standard model-like couplings is chosen as the benchmark model for this search. No statistically significant excess of events is observed in the data, and upper limits on the cross section times the branching ratio of W′R resonances at 95% C.L. lie in the range of 6.1–1.0 pb for W′R masses ranging from 0.5 to 2.0 TeV. These limits are translated into a lower bound on the allowed right-handed W′R mass, giving mW′R>1.13  TeV at 95% C.L

Search for Pair Production of a New b' Quark that Decays into a Z Boson and a Bottom Quark with the ATLAS Detector

A search is reported for the pair production of a new quark b′ with at least one b′ decaying to a Z boson and a bottom quark. The data, corresponding to 2.0  fb^(-1) of integrated luminosity, were collected from pp collisions at √s=7  TeV with the ATLAS detector at the CERN Large Hadron Collider. Using events with a b-tagged jet and a Z boson reconstructed from opposite-charge electrons, the mass distribution of large transverse momentum b′ candidates is tested for an enhancement. No evidence for a b′ signal is detected in the observed mass distribution, resulting in the exclusion at a 95% confidence level of b′ quarks with masses m_b′<400  GeV that decay entirely via b′→Z+b. In the case of a vectorlike singlet b′ mixing solely with the third standard model generation, masses m_b′<358  GeV are excluded

Observation of a New X_b State in Radiative Transitions to Y(1S) and Y(2S) at ATLAS

The χ_b(nP) quarkonium states are produced in proton-proton collisions at the Large Hadron Collider at √s=7  TeV and recorded by the ATLAS detector. Using a data sample corresponding to an integrated luminosity of 4.4  fb^(-1), these states are reconstructed through their radiative decays to Υ(1S,2S) with Υ→μ^+μ^-. In addition to the mass peaks corresponding to the decay modes χ_b(1P,2P)→Υ(1S)γ, a new structure centered at a mass of 10.530±0.005(stat)±0.009(syst)  GeV is also observed, in both the Υ(1S)γ and Υ(2S)γ decay modes. This structure is interpreted as the χ_b(3P) system

Search for Scalar Bottom Quark Pair Production with the ATLAS Detector in pp Collisions at √s = 7 TeV

The results of a search for pair production of the scalar partners of bottom quarks in 2.05  fb^(-1) of pp collisions at √s=7  TeV using the ATLAS experiment are reported. Scalar bottom quarks are searched for in events with large missing transverse momentum and two jets in the final state, where both jets are identified as originating from a bottom quark. In an R-parity conserving minimal supersymmetric scenario, assuming that the scalar bottom quark decays exclusively into a bottom quark and a neutralino, 95% confidence-level upper limits are obtained in the b_1-χ1_1^0 mass plane such that for neutralino masses below 60 GeV scalar bottom masses up to 390 GeV are excluded

Search for tb Resonances in Proton-Proton Collisions at √s = 7 TeV with the ATLAS Detector

This Letter presents a search for tb resonances in 1.04  fb^(-1) of LHC proton-proton collision data collected by the ATLAS detector at a center-of-mass energy of 7 TeV. Events with a lepton, missing transverse momentum, and two jets are selected and the invariant mass of the corresponding final state is reconstructed. The search exploits the shape of the tb invariant mass distribution compared to the expected standard model backgrounds. The model of a right-handed W'_R with standard model-like couplings is chosen as the benchmark model for this search. No statistically significant excess of events is observed in the data, and upper limits on the cross section times the branching ratio of W'_R resonances at 95% C.L. lie in the range of 6.1–1.0 pb for W'_R masses ranging from 0.5 to 2.0 TeV. These limits are translated into a lower bound on the allowed right-handed W'_R mass, giving m_(W'R)>1.13  TeV at 95% C.L