Results are presented of searches for the production of supersymmetric particles decaying into final states with missing transverse momentum and exactly two isolated leptons in \(\sqrt {s}=7~\mbox{TeV}\) proton–proton collisions at the Large Hadron Collider. Search strategies requiring lepton pairs with identical-sign or opposite-sign electric charges are described. In a data sample corresponding to an integrated luminosity of 35 pb−1 collected with the ATLAS detector, no significant excesses are observed. Based on specific benchmark models, limits are placed on the squark mass between 450 and 690 GeV for squarks approximately degenerate in mass with gluinos, depending on the supersymmetric mass hierarchy considered.

A search is presented for a high mass neutral particle that decays directly to the e ± μ ∓ final state. The data sample was recorded by the ATLAS detector in \(\sqrt{s}=7\mbox{~TeV}\) pp collisions at the LHC from March to June 2011 and corresponds to an integrated luminosity of 1.07 fb−1. The data are found to be consistent with the Standard Model background. The high e ± μ ∓ mass region is used to set 95% confidence level upper limits on the production of two possible new physics processes: tau sneutrinos in an R-parity violating supersymmetric model and Z′-like vector bosons in a lepton flavor violating model.

A search for pair-produced scalar particles decaying to a four-jet final state is presented. The analysis is performed using an integrated luminosity of 34 pb−1 recorded by the ATLAS detector in 2010. No deviation from the Standard Model is observed. For a scalar mass of 100 GeV (190 GeV) the limit on the scalar gluon pair production cross section at 95% confidence level is 1 nb (0.28 nb). When these results are interpreted as mass limits, scalar-gluons (hyperpions) with masses of 100 to 185 GeV (100 to 155 GeV) are excluded at 95% confidence level with the exception of a mass window of width about 5 GeV (15 GeV) around 140 GeV.

First measurements of the W → ℓν and Z/γ * → ℓℓ (ℓ = e, μ) production cross sections in proton-proton collisions at \( \sqrt {s} = 7\;{\text{TeV}} \) are presented using data recorded by the ATLAS experiment at the LHC. The results are based on 2250 W → ℓν and 179 Z/γ * → ℓℓ candidate events selected from a data set corresponding to an integrated luminosity of approximately 320 nb. The measured total W and Z/γ ∗-boson production cross sections times the respective leptonic branching ratios for the combined electron and muon channels are \( \sigma_W^{\text{tot}} \). BR(W → ℓν) = 9.96 ± 0.23(stat) ± 0.50(syst) ± 1.10(lumi) nb and \( \sigma_{{{Z} \left/ {\gamma } \right.}}^{\text{tot}} \) BR(Z/γ ∗ → ℓℓ) = 0.82 ± 0.06 (stat) ± 0.05 (syst) ± 0.09(lumi) nb (within the invariant mass window 66 < m ℓℓ < 116GeV). The W/Z cross-section ratio is measured to be 11.7 ± 0.9(stat) ± 0.4(syst). In addition, measurements of the W + and W − production cross sections and of the lepton charge asymmetry are reported. Theoretical predictions based on NNLO QCD calculations are found to agree with the measurements.

In 2004, a combined system test was performed in the H8 beam line at the CERN SPS with a setup reproducing the geometry of sectors of the ATLAS Muon Spectrometer, formed by three stations of Monitored Drift Tubes (MDT). The full ATLAS analysis chain was used to obtain the results presented in this paper. The basic design performances of the Muon Spectrometer were verified. The stability of MDT calibration constants, the alignment system using optical devices and high energy tracks, as well as the intrinsic sagitta resolution of the Muon Spectrometer were studied and found to agree with expectations. The reconstruction of muon tracks using the combined information from both the Inner Detector and the Muon Spectrometer are also presented.

The diiractive production of 0 (770) mesons in muon-proton interactions is studied in the kinematic region 0.15 GeV 2 < Q 2 < 20 GeV 2 and 20 GeV < < 420 GeV. The data were obtained in the Fermilab xed-target experiment E665 with primary muons of 470 GeV energy. Results are presented on the Q 2 , x and dependence of the cross section, on the shape of the + ? mass spectrum, on the slope of the diiraction peak and on the production and decay angular distributions of the 0 (770). The cross section for diiractive production of 0 by virtual photons on protons depends mainly on Q 2. At xed Q 2 , no signiicant dependence on x or is observed. The extrapolation to Q 2 = 0 yields a photoproduction cross section of (10:30 0:33) b. The slope of the t 0 distribution has a value of (7:00:2) GeV ?2 , with a tendency to decrease as Q 2 increases. The production and decay angular distributions of the 0 depend strongly on Q 2 and are consistent with s-channel helicity conservation. The ratio R = L == T deduced from the decay angular distributions rises strongly with Q 2 , passing the value of 1 at Q 2 2 GeV 2 .

For the muon spectrometer of the ATLAS detector at the Large Hadron Collider (LHC), large drift chambers consisting of 6-8 layers of pressurized drift tubes are used for precision tracking covering an active area of 5000 M-2 in the toroidal field of superconducting air-core magnets. The chambers have to provide a spatial resolution of 41 mum with Ar:CO2 (93:7) gas mixture at an absolute pressure of 3 bar and gas gain of 2 x 10(4). The environment in which the chambers will be operated is characterized by high neutron and gamma background with counting rates of up to 100 s(-1) cm(-2). The resolution and efficiency of a chamber from the serial production for ATLAS has been investigated in a 100 GeV muon beam at photon irradiation rates as expected during LHC operation. A silicon strip detector telescope was used as external reference in the beam. The spatial resolution of a chamber is degraded by 4 mum at the highest backround rate. The detection efficiency of the drift tubes is unchanged under irradiation. A tracking efficiency of 98% at the highest rates has been demonstrated. (C) 2003 Published by Elsevier B.V.

Precision drift tube chambers with a sense wire positioning accuracy of better than 20 μm are under construction for the ATLAS muon spectrometer. 70% of the 88 large chambers for the outermost layer of the central part of the spectrometer have been assembled. Measurements during chamber construction of the positions of the sense wires and of the sensors for the optical alignment monitoring system demonstrate that the requirements for the mechanical precision of the chambers are fulfilled.