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The measurement of missing transverse momentum in the ATLAS detector, described in this paper, makes use of the full event reconstruction and a calibration based on reconstructed physics objects. The performance of the missing transverse momentum reconstruction is evaluated using data collected in pp collisions at a centre-of-mass energy of 7 TeV in 2010. Minimum bias events and events with jets of hadrons are used from data samples corresponding to an integrated luminosity of about 0.3 nb 1 and 600 nb 1 respectively, together with events containing a Z boson decaying to two leptons (electrons or muons) or aW boson decaying to a lepton (electron or muon) and a neutrino, from a data sample corresponding to an integrated luminosity of about 36 pb 1 . An estimate of the systematic uncertainty on the missing transverse momentum scale is presented.

A search for doubly charged Higgs bosons decaying to pairs of electrons and/or muons is presented. The search is performed using a data sample corresponding to an integrated luminosity of 4.7 fb −1 of pp collisions at √s = 7 TeV collected by the ATLAS detector at the LHC. Pairs of prompt, isolated, high-pT leptons with the same electric charge (e ± e ± , e ± μ ± , μ ± μ ± ) are selected, and their invariant mass distribution is searched for a narrow resonance. No significant excess over Standard Model background expectations is observed, and limits are placed on the cross section times branching ratio for pair production of doubly charged Higgs bosons. The masses of doubly charged Higgs bosons are constrained depending on the branching ratio into these leptonic final states. Assuming pair production, coupling to left-handed fermions, and a branching ratio of 100% for each final state, masses below 409 GeV, 375 GeV, and 398 GeV are excluded for e ± e ± , e ± μ ± , and μ ± μ ± , respectively.

The process pp -> W (+/-) J/psi provides a powerful probe of the production mechanism of charmonium in hadronic collisions, and is also sensitive to multiple parton interactions in the colliding protons. Using the 2011 ATLAS dataset of 4.5 fb(-1) of root s= 7 TeV pp collisions at the LHC, the first observation is made of the production of W (+/-) + prompt J/psi events in hadronic collisions, using W (+/-) -> mu nu(mu) and J/psi -> mu(+)mu(-) center dot A yield of W (+/-) + prompt J/psi events is observed, with a statistical significance of 5.1 sigma. The production rate as a ratio to the inclusive W (+/-) boson production rate is measured, and the double parton scattering contribution to the cross section is estimated.

A measurement of the jet activity in tt̄ events produced in proton–proton collisions at a centre-of-mass energy of 7 TeV is presented, using 2.05 fb−1 of integrated luminosity collected by the ATLAS detector at the Large Hadron Collider. The tt̄ events are selected in the dilepton decay channel with two identified b-jets from the top quark decays. Events are vetoed if they contain an additional jet with transverse momentum above a threshold in a central rapidity interval. The fraction of events surviving the jet veto is presented as a function of this threshold for four different central rapidity interval definitions. An alternate measurement is also performed, in which events are vetoed if the scalar transverse momentum sum of the additional jets in each rapidity interval is above a threshold. In both measurements, the data are corrected for detector effects and compared to the theoretical models implemented in MC@NLO, Powheg, Alpgen and Sherpa. The experimental uncertainties are often smaller than the spread of theoretical predictions, allowing deviations between data and theory to be observed in some regions of phase space.

The results of a search for the production of second generation scalar leptoquarks are presented for final states consisting of either two muons and at least two jets or a muon plus missing transverse momentum and at least two jets. A total of 1. 03 fb -1 integrated luminosity of proton-proton collision data produced by the Large Hadron Collider at √s = 7 TeV and recorded by the ATLAS detector is used for the search. The event yields in the signal regions are found to be consistent with the Standard Model background expectations. The production of second generation leptoquarks is excluded for a leptoquark mass m LQ <594 (685) GeV at 95 % confidence level, for a branching ratio of 0. 5 (1. 0) for leptoquark decay to a muon and a quark.