Proton-proton collisions at √ s = 7 TeV and heavy ion collisions at √ s NN = 2.76 TeV were produced by the LHC and recorded using the ATLAS experiment's trigger system in 2010. The LHC is designed with a maximum bunch crossing rate of 40 MHz and the ATLAS trigger system is designed to record approximately 200 of these per second. The trigger system selects events by rapidly identifying signatures of muon, electron, photon, tau lepton, jet, and B meson candidates, as well as using global event signatures, such as missing transverse energy. An overview of the ATLAS trigger system, the evolution of the system during 2010 and the performance of the trigger system components and selections based on the 2010 collision data are shown. A brief outline of plans for the trigger system in 2011 is presented.

The production of K 0 S and à hadrons is studied in pp collision data at ffiffi ffi s p ¼ 0:9 and 7 TeV collected with the ATLAS detector at the LHC using a minimum-bias trigger. The observed distributions of transverse momentum, rapidity, and multiplicity are corrected to hadron level in a model-independent way within well-defined phase-space regions. The distribution of the production ratio of " à to à baryons is also measured. The results are compared with various Monte Carlo simulation models. Although most of these models agree with data to within 15% in the K 0 S distributions, substantial disagreements are found in the à distributions of transverse momentum.

A measurement of spin correlation in t " t production is reported using data collected with the ATLAS detector at the LHC, corresponding to an integrated luminosity of 2:1 fb À1 . 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 ¼ 0:40 þ0:09 À0:08 , in agreement with the next-to-leading-order standard model prediction. The hypothesis of zero spin correlation is excluded at 5.1 standard deviations.

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 À1 of proton-proton collisions at ffiffi ffi s p ¼ 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. This Letter presents the first ATLAS search for the Higgs boson decay, h 0 ! v v , to two identical neutral particles ( v ) that have a displaced decay to fermionantifermion pairs. As a benchmark, we take a HV model [9] in which the SM is weakly coupled, by a heavy communicator particle, to a hidden sector that includes a pseudoscalar, the v . Because of the helicity suppression of pseudoscalar decays to low-mass f " f pairs, the v decays predominantly to heavy fermions, b " b, c " c, and þ À in the ratio 85:5:8%. The weak coupling between the two sectors leads the v to have a long lifetime. Other, non-HV, models with the identical signature, where the v is replaced with another weakly interacting scalar or pseudoscalar particle, are discussed in Refs. [4,10]. Both Tevatron experiments, CDF and D0, performed similar searches for displaced decays in their respective tracking volumes, which limited the proper decay length range they could explore to a few hundred millimeters [11,12].In many of these beyond-the-SM scenarios, the lifetime of the neutral states is not specified and can have a very large range. The current search covers a range of expected proper decay lengths extending to about 20 m by exploiting the size and layout of the ATLAS muon spectrometer.

Using data recorded in 2011 with the ATLAS detector at the Large Hadron Collider, a search for evidence of extra spatial dimensions has been performed through an analysis of the diphoton final state. The analysis uses data corresponding to an integrated luminosity of 2.12 fb−12.12 fb−1 of √s=7 TeV proton–proton collisions. The diphoton invariant mass (mγγmγγ) spectrum is observed to be in good agreement with the expected Standard Model background. In the large extra dimension scenario of Arkani-Hamed, Dimopoulos and Dvali, the results provide 95% CL lower limits on the fundamental Planck scale between 2.27 and 3.53 TeV, depending on the number of extra dimensions and the theoretical formalism used. The results also set 95% CL lower limits on the lightest Randall–Sundrum graviton mass of between 0.79 and 1.85 TeV, for values of the dimensionless coupling k/MPl varying from 0.01 to 0.1. Combining with previously published ATLAS results from the dielectron and dimuon final states, the 95% CL lower limit on the Randall–Sundrum graviton mass for k/MPl=0.01 (0.1) is 0.80 (1.95) TeV