By using the ATLAS detector, observations have been made of a centrality-dependent dijet asymmetry in the collisions of lead ions at the Large Hadron Collider. In a sample of lead-lead events with a per-nucleon center of mass energy of 2.76 TeV, selected with a minimum bias trigger, jets are reconstructed in fine-grained, longitudinally segmented electromagnetic and hadronic calorimeters. The transverse energies of dijets in opposite hemispheres are observed to become systematically more unbalanced with increasing event centrality leading to a large number of events which contain highly asymmetric dijets. This is the first observation of an enhancement of events with such large dijet asymmetries, not observed in proton-proton collisions, which may point to an interpretation in terms of strong jet energy loss in a hot, dense medium.

Using 1:8 fb À1 of pp collisions at a center-of-mass energy of 7 TeV recorded by the ATLAS detector at the Large Hadron Collider, we present measurements of the production cross sections of Çð1S; 2S; 3SÞ mesons. Ç mesons are reconstructed using the dimuon decay mode. Total production cross sections for p T < 70 GeV and in the rapidity interval jy Ç j <

The production of W bosons in association with two jets in proton-proton collisions at a centre-of-mass energy of √ s = 7 TeV has been analysed for the presence of double-parton interactions using data corresponding to an integrated luminosity of 36 pb −1 , collected with the ATLAS detector at the Large Hadron Collider. The fraction of events arising from double-parton interactions, f (D) DP , has been measured through the p T balance between the two jets and amounts to f (D) DP = 0.08 ± 0.01 (stat.) ± 0.02 (sys.) for jets with transverse momentum p T > 20 GeV and rapidity |y| < 2.8. This corresponds to a measurement of the effective area parameter for hard double-parton interactions of σ eff = 15 ± 3 (stat.) +5 −3 (sys.) mb.

Measurement of the production cross section of prompt J/ψ mesons in association with a W ± boson in pp collisions at √ s = 7 TeV with the ATLAS detectorThe ATLAS collaboration E-mail: atlas.publications@cern.ch Abstract: The process pp → W ± J/ψ 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 √ s = 7 TeV pp collisions at the LHC, the first observation is made of the production of W ± + prompt J/ψ events in hadronic collisions, using W ± → µν µ and J/ψ → µ + µ − . A yield of 27.4 +7.5 −6.5 W ± + prompt J/ψ events is observed, with a statistical significance of 5.1σ. 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. The ATLAS collaboration 20 IntroductionStudy of the production of a W boson in association with a prompt J/ψ meson offers new tests of Quantum Chromodynamics (QCD) at the perturbative/non-perturbative boundary as well as developing the framework for future probes of the Higgs sector and beyond-thestandard-model searches in such final states. Perturbative calculations of heavy quarkonium production in hadronic collisions distinguish between terms that produce a heavy quark system (QQ) in a colour-singlet (CS) or a colour-octet (CO) state. The relative importance of these terms for inclusive J/ψ production is a subject of debate [1][2][3][4][5][6][7]. In the case of prompt J/ψ production in association with a W ± boson, the relative contributions of CS and CO processes differ from the inclusive process. Some theoretical studies [8,9] suggest W ± + prompt J/ψ production should be dominated by colour-octet processes, and thus be a distinctive test of the non-relativistic QCD (NRQCD) framework [10,11]. In contrast, recent work [12] suggests that in 7 TeV pp collisions, CO and CS (in particular, electromagnetic W ± γ * →W ± J/ψ) contributions to the W ± + prompt J/ψ cross section are comparable. Measurements of the production cross sections can help distinguish between these models. A search for the related processes W ± +Υ(1S) and Z + Υ(1S) performed by the CDF experiment saw no excess of events above the expected background and set upper limits on the production rate [13].Observation and measurement of W ± + prompt J/ψ production for the first time represents a step in our understanding toward measurements of the Higgs boson in rare quarkonia and associated vector boson decay modes, first proposed in ref. [14]. Recent -1 - JHEP04(2014)172phenomenological studies [15] have emphasised the value of these rare decay modes to provide a unique probe of the Higgs boson charm couplings. Such final states can also be sensitive probes of beyond-the-standard-model (BSM) frameworks. The presence of an anomalous rate of W ± /Z + prompt J/ψ/Υ associated production over standard model predictions can, for example, be an indication of a sig...

This paper presents a summary of beam-induced backgrounds observed in the ATLAS detector and discusses methods to tag and remove background contaminated events in data. Triggerrate based monitoring of beam-related backgrounds is presented. The correlations of backgrounds with machine conditions, such as residual pressure in the beam-pipe, are discussed. Results from dedicated beam-background simulations are shown, and their qualitative agreement with data is evaluated. Data taken during the passage of unpaired, i.e. non-colliding, proton bunches is used to obtain background-enriched data samples. These are used to identify characteristic features of beam-induced backgrounds, which then are exploited to develop dedicated background tagging tools. These tools, based on observables in the Pixel detector, the muon spectrometer and the calorimeters, are described in detail and their efficiencies are evaluated. Finally an example of an application of these techniques to a monojet analysis is given, which demonstrates the importance of such event cleaning techniques for some new physics searches.