Measurements of inclusive jet production are performed in pp and Pb+Pb collisions at √(s)NN=2.76  TeV with the ATLAS detector at the LHC, corresponding to integrated luminosities of 4.0 and 0.14  nb(-1), respectively. The jets are identified with the anti-k(t) algorithm with R=0.4, and the spectra are measured over the kinematic range of jet transverse momentum 32

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Measurements are presented of the properties of high transverse momentum jets, produced in protonproton collisions at a center-of-mass energy of ffiffi ffi s p ¼ 7 TeV. The data correspond to an integrated luminosity of 35 pb À1 and were collected with the ATLAS detector in 2010. Jet mass, width, eccentricity, planar flow and angularity are measured for jets reconstructed using the anti-k t algorithm with distance parameters R ¼ 0:6 and 1.0, with transverse momentum p T > 300 GeV and pseudorapidity jj < 2. The measurements are compared to the expectations of Monte Carlo generators that match leading-logarithmic parton showers to leading-order, or next-to-leading-order, matrix elements. The generators describe the general features of the jets, although discrepancies are observed in some distributions.

Measurements of the centrality and rapidity dependence of inclusive jet production in sNN=5.02 TeV proton–lead ( p+Pb ) collisions and the jet cross-section in s=2.76 TeV proton–proton collisions are presented. These quantities are measured in datasets corresponding to an integrated luminosity of 27.8 nb −1 and 4.0 pb −1 , respectively, recorded with the ATLAS detector at the Large Hadron Collider in 2013. The p+Pb collision centrality was characterised using the total transverse energy measured in the pseudorapidity interval −4.9<η<−3.2 in the direction of the lead beam. Results are presented for the double-differential per-collision yields as a function of jet rapidity and transverse momentum ( pT ) for minimum-bias and centrality-selected p+Pb collisions, and are compared to the jet rate from the geometric expectation. The total jet yield in minimum-bias events is slightly enhanced above the expectation in a pT -dependent manner but is consistent with the expectation within uncertainties. The ratios of jet spectra from different centrality selections show a strong modification of jet production at all pT at forward rapidities and for large pT at mid-rapidity, which manifests as a suppression of the jet yield in central events and an enhancement in peripheral events. These effects imply that the factorisation between hard and soft processes is violated at an unexpected level in proton–nucleus collisions. Furthermore, the modifications at forward rapidities are found to be a function of the total jet energy only, implying that the violations may have a simple dependence on the hard parton–parton kinematics

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...