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This study presents a measurement of the double-differential cross section for the Drell-Yan Z/[gamma]* → l+l- and photon-induced [gamma][gamma] → l+l- processes where l is an electron or muon. The measurement is performed for invariant masses of the lepton pairs, mll, between 116 GeV and 1500 GeV using a sample of 20.3 fb-1 of pp collisions data at centre-of-mass energy of √s = 8 TeV collected by the ATLAS detector at the LHC in 2012. The data are presented double differentially in invariant mass and absolute dilepton rapidity as well as in invariant mass and absolute pseudorapidity separation of the lepton pair. The single-differential cross section as a function of mll is also reported. The electron and muon channel measurements are combined and a total experimental precision of better than 1% is achieved at low mll. A comparison to next-to-next-to-leading order perturbative QCD predictions using several recent parton distribution functions and including next-to-leading order electroweak effects indicates the potential of the data to constrain parton distribution functions. In particular, a large impact of the data on the photon PDF is demonstrated.
Charged-hadron transverse-momentum and pseudorapidity distributions in proton-proton collisions at √s = 7 TeV are measured with the inner tracking system of the CMS detector at the LHC. The charged-hadron yield is obtained by counting the number of reconstructed hits, hit-pairs, and fully reconstructed charged-particle tracks. The combination of the three methods gives a charged-particle multiplicity per unit of pseudorapidity dN{sub ch}/d[eta].
We show that initial-state interactions contribute to the cos 2[phi] distribution in unpolarized Drell-Yan lepton pair production p p and p[bar p][yields][ell][sup+][ell][sup -] X, without suppression. The asymmetry is expressed as a product of chiral-odd distributions h[sub 1][sup[perpendicular]] (x[sub 1], p[sub[perpendicular]][sup 2]) x[bar h][sub 1][sup[perpendicular]] (x[sub 2], k[sub[perpendicular]][sup 2]), where the quark-transversity function h[sub 1][sup[perpendicular]](x, p[sub[perpendicular]][sup 2]) is the transverse momentum dependent, light-cone momentum distribution of transversely polarized quarks in an unpolarized proton. We compute this (naive) T-odd and chiral-odd distribution function and the resulting cos 2[psi]asymmetry explicitly in a quark-scalar diquark model for the proton with initial-state gluon interaction. In this model the function h[sub 1][sup[perpendicular]] (x[sub 1], p[sub[perpendicular]][sup 2]) equals the T-odd (chiral-even) Sivers effect function f[sub 1T][sup[perpendicular]](x, p[sub[perpendicular]][sup 2]). This suggests that the single-spin asymmetries in the SIDIS and the Drell-Yan process are closely related to the cos 2[phi] asymmetry of the unpolarized Drell-Yan process, since all can arise from the same underlying mechanism. This provides new insight regarding the role of quark and gluon orbital angular momentum as well as that of initial- and final-state gluon exchange interactions in hard QCD processes.
Underlying Event distributions are studied in events containing at least one charged particle jet produced in proton-proton collisions at sqrt(s) = 7 TeV. Jets are reconstructed from charged particles using the anti-Kt algorithm with radius parameter R = 0.6. The jet with the largest transverse momentum (pT) and.
The first measurement of the cross section for top-quark pair production in pp collisions at the LHC at center-of-mass energy sqrt(s)= 7 TeV has been performed using 3.1 {\pm} 0.3 inverse pb of data recorded by the CMS detector. This result utilizes the final state with two isolated, highly energetic charged leptons, large missing transverse energy, and two or more jets. Backgrounds from Drell-Yan and non-W/Z boson production are estimated from data. Eleven events are observed in the data with 2.1 {\pm} 1.0 events expected from background. The measured cross section is 194 {\pm} 72 (stat.) {\pm} 24 (syst.) {\pm} 21 (lumi.) pb, consistent with next-to-leading order predictions.
We report the first measurement of the opening angledistribution between pairs of jets produced in high-energy collisions oftransversely polarized protons. The measurement probes (Sivers)correlations between the transverse spin orientation of a proton and thetransverse momentum directions of its partons. With both beams polarized, the wide pseudorapidity ( -1 leq eta leq +2) coverage for jets permitsseparation of Sivers functions for the valence and sea regions. Theresulting asymmetries are all consistent with zero and considerablysmaller than Sivers effects observed in semi-inclusive deep inelasticscattering (SIDIS). We discuss theoretical attempts to reconcile the newresults with the sizable transverse spin effects seen in SIDIS andforward hadron production in pp collisions.