Physics Letters B 709 (2012) 28–49 Contents lists available at SciVerse ScienceDirect Physics Letters B www.elsevier.com/locate/physletb Measurement of the charge asymmetry in top-quark pair production in proton–proton collisions at √ s = 7 TeV ✩ .CMS Collaboration ⋆ CERN, Switzerland article info abstract Article history: Received 21 December 2011 Received in revised form 31 January 2012 Accepted 31 January 2012 Available online 3 February 2012 Editor: M. Doser Keywords: CMS Physics Top quark The difference in angular distributions between top quarks and antiquarks, commonly referred to as the charge asymmetry, is measured in pp collisions at the LHC with the CMS experiment. The data sample corresponds to an integrated luminosity of 1.09 fb −1 at a centre-of-mass energy of 7 TeV. Top-quark pairs are selected in the final state with an electron or muon and four or more jets. At least one jet is identified as originating from b-quark hadronization. The charge asymmetry is measured in two variables, one based on the pseudorapidities (η) of the top quarks and the other on their rapidities ( y). The results A η C =−0.017 ± 0.032 (stat.) +0.025 −0.036 (syst.) and A y C =−0.013 ± 0.028 (stat.) +0.029 −0.031 (syst.) are consistent within uncertainties with the standard-model predictions.  2012 CERN. Published by Elsevier B.V. All rights reserved. 1. Introduction The top quark is the only fundamental fermion with a mass on the order of the scale of electroweak symmetry breaking, and may therefore play a special role in physics beyond the standard model (BSM). In some BSM theories, top-quark pairs can be pro- duced through the exchange of yet unknown heavy particles, in addition to the production through quark–antiquark annihilation and gluon–gluon fusion. Possible candidates include axigluons [1, 2],Z ′ bosons [3], and Kaluza–Klein excitations of gluons [4,5]. Such new particles can appear as resonances in the t ¯ t invariant mass spectrum in s-channel production of top-quark pairs. If these hypo- thetical particles are exchanged in the t or u channels, alternative approaches are needed to search for new top-quark production modes [6]. One property of t ¯ t production that can be sensitive to the presence of such additional contributions is the difference in angular distributions of top quarks and antiquarks, commonly re- ferred to as the charge asymmetry. In the standard model (SM), a small charge asymmetry in t ¯ t production through quark–antiquark annihilation appears in QCD calculations at next-to-leading order (NLO) [7,8]. The interference between the Born diagram and the box diagram, as well as be- tween initial- and final-state radiation, correlates the flight direc- tions of the top quarks and antiquarks to the directions of motion of the initial quarks and antiquarks, respectively. The asymmetric initial state of proton–antiproton collisions leads to an observ- ✩ © CERN for the benefit of the CMS Collaboration. ⋆ E-mail address: cms-publication-committee-chair@cern.ch. able forward–backward asymmetry at the Tevatron, where the top quarks are emitted preferentially along the direction of motion of the incoming protons and the top antiquarks along the direction of the antiprotons. This asymmetry is observable in the difference in rapidity ( y) of top quarks and antiquarks, y t − y ¯ t . Recent measure- ments [9,10] by the CDF and D0 Collaborations report asymmetries that are about two standard deviations larger than the value of about 0.08 [7,8,11–13] predicted in the SM. At high t ¯ t invariant mass ( M t¯ t > 450 GeV/c 2 ), the CDF Collaboration finds an even larger asymmetry relative to the SM prediction [9], while the D0 Collaboration does not observe a significant mass dependence of the asymmetry. These results have led to speculations that the large asymmetry might be generated by additional axial couplings of the gluon [14] or by heavy particles with unequal vector and axial-vector couplings to top quarks and antiquarks [15–28]. Owing to the symmetric initial state of proton–proton colli- sions at the Large Hadron Collider (LHC), the charge asymmetry does not manifest itself as a forward–backward asymmetry; the rapidity distributions of top quarks and antiquarks are symmetri- cal around y = 0. However, since the quarks in the initial state are mainly valence quarks, while the antiquarks are always sea quarks, the larger average momentum fraction of quarks leads to an excess of top quarks produced in the forward directions. The rapidity distribution of top quarks in the SM is therefore broader than that of the more centrally produced top antiquarks. The same effect is visible in the purely geometrically defined pseudorapid- ity η =− ln(tan θ/2), where θ is the polar angle relative to the counterclockwise beam axis. The charge asymmetry can be ob- served through the difference in the absolute values of the pseu- dorapidities of top quarks and antiquarks, |η|=|η t |−|η ¯ t | [29]. 0370-2693/  2012 CERN. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.physletb.2012.01.078