Measurement of \(t\bar{t}\) production with a veto on additional central jet activity in pp collisions at \(\sqrt{s}=7\) TeV using the ATLAS detector

The ATLAS Collaboration 0 0 CERN, 1211 Geneva 23, Switzerland A measurement of the jet activity in t t events produced in proton-proton collisions at a centre-of-mass energy of 7 TeV is presented, using 2.05 fb1 of integrated luminosity collected by the ATLAS detector at the Large Hadron Collider. The t t events are selected in the dilepton decay channel with two identified b-jets from the top quark decays. Events are vetoed if they contain an additional jet with transverse momentum above a threshold in a central rapidity interval. The fraction of events surviving the jet veto is presented as a function of this threshold for four different central rapidity interval definitions. An alternate measurement is also performed, in which events are vetoed if the scalar transverse momentum sum of the additional jets in each rapidity interval is above a threshold. In both measurements, the data are corrected for detector effects and compared to the theoretical models implemented in MC@NLO, POWHEG, ALPGEN and SHERPA. The experimental uncertainties are often smaller than the spread of theoretical predictions, allowing deviations between data and theory to be observed in some regions of phase space. - Measurements of the top quark provide an important test of the Standard Model (SM) and any observed deviation from the SM predictions could indicate the presence of new physics. However, many top quark measurements have large uncertainties that arise from the theoretical description of quark and gluon radiation in the standard Monte Carlo (MC) event generators. Recent measurements that are affected by such modelling uncertainties include the t t production cross-section [14], the spin correlations in t t events [5], the charge asymmetry [6, 7] and the top quark mass [4]. In addition, a significant disagreement between data and the prediction from MC@NLO [8, 9] was observed by the D0 Collaboration in the transverse momentum distribution of the t t system [10]. This disagreement obscures the interpretation of the observed forward-backward asymmetry in terms of a deviation from SM predictions. Measurements sensitive to the theoretical description of quark and gluon radiation in events containing a t t final state are therefore needed in order to constrain the modelling and reduce the impact on future experimental measurements. In this article, a jet veto is used to quantify the jet activity that arises from quark and gluon radiation produced in association with the t t system. The events are selected in the dilepton decay channel so that the additional jets can be easily distinguished from the t t decay products (two leptons and two jets originating from b-quarks). The variable of interest is the gap fraction, defined as f (Q0) = where N is the number of selected t t events and n(Q0) is the subset of these events that do not contain an additional jet with transverse momentum, pT, above a threshold, Q0, in a central rapidity1 interval. The minimum jet pT used in the measurement is 25 GeV. The measurement is corrected for detector effects and presented in a fiducial region. The gap fraction can then be written as f (Q0) = where is the fiducial cross section for inclusive t t production and (Q0) is the fiducial cross section for t t events produced in the absence of an additional jet with pT > Q0 in 1ATLAS uses a right-handed coordinate system with the z-axis along the beam line. Cylindrical coordinates (r , ) are used in the transverse plane, being the azimuthal angle. Pseudorapidity is defined in terms of the polar angle as = ln[tan(/2)]. Rapidity is defined as y = 0.5 ln[(E + pz)/(E pz)] where E denotes the energy and pz is the component of the momentum along the beam direction. Transverse momentum and energy are defined as pT = p sin and ET = E sin , respectively. the rapidity interval. The gap fraction is measured for multiple values of Q0 and for four jet rapidity intervals: |y| < 0.8, 0.8 |y| < 1.5, 1.5 |y| < 2.1 and |y| < 2.1. The veto criterion can be extended to probe jet activity beyond the leading additional jet. An alternate definition of the gap fraction is used in this case, f (Qsum) = where n(Qsum) is the number of t t events, and (Qsum) is the cross section, in which the scalar transverse momentum sum of the additional jets in the rapidity interval is less than Qsum. The gap fraction defined using Q0 is mainly sensitive to the leading-pT emission accompanying the t t system, whereas the gap fraction defined using Qsum is sensitive to all hard emissions accompanying the t t system. Many of the experimental systematic uncertainties cancel in the ratio, as observed in the ATLAS measurement of the gap fraction in dijet events [11]. The data are therefore expected to constrain the modelling of quark and gluon radiation in t t events and provide useful information about the general theoretical description of jet vetoes, which have been proposed as a tool to enhance new physics signals [12 14], and t (...truncated)