Abstract
Additional jet activity in dijet events is measured using {pp} collisions at ATLAS at a centre-of-mass energy of 7,mathrm{TeV}, for jets reconstructed using the {mathrm{anti}hbox {-}{k_t}} algorithm with radius parameter {R=0.6}. This is done using variables such as the fraction of dijet events without an additional jet in the rapidity interval bounded by the dijet subsystem and correlations between the azimuthal angles of the dijet s. They are presented, both with and without a veto on additional jet activity in the rapidity interval, as a function of the scalar average of the transverse momenta of the dijet s and of the rapidity interval size. The double differential dijet cross section is also measured as a function of the interval size and the azimuthal angle between the dijet s. These variables probe differences in the approach to resummation of large logarithms when performing QCD calculations. The data are compared to powheg, interfaced to the pythia 8 and herwig parton shower generators, as well as to hej with and without interfacing it to the ariadne parton shower generator. None of the theoretical predictions agree with the data across the full phase-space considered; however, powheg+pythia 8 and hej+ariadne are found to provide the best agreement with the data. These measurements use the full data sample collected with the ATLAS detector in 7,mathrm{TeV} {pp} collisions at the LHC and correspond to integrated luminosities of 36.1,mathrm{pb}^{-1} and 4.5,mathrm{fb}^{-1} for data collected during 2010 and 2011, respectively.
Highlights
When studying these phase-space regions, a interesting observable is the gap fraction, f(Q0), defined as f(Q0) = σjj (Q0) /σjj where σjj is the inclusive dijet cross section and σjj (Q0) is the cross section for dijet production in the absence of jets with transverse momentum greater than Q0 in the rapidity interval bounded by the dijet system
This paper presents measurements of the gap fraction and the mean number of jets in the rapidity interval as functions of both the dijet rapidity separation and the scalar average of the transverse momenta of the dijets
Theoretical predictions based on perturbative QCD are tested by studying dijet events in extreme regions of phase space
Summary
Previous measurements have been made of dijet production for which a strict veto, of order of QCD, was imposed on the emission of additional radiation in the inter-jet region, so-called “rapidity gap” events, at HERA [28,29,30] and at the Tevatron [31,32,33,34,35]. This paper presents measurements of the gap fraction and the mean number of jets in the rapidity interval as functions of both the dijet rapidity separation and the scalar average of the transverse momenta of the dijets. Measurements of the first azimuthal angular moment, the ratio of the first two moments and the double-differential cross sections as functions of φ and y are presented, both for an inclusive dijet sample and for events where a jet veto is imposed.
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