Measurement of the jet fragmentation function and transverse profile in proton–proton collisions at a center-of-mass energy of 7 TeV with the ATLAS detector

Abstract

The jet fragmentation function and transverse profile for jets with 25 GeV < p T jet < 500 GeV and |η jet | < 1.2 produced in proton-proton collisions with a center-ofmass energy of 7 TeV are presented. The measurement is performed using data with an integrated luminosity of 36 pb −1 . Jets are reconstructed and their momentum measured using calorimetric information. The momenta of the charged particle constituents are measured using the tracking system. The distributions corrected for detector effects are compared with various Monte Carlo event generators and generator tunes. Several of these choices show good agreement with the measured fragmentation function. None of these choices reproduce both the transverse profile and fragmentation function over the full kinematic range of the measurement. Introduction and overviewThis paper presents measurements of jet properties in proton-proton (pp) collisions at a center of mass energy of 7 TeV at the CERN LHC using the ATLAS detector. Jets are identified and their momenta measured using the calorimeters. Charged particles measured by the tracking system are then associated with these jets using a geometric definition. The structure of the jets is studied using these associated particles. Jets produced at large transverse momentum in protonproton collisions arise from the scattering of proton constituents leading to outgoing partons (quarks and gluons) with large transverse momenta. These manifest themselves as jets of hadrons via a "fragmentation process". While the scattering of the proton constituents is well described by perturbative QCD and leads, at lowest order, to final states of e-mail: atlas.publications@cern.ch gg, gq, and qq, the fragmentation process is more complex. First, fragmentation must connect the outgoing partons with the rest of the event as the jet consists of colourless hadrons while the initiating parton carries colour. Second, the process involves the production of hadrons and takes place at an energy scale where the QCD coupling constant is large and perturbation theory cannot be used. Fragmentation is therefore described using a QCD-motivated model with parameters that must be determined from experiment. The fragmentation function D h i (z, Q) is defined as the probability that a hadron of type h carries longitudinal momentum fraction z of the momentum p i of a parton of type iD(z, Q) depends on z and on the scale Q of the hard scattering process which produced the parton. While the value of D h i (z, Q) cannot be calculated in perturbative QCD, the variation with Q can be predicted provided Q is sufficiently large [1][2][3][4][5][6].In this paper a quantity related to D h i (z, Q) is measured. After jets have been reconstructed, the data are binned for fixed ranges of jet transverse momenta (p T jet ), each bin containing N jet jets; z is then determined for each charged particle associated with the jetwhere p jet is the momentum of the reconstructed jet and p ch the momentum of the charged particle. The following quantity is measu...

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