Jet conversions in a quark-gluon plasma

Abstract

We report our recent work on conversions between gluon and quark jets as they traverse through a quarkgluon plasma (QGP) and their effects on the nuclear modification factors for quark and gluon jets as well as the ratios of p/π + andp/π − at high transverse momentum in ultra-relativistic heavy ion collisions.Keywords: Ultra-relativistic heavy ion collisions; Quark-gluon plasma; Jet quenching; Jet conversions One of the most interesting observations in central heavy ion collisions at Relativistic Heavy Ion Collider (RHIC) is suppressed production of hadrons with large transverse momentum [1]. This phenomenon has been attributed to the radiative energy loss of partonic jets, produced from initial hard scattering of incoming nucleons, as they traverse through the dense partonic matter created during these collisions [2][3][4]. Recent studies have shown that elastic scattering of jets in the produced quark-gluon plasma (QGP) also leads to an appreciable loss of their energies [5,6]. Because of its larger color charge, a gluon jet is expected to lose more energy than quark and antiquark jets [7]. Since the ratio of high momentum protons and antiprotons to pions produced from fragmentation of a gluon jet is much larger than that from a quark jet and there are more gluon than quark jets in proton-proton collisions, a larger energy loss of gluon jets than that of quark jets would lead to smaller p/π + andp/π − ratios at high transverse momentum in central heavy ion collisions than in proton-proton collisions at same energy [8]. This is in contrast to p/π + and p/π − ratios at intermediate transverse momentum where they are enhanced in central heavy ion collisions due to production via quark coalescence or recombination [9][10][11]. Experimentally, data from the STAR collaboration have indicated, however, that the p/π + andp/π − ratios at high transverse momentum in central Au+Au collisions [12] approach those in p+p and d+Au collisions [13], implying that the ratio of final quark and gluon jets at high transverse momentum is similar to that of initial ones. A possible mechanism for reducing the effect due to difference in quark and gluon jet energy losses in QGP is to allow a net conversion of quark jets to gluon jets via both elastic (q(q)g ↔ gq(q)) and inelastic (qq ↔ gg) scattering with thermal quarks and gluons in the QGP, an idea first considered in Refs. [14] for deeply inelastic scattering. In this talk, we report the results from our recent study [15].The conversion rate of a quark jet to a gluon jet or vice versa in a QGP is related to its collisional width due to conversion scattering , i.e., given by the thermal average Γ = ∑ i |M i | 2 of the sum of squared amplitudes for these scattering processes after averaging over the spins and colors of initial partons and summing over those of final partons. For the two-body processes q(q)g ↔ gq(q) and qq ↔ gg, their amplitudes are * On leave from Institute of Particle Physics, Huazhong Normal University, Wuhan 430079, China well-known. To ensure that a quark (gluon)...

Medium-evolved fragmentation functions

“…This possibility has been taken into account in e.g. [14,32]. The effect of these corrections, not included in the present work, is not large and restricted to the small-and moderate-p T part of the spectrum.…”