Parton energy loss in an expanding quark-gluon plasma: Radiative vs. collisional

Abstract

We perform a comparison of the radiative and collisional parton energy losses in an expanding quark-gluon plasma. The radiative energy loss is calculated within the light-cone path integral approach [4]. The collisional energy loss is calculated using the Bjorken method with an accurate treatment of the binary collision kinematics. Our numerical results demonstrate that for RHIC and LHC conditions the collisional energy loss is relatively small in comparison to the radiative one. We find an enhancement of the heavy quark radiative energy loss as compared to that of the light quarks at high energies.1. The suppression of high-p T hadrons in AA-collisions (usually called jet quenching) observed in the experiments at RHIC (for a review, see [1]) is widely believed to be due to the parton energy loss in the hot quark-gluon plasma (QGP) produced at the initial stage of nucleus-nucleus collision. The parton energy loss may come from the collisional energy loss and the induced gluon radiation. The first estimate of the collisional energy loss in the QGP has been done by Bjorken [2]. The radiative energy loss has been under active investigation in the last years [3,4,5,6,7,8,9] (for a review, see [10]). The calculations of the radiative energy loss within the BDMPS [3,10] and the light-cone path integral (LCPI) [4,6,7]) approaches demonstrate that for high energy partons the energy loss is likely dominated by the induced gluon radiation. The estimates given in [11] show that the collisional energy loss can roughly increase the energy loss by 30-40% for RHIC energies. However, it has recently been claimed [12,13,14] that for RHIC conditions the collisional energy loss may be as important as the radiative one, or even dominate at low energies. But an accurate comparison of the two mechanisms of the energy losses so far has not been performed, say, in [14] even for the plasma with a uniform density the radiative energy loss was calculated incorrectly. The authors have used the kinematical suppression factor for the radiative energy loss obtained in [8] which strongly overestimates the kinematic suppression [15]. The question of the importance of the collisional energy loss is becoming of special current interest in connection with the recent data on the non-photonic electrons [16] indicating that the nuclear suppression for heavy quarks may be similar to that for light ones. This fact seems to be inconsistent with purely radiative energy loss which, at low energies, should be suppressed for heavy quarks by the mass effects [17]. Unfortunately, presently for both the radiative and collisional 1 losses the uncertainties in the theoretical predictions are rather big. Say, the results are very sensitive to the choice of α s (running or constant [11,18]), to the infrared effects [11]. For clarifying the situation with the relative contributions of the radiative and collisional energy loss it is important to perform the calculations with the same parametrizations of the coupling constant and the infrared cutoffs. In the ...

Collisional energy loss of heavy quarks

“…An additional contribution of suppression, potentially important for the range of smaller (to medium) momenta, is the collisional energy loss by elastic processes as discussed in [10,11,12] and with further modifications in [13,14,15,16,17,18]. It will turn out that in our approach a fairly large collisional energy loss is predicted, although some comparisons between collisional and radiative contributions [19,20] indicate another picture where the collisional loss should be relatively small (∼ 20%) compared to the radiative one. This conclusion was, however, based on a fixed coupling approach, and using a naive approximation for the required infrared regulator in the QCD cross section.…”

Medium Modification of Photon-Tagged and Inclusive Jets in High-Multiplicity Proton-Proton Collisions

“…The results are not very sensitive to variation of α f r s in the physically reasonable domain α f r s ∼ 0.5 − 0.7. The point is that for a small-size plasma the hardness Q of induced gluon emission can attain quite large values since Q 2 ∼ > 2ω/L [16]. For radiation of gluons with energy ω ∼ 1 − 3 GeV and L ∼ 1 fm Q ∼ > 0.6 − 1 GeV, where the freezing of α s is not very important.…”

“…Although R pp is unobservable directly it is important for the nuclear modification factors R pA and R AA , which, in the presence of the mini-QGP, should be divided by R pp . To illustrate the magnitude of the suppression effect in pp collisions we present our preliminary results for R pp of charged hadrons.The jet quenching is dominated by radiative energy loss [11][12][13][14][15] with relatively small effect from collisional mechanism [13,16,17]. We use the light-cone path integral (LCPI) approach [12,13] to induced gluon emission.…”

Collisional Energy Loss of a Fast Parton in a QGP

“…In this case explaining the heavy flavour quenching with purely radiative c quark energy loss might still be possible [8]. However, the relative contributions from D and B decays are likely to be of the same order, as in pp collisions [14].…”

“…We will consider a fast parton of velocity v → 1. The appropriate sum and average of |M i | 2 over spin and color is implicit in (8) and in the following.…”