Color Randomization of Fast Gluon-Gluon Pairs in the Quark-Gluon Plasma
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I, PARTICLES, FIELDS, GRAVITATION, AND ASTROPHYSICS
Color Randomization of Fast Gluon-Gluon Pairs in the Quark-Gluon Plasma1 B. G. Zakharov Landau Institute for Theoretical Physics, Chernogolovka, Moscow oblast, 142432 Russia e-mail: [email protected] Received July 18, 2018; revised July 18, 2018; accepted July 19, 2018
Abstract—We study the color randomization of two-gluon states produced after splitting of a primary fast gluon in the quark-gluon plasma. We find that for the LHC conditions the color randomization of the gg pairs is rather slow. At jet energies E = 100 and 500 GeV, for typical jet path length in the plasma in central Pb+Pb collisions, the SU(3)-multiplet averaged color Casimir operator of the gg pair differs considerably from its value 2Nc for a fully randomized gg state. Our calculations of the energy dependence for generation of the nearly collinear decuplet gg states, that can lead to the baryon jet fragmentation, show that the contribution of the anomalous decuplet color states to the baryon production should become small at pT ≳ 10 GeV. DOI: 10.1134/S1063776119020146
1. INTRODUCTION The results of experiments on heavy ion collisions at RHIC and LHC provide strong evidence for formation of a hot quark-gluon plasma (QGP) at the proper time τ0 ~ 0.5–1 fm. One of the major signals of the QGP formation in AA collisions is a strong suppression of high-pT particles (jet quenching (JQ)) as compared to pp collisions. The JQ phenomenon is believed to be a consequence of medium modification of the jet fragmentation functions (FFs) due to radiative [1–6] and collisional [7] parton energy loss in the QGP. The energy loss is dominated by the radiative mechanism, and the effect of the collisional energy loss turns out to be relatively small [8, 9]. The first-principle calculation of the medium modification of the jet FFs in AA collisions remains an unsolved problem. The available approaches to the radiative energy loss [2–5] deal with one gluon emission. In phenomenological applications to the JQ multiple gluon emission is usually treated in the approximation of independent gluon radiation [10], similarly to the radiation of soft photons in QED. This approximation may be reasonable for calculation of the nuclear modification factor R AA that is sensitive mostly to the Sudakov suppression of the FFs at the fractional momenta x close to unity. But this may be unsatisfactory for the soft region x ≪ 1. In principle, the diagram technique of the light-cone path integral (LCPI) [3, 11] approach, originally developed for onegluon emission, allows one to go beyond the one gluon level. However, even at the level of two gluons, and in 1 The article is published in the original.
a crude oscillator approximation [12] (when multiple scattering is described in terms of the transport coefficient qˆ [2]), calculations become extremely complicated [13–15]. And until now no accurate method has been developed for multiple gluon emission that could be used for a robust calculation of the in-medium jet evolution. In the last years
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