Energy flow along the medium-induced parton cascade

Abstract

We discuss the dynamics of parton cascades that develop in dense QCD matter, and contrast their properties with those of similar cascades of gluon radiation in vacuum. We argue that such cascades belong to two distinct classes that are characterized respectively by an increasing or a constant (or decreasing) branching rate along the cascade. In the former class, of which the BDMPS, medium-induced, cascade constitutes a typical example, it takes a finite time to transport a finite amount of energy to very soft quanta, while this time is essentially infinite in the latter case, to which the DGLAP cascade belongs. The medium induced cascade is accompanied by a constant flow of energy towards arbitrary soft modes, leading eventually to the accumulation of the initial energy of the leading particle at zero energy. It also exhibits scaling properties akin to wave turbulence. These properties do not show up in the cascade that develops in vacuum. There, the energy accumulates in the spectrum at smaller and smaller energy as the cascade develops, but the energy never flows all the way down to zero energy. Our analysis suggests that the way the energy is shared among the offsprings of a splitting gluon has little impact on the qualitative properties of the cascades, provided the kernel that governs the splittings is not too singular.

Angular structure of jet quenching within a hybrid strong/weak coupling model

“…We encourage theorists pursuing other approaches to calculate all these observables and compare to data. Consider for example the weakly coupled calculations in which energy is lost to radiated gluons which themselves are quickly degraded down to soft gluons with momenta of order the temperature [150][151][152][153][154][155][156], which leads to rapid hydrodynamization of the emitted energy [156]. These approaches yield qualitative agreement with the / p T -distribution that we have shown in figure 12 [152], but fail to reproduce the soft enhancement of the fragmentation function…”

“…This is an immediate and natural consequence of strong coupling dynamics [123]. Something similar can happen at weak coupling even though the energy is initially lost by gluon radiation because these radiated gluons can experience a cascade of reinteractions that converts the energy into soft particles at large angles [150][151][152][153][154][155][156]. The effects of both transverse momentum broadening [85,86,[107][108][109] and the backreaction of the medium [95,[157][158][159][160][161][162] on jet observables have also been studied within the context of perturbative energy loss mechanisms.…”

“…This degradation may be viewed as a rapid transfer of energy from hard (jet) modes to soft (medium) modes and it has been described recently in detail in refs. [150][151][152][153][154][155][156]. In an infinite medium, this process leads to the eventual (local) thermalization of the lost energy after which the dynamics may be best understood in terms of hydrodynamics.…”

“…Since the wake carries the momentum lost by the jet, the particles in the final state that come from the hadronization of the medium including the wake must have net momentum in the direction of the jet, meaning that some component of what experimentalists reconstruct as a jet, even after background subtraction, must in fact be composed of soft particles coming from the wake in the plasma. Motivated by expectations coming from explicit calculations of the wake in strongly coupled plasma [123,125] and by recent weakly coupled calculations in which the energy lost by a jet cascades into multiple soft particles [150][151][152][153][154][155][156] we analyze the wake upon making the simplifying assumptions that all the energy and momentum lost by the jet is incorporated into hydrodynamic motion and that the resulting perturbations to the temperature and velocity of the hydrodynamic fluid are small. That is, we assume that all of the lost momentum and energy go into a hydrodynamic wake that is as thermalized as it can be by the time of freezeout, yielding only small corrections to the spectra of the hadrons in the final state, a final state that remembers nothing about the origin of this perturbation other than the momentum and energy dumped into the plasma.…”

Thermalization of mini-jets in a quark-gluon plasma

“…qualitatively, within the pQCD picture for medium-induced radiation, which predicts the formation of well-developed gluon cascades, or 'mini-jets', via multiple branching [27][28][29][30][31][32][33][34][35] (see also refs. [36][37][38][39] for earlier, related, studies and the recent review paper [40]).…”