Angular distribution of medium-induced QCD cascades

Abstract

We provide a complete description of the angular distribution of gluons in a medium-induced QCD cascade. We identify two components in the distribution, a soft component dominated by soft multiple scatterings, and a hard component dominated by a few hard scatterings. The typical angle that marks the boundary between these two components is determined analytically as a function of the energy of the observed gluon and the size of the medium. We construct the complete solution (beyond the diffusion approximation) in the regime where multiple branchings dominate the dynamics of the cascade in the form of a power series in the number of collisions with the medium particles. The coefficients of this expansions are related to the moments of the distribution in the diffusion approximation and are determined analytically. The angular distribution may be useful in phenomenological studies of jet shapes in heavy-ion collisions.

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]).…”