Attenuation of jet partons in strongly interacting quark-gluon plasma
Authors:   , Grishmanovskii Ilia 
Published: November 2024

The goal of this thesis is to study the interaction of the jet particle with the strongly interacting QCD medium within the framework of the effective dynamical quasiparticle model (DQPM), which describes the nonperturbative QGP medium in terms of strongly interacting off-shell massive quarks and gluons, whose properties are fitted to reproduce the lattice QCD thermodynamics at zero baryon chemical potential. In addition to the temperature $T$ and chemical potential $\mu_B$ dependencies of quasiparticle masses, the DQPM includes information about their interactions in the sQGP through dynamically generated widths, which also depend on $T$ and $\mu_B$. In chapter 1 of the thesis we discuss the basic DQPM ingredients and provide the details of the calculation of the DQPM propagators, spectral functions, strong coupling, thermal masses and widths, showing that the thermodynamic quantities evaluated within the DQPM are consistent with the lattice QCD results. The chapter 2 is dedicated to the partonic interactions in the QGP. Using the DQPM propagators for massive quarks and gluons, we construct partonic scattering amplitudes for elastic and inelastic reactions. By performing the integration over the phase space, we calculate the differential and total elastic and inelastic cross sections and investigate various observables, demonstrating the following key findings. First, we show that the DQPM amplitudes for both $2 \to 2$ and $2 \to 3$ processes align with the pQCD calculation when quasiparticle masses and widths are zero, thus highlighting the consistency between pQCD and the DQPM. Also, we show that high-energy elastic and radiative processes are dominated by the $t$ channel and their cross sections scale with QCD color factors, which is also consistent with pQCD. For the $qq \to qqg$ process, significant contributions arise from gluon emission from exchange lines, which emphasizes the need for precise evaluations without approximations. Next, we show that $T$ and $\mu_B$ dependencies of the cross sections are mainly driven by the DQPM strong coupling, showing a strong $T$ dependence, but a weaker $\mu_B$ dependence. Whereas elastic cross sections dominate at low energies and high temperatures, radiative cross sections increase at low temperatures and high energies, becoming comparable or even dominant. Furthermore, we find that the partonic cross sections are only slightly affected by the leading parton mass but are strongly sensitive to the mass of the emitted gluon, which indicates its importance for the radiative processes. Finally, we show that the interaction rate and relaxation time in the sQGP are primarily governed by elastic scattering. In chapter 3 we focus on the attenuation of jet partons in the QGP by applying together the DQPM and kinetic theory to calculate the jet transport coefficients, specifically $\hat{q}$ (transverse momentum broadening per unit length) and $dE/dx$ (energy loss per unit length). The calculations are performed for both elastic and inelastic reactions, including all possible contributions and interference terms. Our results demonstrate several key findings. First, we find that both elastic and inelastic $\hat{q}$ and $dE/dx$ rise rapidly as the medium temperature approaches the critical temperature $T_c$. This rise is more pronounced for inelastic interactions and is driven by the behavior of the DQPM strong coupling. We also find that the off-shellness of medium partons generally reduces the value of $\hat{q}$, especially at low temperatures. Furthermore, both elastic and inelastic $\hat{q}$ and $dE/dx$ exhibit a strong dependence on jet momentum. For low temperatures and high jet momenta, the contribution from the inelastic reactions dominates over the elastic one, indicating the importance of radiative processes for jet partons. A comparison of DQPM results with pQCD models shows that elastic energy loss in the QGP medium is stronger than that with massless pQCD partons. Nevertheless, at high temperatures the DQPM results for $\hat{q}$ align qualitatively with pQCD calculation as well as with lattice QCD results and phenomenological estimates. Then, investigating different scenarios for strong couplings in thermal, jet, and emitted gluon vertices, we find that $\hat{q}$ and $dE/dx$ are also highly sensitive to the dependencies of these couplings on energy, momentum transfer, or temperature. In addition, we examine the scaling $\eta/s \approx 1.25\; T^3/\hat{q}$ and show that this scaling is valid in the weak coupling regime at high temperatures but is violated in the strong coupling regime at low temperatures. Lastly, we apply the DQPM together with the soft gluon approximation to study the transport coefficients of a charm quark and show that the inclusion of radiative processes gives a small contribution at low charm momenta and a large contribution at high charm momenta, suggesting the consideration of the non-thermal strong coupling in the charm vertex.