Speaker
Description
Shear and bulk viscosities are two key transport coefficients that characterize the fundamental properties of quark-gluon plasma. They quantify the response of the energy-momentum tensor to the shear flow and divergent flow, serving as crucial input parameters for the phenomenological and transport models that interpret experimental data, e.g. the elliptic flow $v_2$.
However, calculating these inherently non-perturbative viscosities within lattice QCD presents challenges due to strong ultraviolet fluctuations in the relevant operators. The traditional approach using the multi-level algorithm has the limitation that it applies only in the quenched approximation, as done in [1, 2]. Recently, the gradient flow method was introduced to address this issue [3], opening the path to studies in full QCD. However, [3] examined only a single temperature, $1.5T_c$, where $T_c$ is the confinement and deconfinement transition temperature.
We present results extending [3] to a wide temperature range from $0.76T_c $ to $2.25T_c $, with particular focus on the phase transition region and high-temperature regime. The former helps us to understand how the system behaves when subject to critical change, a topic of wide concern in the community. The latter allows us to compare against the NLO perturbative estimate, which becomes more reliable at high temperature. Our preliminary results suggest that $\zeta/T^3$ peaks around $T_c$, while $\eta/T^3$ increases slowly with temperature without a dip structure around $T_c$.
Reference:
[1] N. Astrakhantsev, V. Braguta, and A. Kotov, J. High Energy Phys. 04 (2017) 101.
[2] N. Astrakhantsev, V. Braguta, and A. Kotov, Phys. Rev. D 98, 054515 (2018).
[3] L. Altenkort, A.M. Eller, A. Francis, O. Kaczmarek, L. Mazur, G.D. Moore, and H.-T. Shu, Phys. Rev. D 108,
014503 (2023).
