Seminar by Dr. Harshit Pandey

Abstract: Studying real-time dynamics and the approach to thermalization in gauge theories is one of the primary goals behind the push towards methods involving quantum computing, quantum simulations, and tensor networks.

My talk concerns studying these properties using existing lattice gauge theory techniques to provide some fundamental understanding and benchmark results towards these efforts.

In my talk, I will discuss how we can understand the process of thermalization in the strongly interacting quantum field theory of QCD in terms of the soft momentum modes of gauge fields. At high enough temperatures, there is a clear separation of scales into these soft (magnetic) modes, intermediate electric modes, and modes at the scale of the temperature. The soft modes behave predominantly classically owing to their high occupancies, which simplifies studying their dynamics, for which we use ab-initio lattice techniques.

Evolving these soft modes and quantifying the divergence of their phase-space trajectories, we find that the maximal Lyapunov exponent λL of physical states in a non-Abelian gauge theory consisting of soft momentum modes both in- and out-of-thermal equilibrium conditions is positive definite and similar in magnitude, signifying their chaotic nature. Consequently, we use these observations to provide estimates of thermalization time for these modes. By measuring the Kolmogorov-Sinai entropy rate in terms of the spectrum of positive λL, we estimate a typical time-scale of ∼ 0.50(3) fm/c to achieve thermalization at T ∼ 600 MeV starting from a particular non-thermal state consisting of over-occupied gluons, believed to exist in the early stages of heavy-ion collisions.

References: 

1. Pandey et al., Nucl.Phys.B 1025 (2026) 117401.

2. Guin et al., Phys.Lett.B 866 (2025) 139490.

Venue: Seminar Room (202), Physics Department IIT Bombay, Powai, Mumbai