Entropy Production and Thermalization in Heavy-Quark Langevin Dynamics

Heavy quarks are often treated as Brownian probes of the quark-gluon plasma. We formulate the irreversible entropy balance of their momentum distribution and separate the production generated by collisions from the driving caused by an evolving medium. For linear dynamics the production splits into two positive contributions, from directed momentum and from the momentum width. A static bath obeys an exact H-theorem. A cooling bath adds an explicit driving term, and longitudinal Bjorken expansion enters as a separate kinematic drive rather than as collisional production. For a Bjorken temperature protocol we derive closed-form strong- and weak-coupling limits of the total production. At strong coupling it exceeds the initial relative entropy by a term linear in the relaxation time, whose quasistatic part has the thermodynamic-length form. At weak coupling it vanishes inversely with the relaxation time. A positive strong-coupling slope therefore guarantees maximal dissipation at intermediate coupling. The same structure holds for a relativistic Fokker-Planck equation with a Jüttner stationary state, solved numerically for charm and bottom quarks under the temperature protocol. Linear-response coefficients computed from the static generator reproduce both limits, and the numerical entropy balance is satisfied to high accuracy.