Dynamics of a tracer granular particle as a nonequilibrium Markov process.

The dynamics of a tracer particle in a stationary driven granular gas is investigated. We show how to transform the linear Boltzmann equation, describing the dynamics of the tracer into a master equation for a continuous Markov process. The transition rates depend on the stationary velocity distribution of the gas. When the gas has a Gaussian velocity probability distribution function (PDF), the stationary velocity PDF of the tracer is Gaussian with a lower temperature and satisfies detailed balance for any value of the restitution coefficient alpha. As soon as the velocity PDF of the gas departs from the Gaussian form, detailed balance is violated. This nonequilibrium state can be characterized in terms of a Lebowitz-Spohn action functional W(tau) defined over trajectories of time duration tau. We discuss the properties of this functional and of a similar functional W(tau), which differs from the first for a term that is nonextensive in time. On the one hand, we show that in numerical experiments (i.e., at finite times tau), the two functionals have different fluctuations and W always satisfies an Evans-Searles-like symmetry. On the other hand, we cannot observe the verification of the Lebowitz-Spohn-Gallavotti-Cohen (LS-GC) relation, which is expected for W(tau) at very large times tau. We give an argument for the possible failure of the LS-GC relation in this situation. We also suggest practical recipes for measuring W(tau) and W(tau) in experiments.