  We investigate the possible thermalization process of the highly occupied and
weakly coupled Yang-Mills fields expanding along the beam axis through an
evaluation of the entropy, particle number, and pressure anisotropy. The time
evolution of the system is calculated by solving the equation of motion for the
Wigner function in the semiclassical approximation with initial conditions
mimicking the glasma. For the evaluation of the entropy, we adopt the
Husimi-Wehrl (HW) entropy, which is obtained by using the Husimi function, a
positive semidefinite quantum distribution function given by smearing the
Wigner function. By numerical calculations at $g=0.1$ and $0.2$, the entropy
production is found to occur together with the particle creation in two
distinct stages: In the first stage, the particle number and the entropy at low
longitudinal momenta grow rapidly. In the second stage, the particle number and
the entropy of higher longitudinal momentum modes show slower increase. The
pressure anisotropy remains in our simulation and implies that the system is
still out-of-equilibrium.
