Emergence of hydrodynamical behavior in expanding quark-gluon plasmas

Understand

We use a set of simple angular moments to solve the Boltzmann equation in the relaxation time approximation for a boost invariant longitudinally expanding gluonic plasma.

• The transition from the free streaming regime at early time to the hydrodynamic regime at late time is well captured by the first two-moments, corresponding to the monopole and quadrupole components of the momentum distribution, or equivalently to the energy density and the difference between the longitudinal and the transverse pressures.
• We relate this property to the existence of fixed points in the infinite hierarchy of equations satisfied by the moments.
• These fixed points are already present in the two-moment truncations and are only moderately affected by the coupling to higher moments.