Speakers
Description
A challenging goal in relativistic heavy-ion physics is the
investigation of the phase diagram of strongly interacting matter and
the determination of its phase structure, governed by the approximate
chiral symmetry of the light-quark sector of QCD. In this study [1] we
investigate a linear quark-meson $\sigma$ model in and out of
equilibrium employing Schwinger-Keldysh real-time techniques to derive a
set of coupled Boltzmann-Uehling-Uhlenbeck (BUU) equations for the
$\sigma$-mean field (the order parameter of the phase transition) and
the quark- and meson phase-space distribution function from a
$\Phi$-derivable approximation, which is then numerically solved using
an accurate quadrature algorithm to evaluate the collision terms. This
numerical scheme is used to evaluate the grand-canonical baryon-number
fluctuations for an expanding fireball. Even when starting with a purely
Gaussian initial distribution the evolution results in a temporary
buildup of higher-order fluctuations of the net-baryon number like the
curtosis at low momenta when the system is evolving close to the
critical point or the first-order phase-transition line. This is mainly
caused by the slowly evolving $\sigma$-mean field, i.e., the order
parameter of the phase transition. This is partially counterballanced by
the further dissipative evolution due to collisions of the quarks,
mesons, and the mean field, leading to a considerable weakening of the
final fluctuations, depending on the expansion rate of the fireball.
[1] Annals of Physics 431, 168555 (2021)
