  The mass sensitivity of the chiral phase transition of QCD with and without
axial $U_A(1)$-symmetry breaking at vanishing and finite quark chemical
potential is investigated. To focus on the low-energy sector of QCD, a
quark-meson model with three dynamical quark flavors is employed.
Non-perturbative quantum fluctuations are taken into account with the
functional renormalization group. The inherent ambiguities in fixing the
low-energy model parameters away from the physical mass point and their
consequences for spontaneous chiral symmetry breaking are discussed in detail
and a heuristic parameter fixing scheme motivated by chiral perturbation theory
is proposed. The influence of vacuum and thermal fluctuations of quarks and
mesons on the order of the chiral phase transition is additionally assessed
with a mean-field analysis.
