NANOGrav Signal from First-Order Confinement/Deconfinement Phase
  Transition in Different QCD Matters
  Recently, an indicative evidence of a stochastic process, reported by the
NANOGrav Collaboration based on the analysis of 12.5-year pulsar timing array
data which might be interpreted as a potential stochastic gravitational wave
signal, has aroused keen interest of theorists. The first-order color charge
confinement phase transition at the QCD scale could be one of the cosmological
sources for the NANOGrav signal. If the phase transition is flavor dependent
and happens sequentially, it is important to find that what kind of QCD matter
in which the first-order confinement/deconfinement phase transition happens is
more likely to be the potential source of the NANOGrav signal during the
evolution of the universe. In this paper, we would like to illustrate that the
NANOGrav signal could be generated from confinement/deconfinement transition in
either heavy static quarks with a zero baryon chemical potential, or quarks
with a finite baryon chemical potential. In contrast, the gluon confinement
could not possibly be the source for the NANOGrav signal according to the
current observation. Future observation will help to distinguish between
different scenarios.
