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 cosmological confinement/deconfinement 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 the dominant phase transition producing the gravitational waves that match 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, or a pure gluon system. Moreover, the quark confinement as compared with the gluon confinement is more likely to be the source for the NANOGrav signal regardless of whether the chemical potential is finite or not. Future observation will help to distinguish between different scenarios.