NANOGrav hints for first-order confinement-deconfinement phase transition in different QCD-matter scenarios

Recent observations from several pulsar timing array (PTA) collaborations have unveiled compelling evidence for a stochastic signal in the nanohertz band. This signal aligns remarkably with a gravitational wave (GW) background, potentially originating from the first-order color charge confinement phase transition. Distinct quantum chromodynamics (QCD) matters, such as quarks or gluons, and diverse phase transition processes thereof can yield disparate GW energy density spectra. In this paper, employing the Bayesian analysis on the NANOGrav 15-yr dataset, we explore the compatibility with the observed PTA signal of the GWs from phase transitions of various QCD-matter scenarios in the framework of the holographic QCD. We find that the PTA signal can be effectively explained by the GWs from the confinement-deconfinement phase transition of pure quark systems in a hard-wall model of the holographic QCD where the bubble dynamics, one important source of the GWs, is of the Jouguet detonations. Notably, our analysis decisively rules out the plausibility of the pure gluon QCD-matter scenario and the nonrunaway bubble dynamics model for the phase transition in explaining the observed PTA signal.