Fate of first-order chiral phase transition in QCD: Implications to dark QCD

Abstract

The first-order nature of the chiral phase transition in QCD-like theories can play crucial roles to address a dark side of the Universe, where the created out-of equilibrium is essential to serve as cosmological and astrophysical probes such as gravitational wave productions, which have extensively been explored. This interdisciplinary physics is built based on a widely-accepted conjecture that the thermal chiral phase transition in QCDlike theories with massless (light) three flavors is of first order. We find that such a first order feature may not hold, when ordinary or dark quarks are externally coupled to a weak enough background field of photon or dark photon (which we collectively call a "magnetic" field). We assume that a weak "magnetic" background field could be originated from some "magnetogenesis" in the early Universe. We work on a low-energy effective model which can describe the chiral phase transition in a wide class of QCD-like theories. We show that in the case with massless (light) three flavors, the first-order feature goes away when 2f 2 π eB( (4πf π ) 2 ), where eB is the "magnetic" field strength and f π the pion decay constant at the vacuum. This disappearance is the generic consequence of the presence of the "magnetically" induced scale anomaly and the "magnetic" catalysis for the chiral symmetry breaking, and would impact or constrain modeling dark QCD coupled to an external "magnetic" field.