New Aspects Of Thermomagnetic QCD Phase Structure

The phase structure of thermomagnetic QCD has been one of the important research topics in high-energy physics,and recently,we have some new findings in the field of thermo-magnetic QCD.We find that the inverse magnetic catalysis(IMC)for U(1)axial symmetry can emerge around the chiral crossover regime in the thermomagnetic QCD with 2+1 fla-vors at physical point.This phenomenon can be correlated with the IMC for the chiral SU(2)_L×SU(2)_Rsymmetry.We explicitly observe the axial inverse magnetic catalysis(AIMC)based on a Nambu-Jona-Lasinio model with 2+1 quark flavors,where introduced anomalous magnetic moments of the quarks play the essential role to drive both the chiral IMC and AIMC.Our finding has been tested in the lattice QCD,and in this paper possible phenomenological and cosmological implications are also briefly addressed.We also find that the first-order nature of the chiral phase transition in QCD-like the-ories 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 gravi-tational wave productions,which have extensively been explored.This interdisciplinary physics is built based on a widely-accepted conjecture that the thermal chiral phase transi-tion in QCD like 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 cou-pled 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 still work on a NJL 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_π~2?e B?(4πf_π)~2,where e B is the“magnetic”field strength and f_πthe 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.