2025/07/03 by Zhang, Lei, Liu, Lu-Meng, Huang, Mei
#FOS: Physical sciences #High Energy Physics - Phenomenology (hep-ph) #Nuclear Theory (nucl-th)
paper · doi:10.48550/arxiv.2507.02697
We investigate the impact of the CP-violating θ term on isospin symmetry breaking in quark matter and compact star properties using a two-flavor Nambu-Jona-Lasinio (NJL) model. By incorporating the θ parameter through the Kobayashi-Maskawa-'t Hooft (KMT) determinant interaction, we derive the thermodynamic potential and gap equations under finite temperature, baryon chemical potential, and isospin chemical potential. At zero temperature and baryon density, θ suppresses conventional chiral (σ) and pion (π) condensates while promoting pseudo-scalar (η) and scalar-isovector (δ) condensates, thereby reducing the critical isospin chemical potential μIcrit for spontaneous symmetry breaking. For θ=π, a first-order phase transition emerges at μIcrit = 0.021 GeV, accompanied by CP symmetry restoration. Extending the investigation to finite temperature and baryon chemical potential reveals that these θ-term-induced effects persist. Axion effects (modeled via θ≡ a/fa) stiffen the equation of state (EOS) of non-strange quark stars, increasing their maximum mass and radii, in agreement with multimessenger constraints from pulsar observations and gravitational wave events. These results establish θ as a critical parameter modulating both the Quantum Chromodynamics (QCD) phase structure and compact star observables.