How the QCD trace anomaly behaves at the core of twin stars?

Abstract

We investigate the behavior of the dense and cold (normalized) QCD trace anomaly, $\Delta$, in the interior of twin neutron stars (obtained from several sets of equations of state in agreement with modern compact-star and multimessenger data) satisfying static and dynamic stability conditions. We scan the formed twin-star parameter space in order to look for effects caused by the presence of a strong first-order phase transition connecting hadron and quark phases by means of a Maxwell construction. We found robustly that $\Delta$ suffers an abrupt decrease around the transition point, even reaching large negative values ($\Delta\simeq-0.35$), in marked contrast to current studies pointing out a smooth behavior with $\Delta\gtrsim 0$ at all densities. Besides, we characterize the behavior of {the recently defined} conformal factor, $d_{c}$, in our four categories of twin stars for which we perform comparisons with theoretical constraints put, e.g. by Bayesian studies sometimes adjusted to agree with perturbative QCD at high densities. These analyses allow us to hypothesize modifications in the strong QCD coupling in dense nuclear matter with a strong thermodynamic discontinuity.