Schwinger effect in dynamical holographic QCD with a supercritical region

Abstract

We study the phase structure of QCD matter using a dynamical Einstein--Maxwell--Dilaton holographic model, using both thermodynamic and dynamical observables. Depending on the warp factor, the model admits either a standard confinement/deconfinement transition or a first-order \textit{specious confinement}/deconfinement transition ending at a critical end point (CEP), giving rise to a rich phase diagram with a supercritical region. We probe this structure using both thermodynamic (heat capacity) and dynamical (squared speed of sound, IR wall) observables. We show that the loci of maxima in the heat capacity and minima in the sound speed define two distinct crossover lines emanating from the CEP and extending into the supercritical region, each tracing a different separation between confined-like and deconfined-like matter. As a dynamical probe, the persistence of the IR wall introduces another separation line, not emanating from the CEP, and reveals a triangular region, before the CEP, where the system is thermodynamically confined but dynamically deconfined. We further study the Schwinger effect as a nonperturbative probe of vacuum instability, determining the critical and threshold electric fields in both confined and deconfined phases. In the specious confined phase and in the confined-like phase beyond the critical end point, these fields depend on temperature and chemical potential, unlike in the standard confined phase, and we are able to trace their behavior for the first time including the supercritical region. Our results highlight the complementarity of thermodynamic and dynamical probes in mapping the QCD phase diagram and, in particular, establish the Schwinger threshold and critical fields as sensitive diagnostics of confinement not only in the known phase transitions but also in the supercritical regime.