Soft mode dynamics associated with QCD critical point and color superconductivity -- pseudogap, anomalous dilepton production and electric conductivity

TL;DR

This paper explores the soft mode dynamics near the QCD critical point and color superconductivity, revealing their impact on electromagnetic signals in heavy-ion collisions, including pseudogap phenomena and enhanced dilepton production.

Contribution

It systematically analyzes the soft modes associated with QCD phase transitions using the NJL model and links them to observable electromagnetic effects in heavy-ion collisions.

Findings

Soft modes exhibit prominent spectral strength near phase transitions.

Diquark soft mode causes a pseudogap in quark spectra.

Soft modes lead to enhanced electric conductivity and dilepton production.

Abstract

We give a systematic account of the soft mode dynamics of QCD critical point and the two-flavor color-superconductivity based on the 2-flavor Nambu--Jona-Lasinio model, and investigate their effects on electromagnetic observables in relativistic heavy-ion collisions (HIC). We first demonstrate that the collective excitations coupled to the fluctuations of the respective order parameters are the soft modes associated with the phase transitions, in the sense that they acquire a prominent spectral strength in the low-energy and low-momentum region near the phase transitions, and the peak energy goes down, i.e., gets softened, and eventually vanishes at the critical point. It is shown that the diquark soft mode of the 2SC gives rise to the pseudogap, i.e., a depression in the density of states of the quark spectra around the Fermi surface above but in the vicinity of the critical temperature. Then, exploiting the ideas that were developed in condensed matter physics for describing the `para-conductivity' in the normal phase of metal superconductors, we show that the soft modes cause an anomalous enhancement of electric conductivity and the dilepton production rate, and discuss their relevance to HIC.