Conserved Charge Fluctuations and Susceptibilities in Strongly Interacting Matter

TL;DR

This paper investigates conserved charge fluctuations in strongly interacting matter using holographic models, revealing suppression of higher order susceptibilities at strong coupling and providing insights into the nature of the quark-gluon plasma near the transition temperature.

Contribution

It compares holographic models for QCD to study charge susceptibilities, showing suppression of higher order fluctuations and supporting the existence of bound states in the quark-gluon plasma.

Findings

Higher order susceptibilities are suppressed at strong coupling.

Conserved charge fluctuations tend to become Gaussian at strong coupling.

Bound states are present and significant in the 1-2 Tc temperature range.

Abstract

We study the conserved charge fluctuations, as quantified by the corresponding susceptibilities, in strongly interacting matter as motived by the quark-gluon plasma. Using the gauge-gravity correspondence approach, we study the patterns of conserved charge fluctuations in two types of holographic models for QCD, the D4/D8 and the D3/D7 models. We compute and compare the quark number susceptibilities in both models and find an interesting common feature of the two: at very strong coupling higher order susceptibilities are suppressed and the conserved charge fluctuations become purely Guassian. In light of the state-of-the-art lattice QCD results we also discuss what we can learn from these susceptibilities about the underlying degrees of freedom in the $1\sim 2T_c$ quark-gluon plasma and examine the viability of different ideas such as holography, quasi-particles, as well as bound states. From analyzes of second order susceptibilities we conclude that the bound states exist and are important in the $1\sim 2T_c$ region. We further construct and make predictions for several ratios of fourth-order susceptibilities that can sensitively reveal such bound states.