On the Square Speed of Sound in High-Energy Collisions: Range of Values and How to Understand It

TL;DR

This paper reviews and investigates the speed of sound in hadronic matter produced in high-energy collisions, relating it to rapidity distributions and explaining why it generally falls within a specific range.

Contribution

It provides a comprehensive analysis of the factors affecting the sound speed in high-energy collision matter and offers explanations for values exceeding the typical range.

Findings

Sound speed squared ranges from 0 to 1/3 in most cases.

The width of the rapidity distribution correlates with the sound speed.

Explanations are provided for scenarios where the sound speed exceeds 1/3.

Abstract

After reviewing the sound speeds in various forms and conditions of matter, we investigate the sound speed of hadronic matter that has decoupled from the hot and dense system formed during high-energy collisions. We comprehensively consider factors such as energy loss of the incident beam, rapidity shift of leading nucleons, and the Landau hydrodynamic model for hadron production. The sound speed is related to the width or standard deviation of the Gaussian rapidity distribution of hadrons. The extracted square speed of sound lies within a range from 0 to 1/3 in most cases. For scenarios exceeding this limit, we also provide an explanation.