Critical net-baryon fluctuations in an expanding system

TL;DR

This paper investigates how a longitudinal expansion and cooling scenario affect net-baryon fluctuations near the QCD critical point, highlighting the importance of diffusion length and freeze-out conditions for observable signals.

Contribution

It introduces a 1+1D non-linear model incorporating expansion and cooling effects on baryon fluctuation dynamics near the critical point.

Findings

Critical signals are visible but broadened by expansion.

Small diffusion length reduces fluctuation amplitudes.

Fourth-order cumulant shows a pronounced minimum at intermediate rapidities.

Abstract

In this work we study the consequences of a longitudinal Bjorken expansion and a Hubble-like temperature cooling scenario on a 1+1D non-linear model of the diffusive dynamics of fluctuations in the net-baryon density. The equilibrium behavior of the fluctuations is fully encoded in the temperature dependence of the susceptibilities on the crossover side both in the vicinity of the assumed location of the critical point and at vanishing baryo-chemical potential in-line with lattice QCD calculations. We demonstrate the great sensitivity of the fluctuation observables on the dynamics, in particular on the diffusion length and the freeze-out conditions. While the critical signals are visible and the critical region is broadened by the expansion, a too small diffusion length can strongly reduce the amplitude of the signals. We propose to search for significant anti-correlations of baryons at intermediate rapidity experimentally and to map out the rapidity dependence of the fourth-order cumulant, which in the presence of a critical point (and only in its presence) has a pronounced minimum at intermediate rapidities.