Centrality dependence of proton and light nuclei yields as a consequence of baryon annihilation in the hadronic phase

TL;DR

This paper investigates how baryon annihilation in the hadronic phase affects proton and light nuclei yields in heavy-ion collisions, explaining the centrality dependence of observed particle ratios and estimating the annihilation freeze-out temperature.

Contribution

It introduces a generalized partial chemical equilibrium framework to estimate baryon annihilation freeze-out temperatures across collision centralities, highlighting its significance before kinetic freeze-out.

Findings

Baryon annihilation causes suppression of proton to pion ratios in central collisions.

Estimated annihilation freeze-out temperature decreases with increasing multiplicity, around 132 MeV in central collisions.

Baryon annihilation remains relevant before kinetic freeze-out, affecting light nuclei yields.

Abstract

The centrality dependence of the $p/\pi$ ratio measured by the ALICE Collaboration in 5.02 TeV Pb-Pb collisions indicates a statistically significant suppression with the increase of the charged particle multiplicity once the centrality-correlated part of the systematic uncertainty is eliminated from the data. We argue that this behavior can be attributed to baryon annihilation in the hadronic phase. By implementing the $B\bar{B} \leftrightarrow 5\pi$ reaction within a generalized partial chemical equilibrium framework, we estimate the annihilation freeze-out temperature at different centralities, which decreases with increasing charged multiplicity and yields $T_{\rm ann} = 132 \pm 5$ MeV in 0-5% most central collisions. This value is considerably below the hadronization temperature of $T_{\rm had} \sim 160$ MeV but above the thermal (kinetic) freeze-out temperature of $T_{\rm kin} \sim 100$ MeV. Baryon annihilation reactions thus remain relevant in the initial stage of the hadronic phase but freeze out before (pseudo-)elastic hadronic scatterings. One experimentally testable consequence of this picture is a suppression of various light nuclei to proton ratios in central collisions of heavy ions.