Baryon annihilation and multiplicity dependence of p/$\pi$ and light nuclei ratios

Abstract

The centrality dependence of the 𝑝/𝜋 ratio measured by the ALICE Collaboration in 5.02 TeV Pb-Pb collisions indicates a statistically significant suppression with the increase of the charged multiplicity, once the centrality-correlated part of the systematic uncertainty is eliminated from the data. We argue that this behavior can be attributed to the presence of baryon annihilation in the hadronic phase. By implementing the $B\bar{B} \leftrightarrow 5\pi$ reactions in the hadronic phase within a generalized partial chemical equilibrium framework, we estimate the annihilation freeze-out temperature $T_{\rm ann}$ at different centralities. $T_{\rm ann}$ is found to decrease with charged multiplicity to the value of $T_{\rm ann}$=135±5 MeV in 0-5% most central collisions, considerably below the hadronization temperature of $T_{\rm had}$∼160 MeV but above the thermal (kinetic) freeze-out temperature of $T_{\rm kin}$∼100 MeV. One experimentally testable consequence of this picture is the suppression of various light nuclei to proton ratios in central collisions of heavy ions.