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

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 $T_{\mathrm{ann}}=132\pm 5$ MeV in 0-5% most central collisions. This value is considerably below the hadronization temperature of $T_{\mathrm{had}}\sim 160$ MeV but above the thermal (kinetic) freeze-out temperature of $T_{\mathrm{kin}}\sim 100$ 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.

arXiv comment7 pages, 5 figures, to be published in Physics Letters B

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