Psi(2S) Production at the LHC

TL;DR

This paper models the production and regeneration of $ ext{psi}(2S)$ mesons in Pb-Pb collisions at the LHC, revealing a sequential regeneration pattern that affects their transverse momentum spectra and nuclear modification factors.

Contribution

It introduces a transport model with temperature-dependent reaction rates to study $ ext{psi}(2S)$ regeneration, highlighting the impact of sequential regeneration on observable spectra and ratios.

Findings

$ ext{psi}(2S)$ regeneration occurs later in the fireball evolution.

Regenerated $ ext{psi}(2S)$ have harder $p_T$ spectra than $J/ ext{psi}$.

Double ratio of $R_{AA}$ can exceed unity at high $p_T$.

Abstract

We calculate the production of $\psi(2S)$ and the pertinent double ratio of its nuclear modification factor ($R_{\rm AA}$) over that of the $J/\psi$ in Pb-Pb collisions at the LHC. Based on a transport model with temperature dependent reaction rates, a sequential regeneration pattern emerges: the larger $\psi(2S)$ width, relative to the $J/\psi$, around and below the critical temperature, implies that most of the $\psi(2S)$ states are regenerated later in the evolution of the fireball. This has noticeable consequences for the transverse-momentum ($p_T$) spectra of the regenerated charmonia. While the total yield of $\psi(2S)$ meson remains smaller than those of $J/\psi$'s, their harder $p_T$ spectra can produce a double ratio above unity for a $p_T>3$GeV cut, as applied by the CMS collaboration. A significant uncertainty in our calculations is associated with the values of the temperature where most of the $\psi(2S)$ regeneration occurs, the quantitative temperature dependence of its inelastic width.