ψ(2S) Production at the LHC

Abstract

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