$\psi(2S)$ enhancement in p-Pb collision as an indication of QGP formation at the LHC

Abstract

Proton-nucleus collisions serve as an important baseline for the understanding and interpretation of the nucleus-nucleus collisions. These collisions have been employed to characterize the cold nuclear matter effects at SPS and RHIC energies for the past several years, as it was thought that Quark-Gluon Plasma (QGP) is not formed in such collisions. However, at the Large Hadron Collider (LHC), there seems a possibility that QGP is formed during proton-lead (p$-$Pb) collisions. In this work, we have derived an expression for gluon induced excitation of $J/\psi$ to $\psi(2S)$, using pNRQCD, and show that the relative enhancement of $\psi(2S)$ vis a vis $J/\psi$, especially at high $p_T$, gives further indication that the QGP is indeed formed in p$-$Pb collisions at the most central collisions at LHC energy. $J/\psi$ and $\psi(2S)$ suppression effects seen at ALICE are also qualitatively explained.