Measurement of $\psi(2S)$ nuclear modification at backward and forward rapidity in $p$$+$$p$, $p$$+$Al, and $p$$+$Au collisions at $\sqrt{s_{_{NN}}}=200$ GeV

TL;DR

This paper measures the nuclear modification of $ ext{psi}(2S)$ in small collision systems at 200 GeV, revealing potential final-state effects beyond cold nuclear matter influences, with results compared to theoretical models and higher-energy data.

Contribution

It provides the first detailed measurements of $ ext{psi}(2S)$ nuclear modification in $p$+Al and $p$+Au collisions at RHIC energies, highlighting possible final-state effects.

Findings

$ ext{psi}(2S)$ shows suppression in small systems.

Results differ from shadowing and transport model predictions.

Comparison with LHC data suggests energy-dependent effects.

Abstract

Suppression of the $J/\psi$ nuclear-modification factor has been seen as a trademark signature of final-state effects in large collision systems for decades. In small systems, the nuclear modification was attributed to cold-nuclear-matter effects until the observation of strong differential suppression of the $\psi(2S)$ state in $p/d$$+$$A$ collisions suggested the presence of final-state effects. Results of $J/\psi$ and $\psi(2S)$ measurements in the dimuon decay channel are presented here for $p$$+$$p$, $p$$+$Al, and $p$$+$Au collision systems at $\sqrt{s_{_{NN}}}=200$ GeV. The results are predominantly shown in the form of the nuclear-modification factor, $R_{pA}$, the ratio of the $\psi(2S)$ invariant yield per nucleon-nucleon collision in collisions of proton on target nucleus to that in $p$$+$$p$ collisions. Measurements of the $J/\psi$ and $\psi(2S)$ nuclear-modification factor are compared with shadowing and transport-model predictions, as well as to complementary measurements at Large-Hadron-Collider energies.