First measurement of the ratio of $\psi (2S)$-to-$J/\psi$ inclusive production in $p\mathrm{Ar}$ and $pp$ collisions at $\sqrt{s_{\mathrm{NN}}}=113\mathrm{GeV}$ with SMOG2

Abstract

A measurement of the $\psi (2S)$-to-$J/\psi$ production cross-section ratio is performed in proton-argon ($p\mathrm{Ar}$) and proton-proton ($pp$) collisions in fixed-target mode at $\sqrt{s_{\mathrm{NN}}}=113\mathrm{GeV}$. Data samples were collected by the LHCb experiment during argon and hydrogen gas injections in the SMOG2 storage cell, resulting in $p\mathrm{Ar}$ and $pp$ collisions, respectively. The $\psi (2S)$-to-$J/\psi$ production cross-section ratio is measured as a function of the charmonium transverse momentum, $p_{\mathrm{T}}$, and rapidity in the centre-of-mass system, $y^{\ast }$. The $\psi (2S)$-to-$J/\psi$ ratio in $p\mathrm{Ar}$ collisions over that in $pp$ collisions is measured to be $0.90\pm 0.04\pm 0.02$ for $-2.3<y^{\ast }<0.0$ and $0<p_{\mathrm{T}}<8\mathrm{GeV}/c$, indicating the emergence of nuclear effects in the $p\mathrm{Ar}$ system. This study acts as a baseline for the interpretation of future measurements with larger systems accessible by the LHCb experiment.

All figures and tables, along with any supplementary material and additional information, are available at https://lhcb-glance.cern.ch/alcm/public/analysis/full-details/6462/ (LHCb public pages)