Measurement of the prompt-production cross-section ratio $\sigma(\chi_{c2})/\sigma(\chi_{c1})$ in $p$Pb collisions at $\sqrt{s_{NN}}$ = 8.16 TeV

TL;DR

This paper presents the first measurement of the ratio of prompt $ ext{χ}_c2$ to $ ext{χ}_c1$ production cross-sections in proton-lead collisions at 8.16 TeV, providing insights into nuclear effects on charmonium states at LHC energies.

Contribution

It introduces the first measurement of the $ ext{χ}_c$ cross-section ratio in $p$Pb collisions at LHC energies, comparing it to $pp$ results to study nuclear effects.

Findings

The ratio $\sigma( ext{χ}_c2)/\sigma( ext{χ}_c1)$ is consistent with unity.

Nuclear effects on $ ext{χ}_c$ states are similar in $p$Pb and $pp$ collisions.

The measurement was performed via $ ext{J}/ extpsi$ decays to muons and photon reconstruction.

Abstract

This Letter reports the first measurement of prompt $\chi_{c1}$ and $\chi_{c2}$ charmonium production in nuclear collisions at Large Hadron Collider energies. The cross-section ratio $\sigma(\chi_{c2}) / \sigma(\chi_{c1})$ is measured in $p$Pb collisions at $\sqrt{s_{NN}}$ = 8.16 TeV, collected with the LHCb experiment. The $\chi_{c1,2}$ states are reconstructed via their decay to a $\rm{J}/\psi$ meson, subsequently decaying into a pair of oppositely charged muons, and a photon, which is reconstructed in the calorimeter or via its conversion in the detector material. The cross-section ratio is consistent with unity in the two considered rapidity regions. Comparison with a corresponding cross-section ratio previously measured by the LHCb collaboration in $pp$ collisions suggests that $\chi_{c1}$ and $\chi_{c2}$ states are similarly affected by nuclear effects occurring in $p$Pb collisions.