Coherent $\psi$(2S) photo-production in ultra-peripheral Pb-Pb collisions at $\sqrt{s}_{\rm NN}$ = 2.76 TeV

TL;DR

This paper reports the first measurement of the coherent $ ext{psi}(2S)$ photo-production cross section in ultra-peripheral Pb-Pb collisions at the LHC, providing new experimental data for charmonium production in nuclear environments.

Contribution

It presents the first experimental measurement of $ ext{psi}(2S)$ coherent photo-production cross section in ultra-peripheral heavy-ion collisions at the LHC.

Findings

Measured cross section: 0.83 ± 0.19 mb in the rapidity range -0.9 to 0.9.

Determined the $ ext{psi}(2S)$ to J/$ ext{psi}$ cross section ratio as 0.34^{+0.08}_{-0.07}.

Results are compared with theoretical model predictions.

Abstract

We have has performed the first measurement of the coherent $\psi$(2S) photo-production cross section in ultra-peripheral Pb-Pb collisions at the LHC. This charmonium excited state is reconstructed via the $\psi$(2S) $\rightarrow l^{+}l^{-}$ and $\psi$(2S) $\rightarrow$ J/$\psi \pi^{+}\pi^{-}$ decays, where the J/$\psi$ decays into two leptons. The analysis is based on an event sample corresponding to an integrated luminosity of about 22 $\mu\rm{b}^{-1}$. The cross section for coherent $\psi$(2S) production in the rapidity interval $-0.9<y<0.9$ is $\mathrm{d}\sigma_{\psi{\rm(2S)}}^{\rm coh}/\mathrm{d}y =0.83\pm 0.19\big(\mathrm{\rm{stat}+{\rm syst}}\big)$ mb. The $\psi$(2S) to J/$\psi$ coherent cross section ratio is $0.34^{+0.08}_{-0.07}(\rm{stat}+{\rm syst})$. The obtained results are compared to predictions from theoretical models.