Charmonium and $e^+e^-$ pair photoproduction at mid-rapidity in ultra-peripheral Pb-Pb collisions at $\sqrt{s_{\rm NN}}$=2.76 TeV

TL;DR

This paper reports measurements of charmonium photoproduction and electron-positron pair production in ultra-peripheral Pb-Pb collisions at 2.76 TeV, providing data that supports models with moderate nuclear gluon shadowing.

Contribution

First measurement of J/ψ and ψ' photoproduction cross sections at mid-rapidity in ultra-peripheral heavy-ion collisions at LHC energies.

Findings

Measured coherent and incoherent J/ψ cross sections consistent with models including moderate gluon shadowing.

Found the e+e- pair production cross section agrees with leading-order QED predictions.

Results support theoretical models with nuclear gluon shadowing effects.

Abstract

The ALICE Collaboration at the LHC has measured the J/$\psi$ and $\psi^{'}$ photoproduction at mid-rapidity in ultra-peripheral Pb-Pb collisions at $\sqrt{s_{\rm NN}}=2.76$ TeV. The charmonium is identified via its leptonic decay for events where the hadronic activity is required to be minimal. The analysis is based on an event sample corresponding to an integrated luminosity of about 23 $\mu\rm{b}^{-1}$. The cross section for coherent and incoherent J/$\psi$ production in the rapidity interval $-0.9<y<0.9$, are $\mathrm{d}\sigma_{{\rm J}/\psi}^{\rm coh}/\mathrm{d}y = 2.38^{+0.34}_{-0.24}\big(\mathrm{sta+sys}\big)$mb. and $\mathrm{d}\sigma_{{\rm J}/\psi}^{\rm inc}/\mathrm{d}y = 0.98^{+0.19}_{-0.17}\big(\mathrm{sta+sys}\big)$mb, respectively. The results are compared to theoretical models for J/$\psi$ production and the coherent cross section is found to be in good agreement with those models incorporating moderate nuclear gluon shadowing, such as EPS09 parametrization. In addition the cross section for the process $\gamma\gamma\rightarrow e^+ e^-$ has been measured and found to be in agreement with models implementing QED at leading order.