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

The ALICE Collaboration 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 $\psi(2S)\rightarrow{\rm J}/\psi \pi^+\pi^-$ 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 $\mu$b$^{-1}$. The cross section for coherent $\psi(2S)$ production in the rapidity interval $-0.9 < y < 0.9$ is ${\rm d}\sigma^{\rm coh}_{\psi(2S)}/{\rm d}y = 0.83\pm0.19 {\rm (stat+syst)}$ mb. The $\psi(2S)/{\rm J}/\psi$ coherent cross section ratio is 0.34$^{+0.08}_{-0.07}{\rm (stat+syst)}$. The obtained cross section and the $\psi(2S)/{\rm J}/\psi$ coherent cross section ratio are compared to those from theoretical models.