$J/\psi$ yield enhancement at very low transverse momentum in Pb-Pb collisions at $\sqrt{s_\mathrm{NN}} = \mathrm{5.02 \,TeV}$ with ALICE

Abstract

A large excess in the yield of $\mathrm{J/\psi}$ at very low transverse momentum (${p}_\mathrm{T}<\mathrm{300 MeV/}c$) and forward rapidity (2.5$<y<$4.0) was recently reported by $\mathrm{ALICE}$ using the LHC Run-1 data, in Pb-Pb collisions at $\sqrt{s_\mathrm{NN}} = \mathrm{2.76 \,TeV}$. This is suggestive of coherent $\mathrm{J/\psi}$ photo-production, similar to measurements in ultra-peripheral collisions (UPC), where the nuclei only interact electromagnetically. During the LHC Run-2, the measurement of the very low ${p}_\mathrm{T}$ $\mathrm{J/\psi}$ at mid-rapidity (-0.9$<y<$0.9) in the di-electron decay channel was possible thanks to the large sample of recorded Pb-Pb collisions. The central barrel detectors provide a good momentum resolution, and make the measurement sensitive to the key characteristics of $\mathrm{J/\psi}$ coherent photo-production, like the corresponding transverse momentum spectrum. In this talk, it will focus on the $\mathrm{J/\psi}$ production in the very low-${p}_\mathrm{T}$ range in peripheral Pb-Pb collisions at $\sqrt{s_\mathrm{NN}} = 5.02 \,TeV$. The ${p}_\mathrm{T}$ spectrum shape for coherent production will be shown and compared to UPC measurements.