Orateur
Description
In 2015, the ALICE collaboration reported the first excess in the yield of $J/\psi$ at very low transverse momentum ($p_T<0.3\ GeV/c$) in the forward rapidity region ($ 2.5 < y <4 $) in peripheral Pb-Pb collisions at $\sqrt{s_{NN}} = 2.76$ TeV at the CERN LHC. [1] The coherent photo-production was proposed as the potential underlying physics mechanism. This mechanism is the main responsible for low-$p_T$ $J/\psi$ production in ultra-peripheral collisions but was never observed in more central collisions that are dominated by the hadronic interactions.
If the photo-production is confirmed as the origin of the excess, this will open up fundamental questions on the nature of the coherence in collisions with nuclear overlap. Furthermore, the $J/\psi$ from the coherent photo-production could become a new probe of the Quark and Gluon Plasma.
References
[1] ALICE Collaboration, J. Adam et al., “Measurement of an excess in the yield of $J/\psi$ at very low $p_T$ in Pb-Pb collisions at $\sqrt{s_{NN}} = 2.76$ TeV", Phys.Rev.Lett.116(2016) 222301, arXiv:1509.08802 [nucl-ex].
