Speaker
Description
The photo-production of heavy vector mesons in the electromagnetic interactions
of ultra-relativistic nuclei is thought to be sensitive to the gluon parton
distribution functions in the nucleus and thus to cold nuclear effects like shadowing
or gluon saturation. Besides the well known observations of vector meson
production in ultra-peripheral collisions (UPC), recently there were reports on
J/ψ photo-production in peripheral and semi-central nucleus-nucleus collisions
both at LHC and RHIC energies, observed as an excess over the expected normal
hadronic production at very low $p_T$ ($p_T$<300 MeV/$c$).
The ALICE Collaboration published J/$\psi$ $p_T$-integrated coherent photo-production
cross sections in peripheral and semi-central Pb-Pb collisions at $\sqrt{s_{NN}}$=2.76~TeV
at forward rapidity ($2.5
at mid-rapidity ($|y|<0.9$). Additionally, thanks to the very good tracking resolution
of the central barrel, the extraction of the $p_T$-differential cross section
was also possible, confirming the photo-production origin of the J/$\psi$ excess.
The quantitative understanding of this low-$p_T$ excess poses significant theoretical
challenges since the J/$\psi$ photo-production depends on the collision dynamics as well as on the photon-flux and
the photo-nuclear cross section.
In this talk we will present the latest ALICE measurements on $p_T$-integrated
and $p_T$-differential coherent J/$\psi$ photo-production cross section
in peripheral Pb-Pb collisions at the available LHC energies,
in comparison to the UPC measurements. These results will be discussed and compared to several model calculations of J/$\psi$ photo-production in Pb-Pb collisions with and without nuclear overlap.
