Speaker
Description
Ultra-peripheral collisions (UPC) are events characterised by large impact parameters between the two projectiles, larger than the sum of their radii. In UPCs, the protons and ions accelerated by the LHC do not interact via the strong interaction and can be regarded as sources of quasireal photons.
Vector meson (e.g. \jpsi and \psip) photoproduction in UPC is quite interesting since it is sensitive to the low-$x$ gluon density. The data set collected with pPb UPCs in ALICE is particularly useful to probe for gluon saturation. The energy dependence of the exclusive photoproduction of \jpsi off proton targets as a function of the centre-of-mass energy of the photon-proton system, $W_{\gamma p}$, measured in pPb collisions at 5.02 TeV will be shown, together with the recent points at 8.16 TeV.
The measurements carried out using Pb--Pb data instead address phenomena such as gluon shadowing. Owing to the statistics available from Run 2 data, the ALICE Collaboration has been able to carry out differential measurements, e.g. it has measured the rapidity-differential cross section of coherent \jpsi production in Pb--Pb UPCs, and the $t$-dependence, and compared it with models incorporating nuclear shadowing effects, thus providing a new tool to investigate the gluon structure at low Bjorken-$x$.
In addition, measurements of coherently photoproduced \jpsi in different conditions, characterised by different average impact parameters, e.g. the recent ALICE measurement of coherent \jpsi photoproduction in peripheral collisions, provide promising tools for the resolution of the ambiguity in Bjorken-$x$ which arises in symmetric A--A UPCs.
