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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.
The study of vector meson photoproduction in UPCs is of great interest since it is sensitive to low-x gluons. The data set collected with ultra-peripheral proton-lead collisions in ALICE is particularly useful to probe for gluon saturation. The energy dependence of the exclusive photoproduction of J/ψ mesons off proton targets as a function of the centre-of-mass energy of the photon-proton system, Wγp, measured in pPb collisions at 5.02 TeV will be shown, together with the recent data analysis at 8.16 TeV.
The measurements carried out using the ultra-peripheral Pb–Pb data ad- dress both gluon saturation and nuclear shadowing. Using the Run 2 data, the ALICE Collaboration has carried out various measurements on coherent J/ψ photoproduction, including the rapidity-differential cross section, and the measurement of the neutron-dependence, and that of the polarisation. The data have been compared to models incorporating nuclear shadowing effects and gluon saturation. In addition, the ALICE Collaboration has new results on the t-dependence of incoherent J/ψ photoproduction. The ALICE data provide un- precedented information about the gluonic content of the target, observing for the first time subnucleonic fluctuations and the onset of the gluon shadowing in the lead target. Both of these observations can also be linked to gluon shadowing at low Bjorken-x. These results and prospects for future UPC measurements in the coming years will be presented.
Refreshments will be served at 10:30
Michelangelo Mangano, Jan Fiete Grosse-Oetringhaus and Pedro Silva