Recent results on J/$\psi$ photoproduction in ultra-peripheral collisions with ALICE

Abstract

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. Photoproduction of vector mesons (e.g. J/$\psi$ and $\psi$') in UPC is quite interesting since it is sensitive to the low-$x$ gluon density. Owing to the statistics available from Run 2 data, the ALICE Collaboration has been able to carry out differential measurements, such as the measurement of the rapidity-differential cross section of coherent J/$\psi$ production in Pb--Pb UPCs and the $t$-dependence of coherent and incoherent J/$\psi$, which are sensitive to the average of the spatial distribution of the gluons and its variance, respectively. In addition, the ALICE Collaboration has measured the energy dependence of the photonuclear cross section with a few different strategies, such as by measuring the rapidity-differential cross section of coherent J/$\psi$ production in different impact parameter ranges i.e. in ultra-peripheral and peripheral collisions, and in neutron emission classes. The latter two provide promising tools for the resolution of the ambiguity in Bjorken-$x$ which arises in symmetric A--A UPCs.