Forward J/$\psi$ in Ultraperipheral Collisions with the ALICE detector during LHC Run 2
The ALICE Collaboration has studied coherent J/$\psi$ photoproduction in Pb–Pb ultraperipheral collisions (UPC) since Run 1. The data sample collected during the LHC Run 2 at forward rapidity, i.e. $−4 < y <−2.5$, has opened new possibilities. New differential measurements are feasible, and it is then possible to study the properties of coherent J/$\psi$. A symmetric collision system such as Pb–Pb has an inherent ambiguity on the sign of the rapidity of the J/$\psi$ with respect to the ion which is emitting the photon. The analysis of coherent J/$\psi$ photoproduction in conjunction with neutron emission provides a viable new route to solve this ambiguity, and to probe for nuclear shadowing effects down to a Bjorken-$x$ of about $10^{−5}$. The two solutions with the forward rapidity acceptance of the ALICE detector are then sensitive to nuclear shadowing at Bjorken-$x \sim 10^{−2}$ and $10^{−5}$. In addition, the data set also allows for the study of the angular distributions of the decay muons of the J/$\psi$, thus providing a tool to assess the polarisation of coherently photoproduced J/$\psi$. It is found that coherent J/$\psi$ polarisation is measured to be consistent with transverse polarisation.