Forward J/ $\psi$ in ultraperipheral collisions with the ALICE detector during LHC Run 2

Ragoni, Simone (2022). Forward J/ $\psi$ in ultraperipheral collisions with the ALICE detector during LHC Run 2. University of Birmingham. Ph.D.

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Abstract

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 conjuction 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- $x$ $\simeq 10^{−2}$ and $10^{−5}$ . In addition, the dataset 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 photopro- duced J/ $\psi$ . It is found that coherent J/ $\psi$ polarisation is measured to be consistent with transverse polarisation.