Probing ultra-dense gluonic matter via UPCs at CMS*

Gluons are found to become increasingly dominant constituents of nuclear matter when being probed at higher energies or smaller Bjorken-$x$ values. This has led to the question of the ultimate fate of nuclear gluonic structure and its interaction with external probes at extreme density regimes. In ultrarelativistic heavy ion collisions, the electromagnetic fields surrounding an ion, quantized as linearly polarized quasi-real photons, can interact with the ion at a distance greater than the sum of their radii, known as ultraperipheral collisions (UPCs). In UPCs, the coherent heavy flavor vector meson via photonuclear interactions is of particular interest, as its cross section can directly probe the nuclear gluon density function at leading order. In this talk, we will present recent results on coherent vector meson photoproduction in 2018 UPC PbPb collisions at 5.02 TeV from the CMS experiment including a new measurement of coherent J/$psi$ photoproduction with the forward neutron tagging technique. We will discuss the related physics implications, as well as exciting opportunities in future LHC heavy ion runs.