Speaker
Description
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 at extreme density. In ultraperipheral collisions (UPCs) of relativistic heavy ions, the coherent heavy-flavor vector meson production via photon-nuclear interactions is of particular interest, since its cross section is sensitive to the nuclear gluon density. However, in symmetric UPCs, a two-way ambiguity in determining the photon-emitter and the target prevents the extraction of contributions involving high- and low-energy photon-nucleus interactions. This limitation, therefore, had so far reduced our capability to probe the extremely small-$x$ regime. In this talk, we will present the first measurement of coherent charmonium photoproduction, where the two-way ambiguity is solved by implementing a forward neutron tagging technique using UPC lead-lead collisions recorded by the CMS experiment at 5.02 TeV. Results of coherent J/$\psi$ production and a new measurement of coherent $\psi\mathrm{(2S)}$ production will be presented. The new results on $\psi\mathrm{(2S)}$ provide a heavier mass scale thus complementing the results on J/$\psi$ toward smaller Bjorken-$x$ values. We will discuss the physics implications of these results, as well as exciting opportunities in future LHC heavy ion runs.
| Category | Experiment |
|---|---|
| Collaboration (if applicable) | CMS |
