# Probing gluon pdf at x->0 with ultraperipheral PbPb collisions at 5.02 TeV in CMS

- Institution: CERN
- Department: LHC Seminar
- Speaker: Andre Govinda Stahl Leiton
- Year: 2022
- Language: English
- Source: [CERN](https://videos.cern.ch/record/3015348)
- Licence: [CERN](http://copyright.web.cern.ch)
- Who can watch: Teens profiles and older (sources give no ratings; placed by Invisible Network from the source's own words)

We will describe a new measurement of coherent J/Psi photoproduction in LHC collisions with PbPb collisions at 5.02 TeV collected by CMS. In UPCs, the coherent heavy-flavor vector meson production via photon-nuclear interactions is of particular interest, as its cross section can directly probe the nuclear gluon density function. Since the two nuclei in symmetric UPCs can serve both as a photon-emitter projectile and a target, one has to disentangle contributions involving high- and low-energy photon-nucleus interactions to probe extremely small-x regimes. In this study, a new forward neutron tagging technique is utilized to solve this two-way ambiguity. We first review the technique of controlling the impact parameter of UPCs via forward neutron tagging established in the measurement of photon-photon QED processes. Then, we present the coherent J/psi photoproduction cross section as a function of photon-Pb center-of-mass energy in UPCs up to about a record energy of 400 GeV, corresponding to x ~ 5x10^{-5}. We discuss the physics implication of this new result, as well as opportunities in future LHC heavy ion runs.

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## Lectures

1. [Probing gluon pdf at x->0 with ultraperipheral PbPb collisions at 5.02 TeV in CMS](https://invisiblenetwork.ai/learn/cern/3015348/a1d683d65d31), 59 min (CERN's player)