CMS-PAS-HIN-22-002

CMS-PAS-HIN-22-002
Photon-nucleus energy dependence of coherent J/$ \psi $ cross session in ultraperipheral PbPb collisions at 5.02 TeV with CMS
CMS Collaboration
6 December 2022

Abstract: To understand the structure and dynamics inside an atomic nucleus at its most fundamental level is a subject at the heart of research on the strong interaction and theory of quantum chromodynamics (QCD). Gluons, the strong interaction carrier, are found to be the dominant constituents inside a nucleus when it is probed at ultra-high energies. Quasi-real photons exchanged in the interactions of highly relativistic heavy ions are powerful and clean probes of the gluonic structures of nuclei and/or nucleons. We report the first measurement of the nuclear gluonic structure probed by high-energy photons via coherent J/$ \psi $ photoproduction as a function of photon-nucleon center-of-mass energy ($ W^{Pb}_{\gamma N} $) up to $ W^{Pb}_{\gamma N} \approx $ 400 GeV. Results are obtained with ultraperipheral PbPb collision data collected by the CMS experiment at the LHC during the 2018 PbPb run, corresponding to an integrated luminosity of 1.52 $ \mathrm{nb}^{-1} $. At high $ W^{Pb}_{\gamma N} $ the coherent J/$ \psi $ cross section is observed to rapidly approach an approximate plateau over 40 $ \leq W^{Pb}_{\gamma N}\leq $ 400 GeV. This observation may provide the first experimental evidence of the black disk limit reached in high energy photons scattering off heavy nuclei, or indicate a gluon density saturation inside the nucleus that is greater than predictions from state-of-the-art QCD theoretical models. The findings open a new domain of QCD studies in the high gluonic density limit and motivate the development of new theoretical models.
Links: CDS record (PDF) ; CADI line (restricted) ; These preliminary results are superseded in this paper, Submitted to PRL. The superseded preliminary plots can be found here.

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