CMS-PAS-HIN-23-009

CMS-PAS-HIN-23-009
The Bjorken- $x$ evolution of gluon fields probed via incoherent $\mathrm{J}/\psi$ photoproduction in ultraperipheral PbPb collisions at $\sqrt{\smash[b]{s_{_{\mathrm{NN}}}}} =$ 5.02 TeV
CMS Collaboration
23 September 2024

Abstract: The cross section for incoherent $\mathrm{J}/\psi$ photoproduction off lead nuclei is measured to probe the event-by-event fluctuations of the gluon field density within heavy nuclei across a broad range of Bjorken- $x$ . Results are obtained using PbPb ultraperipheral collisions at a nucleon-nucleon center-of-mass energy of 5.02 TeV collected by the CMS experiment at the CERN LHC, corresponding to an integrated luminosity of 1.52 nb $^{-1}$ . The dependence of the cross section on the photon-nucleus center-of-mass energy per nucleon ( $W^{\mathrm{Pb}}_{\gamma\mathrm{N}}$ ) is studied from $W^{\mathrm{Pb}}_{\gamma\mathrm{N}} \approx$ 40 GeV up to 400 GeV, corresponding to a Bjorken- $x$ interval of 5.8 $\times$ 10 $^{-3}$ $> x >$ 6.5 $\times$ 10 $^{-5}$ . The ratio of incoherent to coherent $\mathrm{J}/\psi$ cross sections is found to be independent of $W^{\mathrm{Pb}}_{\gamma\mathrm{N}}$ and $x$ over the range studied. Significant nuclear suppression of incoherent $\mathrm{J}/\psi$ photoproduction is observed. For $x <$ 10 $^{-4}$ , the suppression is comparable to that observed in coherent $\mathrm{J}/\psi$ photoproduction, becoming more pronounced as $x$ increases.
Links: CDS record (PDF) ; CADI line (restricted) ; These preliminary results are superseded in this paper, Submitted to PRL. The superseded preliminary plots can be found here.

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