Probing Gluon Fluctuations in Nuclei with the First Energy-Dependent Measurement of Incoherent <math display="inline"><mi>J</mi><mo>/</mo><mi>ψ</mi></math> Photoproduction in Ultraperipheral PbPb Collisions

Jain, Sandhya;Bein, Samuel;Benecke, Anna;Bethani, Agni;CMS;et.al.
(2025) Physical Review Letters — Vol. 135 (2025)

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Abstract
Incoherent J/$\psi$ photoproduction in heavy ion ultraperipheral collisions (UPCs), in which the photon interacts with localized, fluctuating gluonic hotspots rather than the entire nucleus, provides a unique probe of those fluctuations. This study presents the first measurement of the dependence of this photoproduction yield on the photon-nucleon center-of-mass energy ($W_\gamma^\mathrm{N}$), using PbPb UPCs at a nucleon-nucleon center-of-mass energy of 5.02 TeV. The data corresponds to an integrated luminosity of 1.52 nb$^{-1}$, marc:recorded by the CMS experiment. The measurement covers a wide $W_\gamma^\mathrm{N}$ range of $\approx$40-400 GeV, probing gluons carrying a fraction of nucleon momentum $x$ in the range between 5.9 $\times$ 10$^{-3}$ and 6.5 $\times$ 10$^{-5}$. The measured incoherent J/$\psi$ photoproduction cross section is suppressed relative to theoretical predictions without nuclear effects. However, the ratio of incoherent to coherent photoproduction remains constant across the probed $W_\gamma^\mathrm{N}$ and $x$ range. Together, these results pose significant challenges to current theoretical models which include gluon saturation or nuclear shadowing effects.
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Jain, S., Bein, S., Benecke, A., Bethani, A., Bruno, G., Caputo, C., de Favereau de Jeneret, J., Delaere, C., Donertas, I. S., Giammanco, A., Guzel, A. O., Jain, S., Lemaitre, V., Lidrych, J., Mastrapasqua, P., Tran, T. T., Turkcapar, S., CMS, & et al. (2025). Probing Gluon Fluctuations in Nuclei with the First Energy-Dependent Measurement of Incoherent <math display=“inline”><mi>J</mi><mo>/</mo><mi>ψ</mi></math> Photoproduction in Ultraperipheral PbPb Collisions. Physical Review Letters, 135. https://doi.org/10.1103/w9kp-f8xr (Original work published 2025)