Probing Gluon Fluctuations in Nuclei with the First Energy-Dependent Measurement of Incoherent J/ψ Photoproduction in Ultraperipheral PbPb Collisions

초록

Incoherent / photoproduction in heavy ion ultraperipheral collisions (UPCs) provides a sensitive probe of localized, fluctuating gluonic structures within heavy nuclei. This Letter reports the first measurement of the photon-nucleon center-of-mass energy (⁢N) dependence of this process in PbPb UPCs at a nucleon-nucleon center-of-mass energy of 5.02 TeV, using 1.52  nb−1 of data recorded by the CMS experiment. The measurement covers a wide ⁢N range of ≈40–400  GeV, probing gluons carrying a fraction of nucleon momentum down to an unexplored regime of 6.5 ×10−5. Compared to baseline predictions neglecting nuclear effects, the measured cross sections exhibit significantly greater suppression at lower . Additionally, the ratio of incoherent to coherent photoproduction is found to be constant across the probed ⁢N and range, disfavoring the establishment of the black disk limit. This Letter provides critical insights into the -dependent evolution of fluctuating gluonic structures within nuclei and calls for further advancements in theoretical models incorporating nuclear shadowing and gluon saturation.

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