Incoherent 𝐽/𝜓 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 (𝑊𝛾N ) 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 𝑊𝛾N range of ≈40–400 GeV probing gluons carrying a fraction 𝑥 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 𝑥 . Additionally, the ratio of incoherent to coherent photoproduction is found to be constant across the probed 𝑊𝛾N and 𝑥 range, disfavoring the establishment of the black disk limit. This Letter provides critical insights into the 𝑥 -dependent evolution of fluctuating gluonic structures within nuclei and calls for further advancements in theoretical models incorporating nuclear shadowing and gluon saturation.
