Probing Gluon Fluctuations in Nuclei with the First Energy-Dependent Measurement of Incoherent (Formula presented) Photoproduction in Ultraperipheral PbPb Collisions

Incoherent (Formula presented) 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 ((Formula presented)) dependence of this process in PbPb UPCs at a nucleon-nucleon center-of-mass energy of 5.02 TeV, using (Formula presented) of data recorded by the CMS experiment. The measurement covers a wide (Formula presented) range of (Formula presented), probing gluons carrying a fraction (Formula presented) of nucleon momentum down to an unexplored regime of (Formula presented). Compared to baseline predictions neglecting nuclear effects, the measured cross sections exhibit significantly greater suppression at lower (Formula presented). Additionally, the ratio of incoherent to coherent photoproduction is found to be constant across the probed (Formula presented) and (Formula presented) range, disfavoring the establishment of the black disk limit. This Letter provides critical insights into the (Formula presented)-dependent evolution of fluctuating gluonic structures within nuclei and calls for further advancements in theoretical models incorporating nuclear shadowing and gluon saturation. © 2025 CERN, for the CMS Collaboration.