Observation of nuclear suppression in coherent Υ (1S) photoproduction off heavy nuclei at the LHC

The first measurement of coherent Υ (1S) meson photoproduction off heavy nuclei is performed using ultraperipheral lead-lead collisions collected by the CMS experiment at a nucleon-nucleon center-of-mass energy of 5. 02 TeV. The nuclear gluonic structure is probed at a nucleon momentum fraction of order x 10^-3, determined by the kinematics of the process. Owing to the large Υ (1S) mass, the measurement reaches the highest scale accessible so far through coherent vector-meson photoproduction, μ² = 22. 4 GeV², where nonlinear quantum chromodynamics effects are expected to be minimal. In the Υ (1S) rapidity range y 1, the ratio of the measured photoproduction cross section to a baseline model prediction that neglects nuclear effects is S⏤ (₁ₒ) = 0. 25 0. 06 (stat) 0. 02 (syst), thereby demonstrating nuclear suppression in this process. Expressed in terms of a nuclear gluon suppression factor, the result yields RgPb (x 10^-3, μ² = 22. 4 GeV²) = 0. 55 0. 12 (stat) 0. 02 (syst). The measured RgPb is only slightly larger than the values previously reported for coherent ϕ photoproduction, despite the probed μ² differing by approximately two orders of magnitude.