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, μ^{2}=22.4  GeV^{2}, 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_{ϒ(1S)}=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 R_{g}^{Pb}(x≈10^{-3},μ^{2}=22.4  GeV^{2})=0.55±0.12(stat)±0.02(syst). The measured S_{ϒ(1S)} is only slightly larger than the values previously reported for coherent ϕ photoproduction, despite the probed μ^{2} differing by approximately 2 orders of magnitude.