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 SΥ(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 RgPb(x≈$ 10$⁻³$, $μ^2$ = 22.4 GeV$^2$) = 0.55 $±$ 0.12 (stat) $±$ 0.02 (syst). The measured $R_g^Pb$ is only slightly larger than the values previously reported for coherent $ϕ$ photoproduction, despite the probed $μ^2$ differing by approximately two orders of magnitude.