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

The first measurement of coherent $\Upsilon$(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 $x$ $\sim$ 10$^{-3}$, determined by the kinematics of the process. Owing to the large $\Upsilon$(1S) mass, the measurement reaches the highest scale accessible so far through coherent vector-meson photoproduction, $\mu^2$ = 22.4 GeV$^2$, where nonlinear quantum chromodynamics effects are expected to be minimal. In the $\Upsilon$(1S) rapidity range $\lvert y\rvert$ $\lt$ 1, the ratio of the measured photoproduction cross section to a baseline model prediction that neglects nuclear effects is $S_{\Upsilon\text{(1S)}}$ = 0.25 $\pm$ 0.06 (stat) $\pm$ 0.02 (syst), thereby demonstrating nuclear suppression in this process. Expressed in terms of a nuclear gluon suppression factor, the result yields $R_\text{g}^\text{Pb}$($x$ $\approx$ 10$^{-3}$, $\mu^2$ = 22.4 GeV$^2$) = 0.55 $\pm$ 0.12 (stat) $\pm$ 0.02 (syst). The measured $R_\text{g}^\text{Pb}$ is only slightly larger than the values previously reported for coherent $\phi$ photoproduction, despite the probed $\mu^2$ differing by approximately two orders of magnitude.