Observation of nuclear suppression in coherent ${\rm Y}$(1S) photoproduction off heavy nuclei at the LHC

Abstract

The first measurement of coherent ${\rm Y}$(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 ${\rm Y}$(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 ${\rm Y}$(1S) rapidity range $|y|$ $<$ 1, the ratio of the measured photoproduction cross section to a baseline model prediction that neglects nuclear effects is $S_{{\rm Y}\text{(1S)}}$ = 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 $\varphi$ photoproduction, despite the probed ${\mu }^2$ differing by approximately two orders of magnitude.

Comments: Submitted to Physical Review Letters. All figures and tables can be found at http://cms-results.web.cern.ch/cms-results/public-results/publications/HIN-24-013 (CMS Public Pages)