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

TL;DR

This paper reports the first measurement of coherent $$ photoproduction off heavy nuclei at the LHC, revealing nuclear suppression effects at high energy scales and probing gluon distributions at very small momentum fractions.

Contribution

It provides the first experimental evidence of nuclear suppression in $$(1S) photoproduction at high scales, advancing understanding of gluon distributions in heavy nuclei.

Findings

Measured nuclear suppression factor $S_{}$(1S) = 0.25 ± 0.06 (stat) ± 0.02 (syst)

Determined gluon suppression factor $R_g^{Pb}$ = 0.55 ± 0.12 (stat) ± 0.02 (syst)

Observed suppression at a scale of 22.4 GeV$^2$, consistent with quantum chromodynamics expectations.

Abstract

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.