Observation of Nuclear Suppression in Coherent $ϒ$(1S) Photoproduction off Heavy Nuclei at the LHC

The first measurement of coherent $ϒ$⁡(1⁢S) 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 $ϒ$⁡(1⁢S) 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 $ϒ$⁡(1⁢S) rapidity range |𝑦| <1, the ratio of the measured photoproduction cross section to a baseline model prediction that neglects nuclear effects is 𝑆$_{ϒ⁡(1⁢S)}$=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$^{Pb}_𝑔$⁡(𝑥≈10$^{−3}$,𝜇$^2$=22.4  GeV$^2$)=0.55±0.12⁢(stat)±0.02(syst). The measured 𝑆$_{ϒ⁡(1⁢S)}$ 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.