The CMS experiment at the Large Hadron Collider reports the first measurement of coherent Υ(1S) meson photoproduction from heavy nuclei. In this process a photon from one lead ion strikes the other lead ion as a whole and produces a Υ(1S) particle. The measured production rate is much lower than a simple baseline that treats the nucleus as a collection of free protons and neutrons. The paper reports a suppression factor S_{Υ(1S)} = 0.25 ± 0.06 (statistical) ± 0.02 (systematic). When translated to a ratio of nuclear to proton gluon densities, the nuclear gluon suppression is R_g^Pb(x ≈ 10^{-3}, μ^2 = 22.4 GeV^2) = 0.55 ± 0.12 (stat) ± 0.02 (syst).

The measurement used ultraperipheral lead-lead collisions, meaning the two nuclei passed each other far enough apart that they interacted electromagnetically rather than by the strong force. These collisions act like photon–nucleus collisions and give a clean way to study how photons make heavy quark–antiquark pairs that form a vector meson. The Υ(1S) signal was reconstructed from its decay into a pair of muons (μ+μ−) using data recorded in 2018 at a nucleon-nucleon energy of 5.02 TeV. The analyzed data correspond to an integrated luminosity of 1.66 ± 0.03 nb−1.

Coherent production is identified by the Υ(1S) having very low transverse momentum (momentum across the beam direction), typically pT < 0.2–0.3 GeV, because the photon interacted with the entire nucleus. The analysis extracted the Υ(1S) yield by fitting the muon-pair mass and transverse-momentum distributions. Simulated events from the STARLIGHT generator and a full CMS detector simulation based on GEANT4 were used to model the signal shapes and to build templates for coherent and incoherent production. The dominant background comes from quantum electrodynamics (QED) production of muon pairs (γγ → μ+μ−), and its contribution was estimated from data sidebands.