CMS reports first observation of suppressed coherent ϕ photoproduction in Pb-Pb UPCs

The CMS Collaboration reports the first observation of coherent ϕ(1020) meson photoproduction off heavy nuclei in ultraperipheral lead–lead collisions at a center-of-mass energy per nucleon pair of √sNN = 5.36 TeV, using Run 3 data. The measured differential cross section as a function of ϕ rapidity is suppressed by roughly a factor of five compared with an impulse-approximation baseline that treats the nucleus as a collection of free nucleons, and that suppression is about 2–3 times larger than predictions from state-of-the-art models that include gluon saturation and nuclear shadowing.

The measurement reconstructs the ϕ via the decay channel ϕ → K+K− and is reported in the rapidity interval 0.3 < |y| < 1.0, a window chosen because acceptance and efficiency limitations for charged-kaon reconstruction limit the measurable coherent yield. “The dataset corresponds to an integrated luminosity of 1.68 ± 0.09 µb−1,” the collaboration states, and the analysis probes gluons in the lead nucleus with momentum fractions around x ≈ 10−4 at the ϕ mass scale.

CMS presents the result as a differential cross section versus ϕ rapidity and contrasts it with multiple theory baselines. Models that incorporate nuclear shadowing or gluon-saturation mechanisms tend to give better agreement than the impulse approximation, but the data show a substantially stronger suppression than those models predict. “The observed suppression is about a factor of 2-3 larger than the state-of-art theoretical models, which consider gluon saturation effects, motivating possible new physics mechanisms at extremely high densities,” the collaboration notes, highlighting a sizable mismatch between measurement and current theory.

Physically the ϕ(1020) sits in an intermediate regime between perturbative and nonperturbative QCD, making this channel a sensitive probe of how gluon densities evolve at small x. CMS frames the measurement as “a powerful new tool to explore the dense nuclear matter in the transition regime from perturbative to nonperturbative QCD,” positioning the result alongside earlier small-x probes such as CMS’s coherent J/ψ study (Phys. Rev. Lett. 131 (2023) 262301) and ALICE’s photoproduction measurements of K+K− pairs (Phys. Rev. Lett. 132 (2024) 222303).

CMS documents the analysis under publication code HIN-24-009, titled "Observation of coherent ϕ(1020) meson photoproduction in ultraperipheral PbPb collisions at √sNN = 5.36 TeV," and reports the results have been published in Physical Review Letters 135, 262301 (2025). The collaboration says the ϕ tables and figures are provided in the collaboration material, and it points to these Run 3 UPC results as guidance for both further LHC measurements and for experiments at the upcoming Electron–Ion Collider.

For practitioners and theorists alike the bottom line is concrete: coherent ϕ photoproduction in Pb–Pb UPCs is observed, the yield in 0.3 < |y| < 1.0 is substantially below naive expectations, and current saturation/shadowing models undershoot that suppression by factors of 2 to 3. That gap is a clear target for follow-up: check the tabulated PRL numbers, scrutinize model inputs at x ≈ 10−4, and extend UPC measurements in Run 3 and beyond to pin down whether the discrepancy signals new high-density gluon dynamics or a missing ingredient in the modeling.