CMS-HIN-24-013

CMS-HIN-24-013 ; CERN-EP-2026-050
Observation of nuclear suppression in coherent $ \Upsilon{\textrm{(1S)}} $ photoproduction off heavy nuclei at the LHC
CMS Collaboration
7 April 2026
Submitted to Physical Review Letters
Abstract: The first measurement of coherent $ \Upsilon{\textrm{(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{\textrm{(1S)}} $ mass, the measurement reaches the highest scale accessible so far through coherent vector-meson photoproduction, $ \mu^2 = 22.4 \text{GeV}^2 $, where nonlinear quantum chromodynamics effects are expected to be minimal. In the $ \Upsilon{\textrm{(1S)}} $ rapidity range $ \|y\| < $ 1, the ratio of the measured photoproduction cross section to a baseline model prediction that neglects nuclear effects is $ S_{\Upsilon{\textrm{(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^{\mathrm{Pb}}_{\mathrm{g}}(x\approx10^{-3},\mu^2=22.4 \text{GeV}^2)= $ 0.55 $ \pm $ 0.12 (stat) $ \pm $ 0.02 (syst). The measured $ R^{\mathrm{Pb}}_{\mathrm{g}} $ 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.
Links: e-print arXiv:2604.05814 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; HepData record ; CADI line (restricted) ;

Figures	Summary	References	CMS Publications

Figures
png pdf	Figure 1: Invariant mass distribution of muon pairs with $ p_{\mathrm{T}} < $ 0.3 GeV (left), where coherent production processes are mostly dominant, and the transverse momentum distribution of muon pairs in the $ \Upsilon{\textrm{(1S)}} $ mass window 9.2 $ < m_{\mu^{+}\mu^{-}} < $ 9.7 GeV (right). The total fit model and the individual fit components are shown as separate curves and are described in the text. The goodness of fit is quantified by $ \chi^2/\text{dof} $, where dof denotes the number of degrees of freedom. In the $ m_{\mu^{+}\mu^{-}} $ fit, the $ \Upsilon{\textrm{(1S)}} $ signal is scaled for visibility whereas the $ \Upsilon{\textrm{(2S)}} $ and $ \Upsilon{\textrm{(3S)}} $ components are not drawn.
png pdf	Figure 1-a: Invariant mass distribution of muon pairs with $ p_{\mathrm{T}} < $ 0.3 GeV (left), where coherent production processes are mostly dominant, and the transverse momentum distribution of muon pairs in the $ \Upsilon{\textrm{(1S)}} $ mass window 9.2 $ < m_{\mu^{+}\mu^{-}} < $ 9.7 GeV (right). The total fit model and the individual fit components are shown as separate curves and are described in the text. The goodness of fit is quantified by $ \chi^2/\text{dof} $, where dof denotes the number of degrees of freedom. In the $ m_{\mu^{+}\mu^{-}} $ fit, the $ \Upsilon{\textrm{(1S)}} $ signal is scaled for visibility whereas the $ \Upsilon{\textrm{(2S)}} $ and $ \Upsilon{\textrm{(3S)}} $ components are not drawn.
png pdf	Figure 1-b: Invariant mass distribution of muon pairs with $ p_{\mathrm{T}} < $ 0.3 GeV (left), where coherent production processes are mostly dominant, and the transverse momentum distribution of muon pairs in the $ \Upsilon{\textrm{(1S)}} $ mass window 9.2 $ < m_{\mu^{+}\mu^{-}} < $ 9.7 GeV (right). The total fit model and the individual fit components are shown as separate curves and are described in the text. The goodness of fit is quantified by $ \chi^2/\text{dof} $, where dof denotes the number of degrees of freedom. In the $ m_{\mu^{+}\mu^{-}} $ fit, the $ \Upsilon{\textrm{(1S)}} $ signal is scaled for visibility whereas the $ \Upsilon{\textrm{(2S)}} $ and $ \Upsilon{\textrm{(3S)}} $ components are not drawn.
png pdf	Figure 2: Differential cross section for coherent $ \Upsilon{\textrm{(1S)}} $ production in the central $ \|y\| < $ 1.0 and forward 1.0 $ < \|y\| < $ 2.4 intervals. The vertical error bars and boxes represent the statistical and systematic uncertainties, respectively. The results are compared with theoretical predictions from the impulse approximation (IA) [13,59], STARLIGHT [59], and LTA model with two nuclear shadowing scenarios [14] in the top panel, and with three NLO pQCD calculations using the nuclear PDFs EPS09 or EPPS21, or a data-driven scaling constrained by $ \gamma\mathrm{p} $ data [14,44] in the middle panel. The bottom panel shows the ratio of the data to the IA prediction.
png pdf	Figure 3: Coherent $ \Upsilon{\textrm{(1S)}} $ photoproduction cross section per $ \gamma\mathrm{Pb} $ interaction as a function of $ W_{\gamma\mathrm{N}} $. The vertical error bars and boxes represent statistical and systematic uncertainties, respectively. The data are compared with calculations based on the impulse approximation (IA) [13,59], STARLIGHT [59], the NLO pQCD + Data-Driven ($ \gamma\mathrm{p} $) approach [44], and several models (named ``IP BFKL'', ``IP BK'' and ``CGC'') within the CGC framework [39,38,73].

Summary

In summary, the first measurement of coherent $ \Upsilon{\textrm{(1S)}} $ meson photoproduction off a heavy nucleus is presented. The analysis is performed by the CMS experiment using ultraperipheral lead-lead collisions at a nucleon-nucleon center-of-mass energy of 5.02 TeV. The cross section is measured in two intervals of the $ \Upsilon{\textrm{(1S)}} $ meson rapidity within $|y|<2.4$. In the $|y|<1$ interval, the ratio of the measured photoproduction cross section to a baseline model that neglects nuclear effects is $S_{ \Upsilon{\textrm{(1S)}} }=0.25\pm0.06$ stat $ \pm0.02$ syst, thereby demonstrating nuclear suppression in this process. The nuclear gluon distribution is probed at a nucleon momentum fraction of order $x{\sim}10^{-3}$ and at the unprecedented scale $\mu^2 = 22.4 $GeV$^2$ for coherent vector-meson photoproduction. The $ \Upsilon{\textrm{(1S)}} $ production cross section per photon-lead interaction yields a nuclear gluon suppression factor of $R_{g}^{Pb}(x\approx10^{-3},\mu^2=22.4$GeV$^2)=0.55\pm0.12$ stat $\pm0.02$ syst at $|y|<0.2$, where the kinematics correspond to a photon-nucleon center-of-mass energy of ${\approx}200$ GeV. The measured $R_{g}^{Pb}$ remains remarkably close to the values observed in coherent $\phi$ photoproduction, despite the probed $\mu^2$ differing by approximately two orders of magnitude. Models incorporating nuclear shadowing or gluon saturation effects tend to underestimate the degree of suppression. Predictions based on next-to-leading-order perturbative quantum chromodynamics calculations are consistent with the data only once the large uncertainties in the poorly constrained nuclear parton distribution functions are taken into account.

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