CMS-PAS-HIN-24-013

CMS-PAS-HIN-24-013
First measurement of coherent photoproduction of $ \Upsilon(\mathrm{1S}) $ mesons in lead-lead ultraperipheral collisions at $ \sqrt{s_{\mathrm{NN}}} = $ 5.02 TeV
CMS Collaboration
6 May 2025

Abstract: Coherent photoproduction of heavy-flavor vector mesons in ultrarelativistic heavy ion collisions (UPCs) provides a direct probe of the gluonic structure of nuclei. This note presents the first measurement of coherent photoproduction of $ \Upsilon(1S) $ off heavy nuclei. The analysis uses 1.66 nb$^{-1}$ of data collected by the CMS experiment in 2018 at the CERN LHC in lead-lead collisions at a center-of-mass energy per nucleon-nucleon pair of 5.02 TeV. The differential cross section for coherent $ \Upsilon(1S) $ is presented as a function of rapidity ($ \|y\| $) up to $ \|y\| < $ 2.4. At photon-nucleus center-of-mass energies per nucleon $ W_{\gamma \mathrm{N}}^{\mathrm{Pb}} \approx $ 200 GeV, the coherent $ \Upsilon(1S) $ photoproduction cross section is found to be significantly suppressed compared to the expected results from $ \gamma $-proton interactions. Results of this study are compared to theoretical calculations based on quantum chromodynamics.
Links: CDS record (PDF) ; CADI line (restricted) ;

Figures & Tables	Summary	References	CMS Publications

Figures
png pdf	Figure 1: Invariant mass distributions of muon pairs for $ p_{\mathrm{T}} < $ 3.0 GeV (left) and $ p_{\mathrm{T}} < $ 0.3 GeV (right). The total fit model (black) as well as the individual components are shown with different curves.
png pdf	Figure 1-a: Invariant mass distributions of muon pairs for $ p_{\mathrm{T}} < $ 3.0 GeV (left) and $ p_{\mathrm{T}} < $ 0.3 GeV (right). The total fit model (black) as well as the individual components are shown with different curves.
png pdf	Figure 1-b: Invariant mass distributions of muon pairs for $ p_{\mathrm{T}} < $ 3.0 GeV (left) and $ p_{\mathrm{T}} < $ 0.3 GeV (right). The total fit model (black) as well as the individual components are shown with different curves.
png pdf	Figure 2: Transverse momentum distributions of muon pairs in the rapidity range of $ \|y\| < $ 1.0 (left) and 1.0 $ < \|y\| < $ 2.4 (right) with invariant mass within the $ \Upsilon{\textrm{(1S)}} $ mass window (9.2 $ < m_{\mu^+\mu^-} < $ 9.7 GeV). The total model (black) as well as each template are shown.
png pdf	Figure 2-a: Transverse momentum distributions of muon pairs in the rapidity range of $ \|y\| < $ 1.0 (left) and 1.0 $ < \|y\| < $ 2.4 (right) with invariant mass within the $ \Upsilon{\textrm{(1S)}} $ mass window (9.2 $ < m_{\mu^+\mu^-} < $ 9.7 GeV). The total model (black) as well as each template are shown.
png pdf	Figure 2-b: Transverse momentum distributions of muon pairs in the rapidity range of $ \|y\| < $ 1.0 (left) and 1.0 $ < \|y\| < $ 2.4 (right) with invariant mass within the $ \Upsilon{\textrm{(1S)}} $ mass window (9.2 $ < m_{\mu^+\mu^-} < $ 9.7 GeV). The total model (black) as well as each template are shown.
png pdf	Figure 3: Differential coherent $ \Upsilon{\textrm{(1S)}} $ cross section as a function of absolute rapidity. The vertical error bars and blue boxes represent the total and systematic uncertainties, respectively. The horizontal bars show the bin width with the bin centers located in the data average values. The data are compared with theoretical predictions from the impulse approximation [7], STARLIGHT [36], pQCD [23] and different treatments of nuclear shadowing [8]. The blue red bands indicate the nPDF-originating uncertainty of the pQCD + data-driven (blue dashed curve) and pQCD + EPPS21 prediction (red solid curve), respectively [23]. The lower panel illustrates the ratio of the data and the prediction by the impulse approximation.
png pdf	Figure 4: Coherent $ \Upsilon{\textrm{(1S)}} $ photoproduction cross section as a function of $ W_{\gamma \rm{N}}^{\rm{Pb}} $. The vertical bar represents the total uncertainty. The data are compared with different predictions using the CGC framework [49,50,51] as well as a data driven prediction which uses $ \Upsilon{\textrm{(1S)}} $ photoproduction at HERA and LHC in ep and pp collisions as input [23]. The impulse approximation and STARLIGHT predictions are included for comparison. On the upper horizontal axis, the corresponding Bjorken-x values are displayed.

Tables
png pdf	Table 1: Summary of systematic uncertainties.

Summary

This note presented the first measurement of exclusive coherent photoproduction of $ \Upsilon{\textrm{(1S)}} $ mesons in ultraperipheral collisions (UPCs) of heavy ions. Rapidity-differential photoproduction cross sections are reported for both central and forward rapidities of the $ \Upsilon{\textrm{(1S)}} $ meson at a center-of-mass energy per nucleon pair of $ \sqrt{\smash[b]{s_{_{\mathrm{NN}}}}} = $ 5.02 TeV. The measured cross section exhibits a substantial suppression relative to the central values of most theoretical models. The precision of the present measurement challenges the precision of some of the theoretical predictions used for the comparisons in this work. Finally, the photonuclear cross section at a photon-nucleus center-of-mass energy per nucleon of $ W_{\gamma \rm{N}}^{\rm{Pb}} \approx $ 200 GeV is extracted from the most central rapidity bin. This value is compared with theoretical predictions calculated in the color glass condensate formalism using the Balitsky--Kovchegov nonlinear evolution equation, as well as with predictions based on the Balitsky-Fadin--Kuraev--Lipatov linear evolution equation describing gluon density growth.

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