CMS-HIN-24-003

CMS-HIN-24-003 ; CERN-EP-2025-095
Measurement of D$^0$ meson photoproduction in ultraperipheral heavy ion collisions
CMS Collaboration
10 September 2025
Submitted to Phys. Rev. Lett.
Abstract: This Letter reports the first measurement of photonuclear D$^0$ meson production in ultraperipheral heavy ion collisions. The study is performed using lead-lead collision data, with an integrated luminosity of 1.34 nb$^{-1}$, collected by the CMS experiment at a nucleon-nucleon center-of-mass energy of 5.36 TeV. Photonuclear events, where one of the colliding nuclei breaks up and the other remains intact, are selected based on breakup neutron emissions and by requiring no particle activity in a large rapidity interval in the direction of the photon-emitting nucleus. The D$^0$ mesons are reconstructed via the $ \mathrm{D^0}\to\mathrm{K^-}\pi^{+} $ decay channel, with the cross section measured as a function of D$^0$ meson transverse momentum and rapidity. The results are compared with next-to-leading-order perturbative QCD calculations that employ recent parametrizations of the lead nuclear parton distribution functions, as well as with predictions based on the color glass condensate framework. This measurement is the first photonuclear collision study characterizing parton distribution functions of lead nuclei for parton fractional momenta $ x $ (relative to the nucleon) ranging approximately from a few 10$^{-4} $ to 10$^{-2} $ for different hard energy scale $ Q^{2} $ selections. This work is dedicated to the memory of our colleague and friend, Michele Arneodo. Michele was a generous mentor, a passionate physicist, and a kind soul who inspired both students and collaborators with his wisdom, warmth, and dedication to science. His contributions to diffractive physics and his support for young scientists leave a lasting legacy. He will be deeply missed.
Links: e-print arXiv:2509.08626 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; CADI line (restricted) ;

Figures	Summary	References	CMS Publications

Figures
png pdf	Figure 1: Invariant mass distribution of D$^0$ mesons with 5 $ < p_{\mathrm{T}} < $ 8 GeV and 0.0 $ < y < $ 1.0. The description of the fit template is provided in the text.
png pdf	Figure 2: Cross sections for D$^0$ production in $ \gamma\mathrm{N} $ events ($ \mathrm{X}\mathrm{n}0\mathrm{n}+0\mathrm{n}\mathrm{X}\mathrm{n} $) in three $ p_{\mathrm{T}} $ intervals. Vertical bars (brackets) indicate statistical (systematic) uncertainties. Sub-panels show theory-over-data ratios: the middle panel compares G$ \gamma $A-FONLL with EPPS21 lead nPDFs and a CGC calculation [15], while the bottom panel shows G$ \gamma $A-FONLL with CT18NLO proton PDFs. Light-shaded bands reflect scale variations, hatched bands represent PDF and nPDF uncertainties. Vertical bars (brackets) represent statistical (systematic) uncertainties on data.
png pdf	Figure 2-a: Cross sections for D$^0$ production in $ \gamma\mathrm{N} $ events ($ \mathrm{X}\mathrm{n}0\mathrm{n}+0\mathrm{n}\mathrm{X}\mathrm{n} $) in three $ p_{\mathrm{T}} $ intervals. Vertical bars (brackets) indicate statistical (systematic) uncertainties. Sub-panels show theory-over-data ratios: the middle panel compares G$ \gamma $A-FONLL with EPPS21 lead nPDFs and a CGC calculation [15], while the bottom panel shows G$ \gamma $A-FONLL with CT18NLO proton PDFs. Light-shaded bands reflect scale variations, hatched bands represent PDF and nPDF uncertainties. Vertical bars (brackets) represent statistical (systematic) uncertainties on data.
png pdf	Figure 2-b: Cross sections for D$^0$ production in $ \gamma\mathrm{N} $ events ($ \mathrm{X}\mathrm{n}0\mathrm{n}+0\mathrm{n}\mathrm{X}\mathrm{n} $) in three $ p_{\mathrm{T}} $ intervals. Vertical bars (brackets) indicate statistical (systematic) uncertainties. Sub-panels show theory-over-data ratios: the middle panel compares G$ \gamma $A-FONLL with EPPS21 lead nPDFs and a CGC calculation [15], while the bottom panel shows G$ \gamma $A-FONLL with CT18NLO proton PDFs. Light-shaded bands reflect scale variations, hatched bands represent PDF and nPDF uncertainties. Vertical bars (brackets) represent statistical (systematic) uncertainties on data.
png pdf	Figure 2-c: Cross sections for D$^0$ production in $ \gamma\mathrm{N} $ events ($ \mathrm{X}\mathrm{n}0\mathrm{n}+0\mathrm{n}\mathrm{X}\mathrm{n} $) in three $ p_{\mathrm{T}} $ intervals. Vertical bars (brackets) indicate statistical (systematic) uncertainties. Sub-panels show theory-over-data ratios: the middle panel compares G$ \gamma $A-FONLL with EPPS21 lead nPDFs and a CGC calculation [15], while the bottom panel shows G$ \gamma $A-FONLL with CT18NLO proton PDFs. Light-shaded bands reflect scale variations, hatched bands represent PDF and nPDF uncertainties. Vertical bars (brackets) represent statistical (systematic) uncertainties on data.

Summary

To summarize, this Letter presented the first measurement of the inclusive, prompt and nonprompt, photonuclear D$^0$ meson cross section as a function of transverse momentum and rapidity in ultraperipheral heavy-ion collisions. Exploiting the clean environment of photonuclear interactions, where final state and hadronization effects are largely reduced compared to hadronic production, this measurement provides novel constraints on nuclear matter over a wide range of parton momentum fraction $ x $ and interaction energy scale $ Q^{2} $. Comparisons with theory provide discrimination among parton distribution function parametrizations and challenge calculations based on nonlinear quantum chromodynamics evolution at high $ Q^{2} $. By demonstrating both its experimental feasibility and theoretical relevance, this study establishes open heavy-flavor production in ultraperipheral LHC collisions as a powerful probe of parton dynamics in nuclei.

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