CMS-PAS-HIN-25-002

CMS-PAS-HIN-25-002
Constraining low-$ x $ gluon densities with low-$ p_{\mathrm{T}} \rm D^{0} $ meson production in ultraperipheral lead-lead collisions at 5.36 TeV
CMS Collaboration
28 April 2025

Abstract: This analysis presents a rapidity-differential measurement of low transverse momentum ($ p_{\mathrm{T}} $) $ \rm D^{0} $ meson production in ultraperipheral lead-lead collisions at $ \sqrt{s_{_{NN}}} = $ 5.36 TeV. The study is performed in the $ \rm D^{0} $ meson $ p_{\mathrm{T}} $ range of 2 $ < p_{\mathrm{T}} < $ 5 GeV and in intervals of $ \rm D^{0} $ meson rapidity $ -$2 $ < y < $ 2. The results are compared to new predictions based on perturbative quantum chromodynamics, which are obtained using the most recent parametrization of the lead nuclear parton distribution functions, and to predictions based on the color glass condensate formalism, where nuclear suppression effects are dynamically generated. The extension of the kinematic range with respect to the previous measurement of photonuclear $ \rm D^{0} $ production, particularly in the low-$ p_{\mathrm{T}} $ regime, provides stronger constraints on nuclear shadowing effects and the possible emergence of gluon saturation at low $ p_{\mathrm{T}} $.
Links: CDS record (PDF) ; CADI line (restricted) ;

Figures & Tables	Summary	References	CMS Publications

Figures
png pdf	Figure 1: Mass fit for the $ \mathrm{D^0} $ yield extraction in the $ p_{\mathrm{T}} $-interval 2-5 GeV for $ \gamma+\textrm{N} $ events. A description of the fitting procedure for the yield extraction is included in the text.
png pdf	Figure 1-a: Mass fit for the $ \mathrm{D^0} $ yield extraction in the $ p_{\mathrm{T}} $-interval 2-5 GeV for $ \gamma+\textrm{N} $ events. A description of the fitting procedure for the yield extraction is included in the text.
png pdf	Figure 1-b: Mass fit for the $ \mathrm{D^0} $ yield extraction in the $ p_{\mathrm{T}} $-interval 2-5 GeV for $ \gamma+\textrm{N} $ events. A description of the fitting procedure for the yield extraction is included in the text.
png pdf	Figure 1-c: Mass fit for the $ \mathrm{D^0} $ yield extraction in the $ p_{\mathrm{T}} $-interval 2-5 GeV for $ \gamma+\textrm{N} $ events. A description of the fitting procedure for the yield extraction is included in the text.
png pdf	Figure 1-d: Mass fit for the $ \mathrm{D^0} $ yield extraction in the $ p_{\mathrm{T}} $-interval 2-5 GeV for $ \gamma+\textrm{N} $ events. A description of the fitting procedure for the yield extraction is included in the text.
png pdf	Figure 2: Mass fit for the $ \mathrm{D^0} $ yield extraction in the $ p_{\mathrm{T}} $-interval 2-5 GeV for $ \textrm{N}+\gamma $ events. A description of the fitting procedure for the yield extraction is included in the text.
png pdf	Figure 2-a: Mass fit for the $ \mathrm{D^0} $ yield extraction in the $ p_{\mathrm{T}} $-interval 2-5 GeV for $ \textrm{N}+\gamma $ events. A description of the fitting procedure for the yield extraction is included in the text.
png pdf	Figure 2-b: Mass fit for the $ \mathrm{D^0} $ yield extraction in the $ p_{\mathrm{T}} $-interval 2-5 GeV for $ \textrm{N}+\gamma $ events. A description of the fitting procedure for the yield extraction is included in the text.
png pdf	Figure 2-c: Mass fit for the $ \mathrm{D^0} $ yield extraction in the $ p_{\mathrm{T}} $-interval 2-5 GeV for $ \textrm{N}+\gamma $ events. A description of the fitting procedure for the yield extraction is included in the text.
png pdf	Figure 2-d: Mass fit for the $ \mathrm{D^0} $ yield extraction in the $ p_{\mathrm{T}} $-interval 2-5 GeV for $ \textrm{N}+\gamma $ events. A description of the fitting procedure for the yield extraction is included in the text.
png pdf	Figure 3: Double-differential $ \rm D^{0} $ photoproduction cross section ($ {\rm{d}}^2\sigma/{\rm{d}}y{\rm{d}}p_T $) for $ \gamma+\textrm{N} $ (left) and $ \textrm{N}+\gamma $ (right) events. Statistical uncertainties are shown in the vertical lines and the shaded regions represent the systematic uncertainties. The results are compared to the previous analysis [1].
png pdf	Figure 3-a: Double-differential $ \rm D^{0} $ photoproduction cross section ($ {\rm{d}}^2\sigma/{\rm{d}}y{\rm{d}}p_T $) for $ \gamma+\textrm{N} $ (left) and $ \textrm{N}+\gamma $ (right) events. Statistical uncertainties are shown in the vertical lines and the shaded regions represent the systematic uncertainties. The results are compared to the previous analysis [1].
png pdf	Figure 3-b: Double-differential $ \rm D^{0} $ photoproduction cross section ($ {\rm{d}}^2\sigma/{\rm{d}}y{\rm{d}}p_T $) for $ \gamma+\textrm{N} $ (left) and $ \textrm{N}+\gamma $ (right) events. Statistical uncertainties are shown in the vertical lines and the shaded regions represent the systematic uncertainties. The results are compared to the previous analysis [1].
png pdf	Figure 4: Cross section of $ \rm D^{0} $ meson production in data (black boxes) compared with the theoretical predictions calculated in the FONLL framework using the CTEQ18 proton PDF parametrization. The prediction with a charm quark mass of $ m_c = $ 1.3 GeV is shown in the left plot, and that with $ m_c = $ 1.5 GeV is shown on the right. The dark shade represents the uncertainty from the nPDF parametrization, whereas the lighter shade (solid lines) represents the uncertainty from scale variations on the FONLL calculation for $ m_c = $ 1.3 GeV ($ m_c = $ 1.5 GeV).
png pdf	Figure 4-a: Cross section of $ \rm D^{0} $ meson production in data (black boxes) compared with the theoretical predictions calculated in the FONLL framework using the CTEQ18 proton PDF parametrization. The prediction with a charm quark mass of $ m_c = $ 1.3 GeV is shown in the left plot, and that with $ m_c = $ 1.5 GeV is shown on the right. The dark shade represents the uncertainty from the nPDF parametrization, whereas the lighter shade (solid lines) represents the uncertainty from scale variations on the FONLL calculation for $ m_c = $ 1.3 GeV ($ m_c = $ 1.5 GeV).
png pdf	Figure 4-b: Cross section of $ \rm D^{0} $ meson production in data (black boxes) compared with the theoretical predictions calculated in the FONLL framework using the CTEQ18 proton PDF parametrization. The prediction with a charm quark mass of $ m_c = $ 1.3 GeV is shown in the left plot, and that with $ m_c = $ 1.5 GeV is shown on the right. The dark shade represents the uncertainty from the nPDF parametrization, whereas the lighter shade (solid lines) represents the uncertainty from scale variations on the FONLL calculation for $ m_c = $ 1.3 GeV ($ m_c = $ 1.5 GeV).
png pdf	Figure 5: Cross section of $ \rm D^{0} $ meson production in data (black boxes) compared with the theoretical predictions calculated in the FONLL framework using the EPPS21 nuclear PDF parametrization. The prediction with a charm quark mass of $ m_c = $ 1.3 GeV is shown in the left plot, and that with $ m_c = $ 1.5 GeV is shown on the right. The dark shade represents the uncertainty from the nPDF parametrization, whereas the lighter shade represents the uncertainty from scale variations on the FONLL calculation.
png pdf	Figure 5-a: Cross section of $ \rm D^{0} $ meson production in data (black boxes) compared with the theoretical predictions calculated in the FONLL framework using the EPPS21 nuclear PDF parametrization. The prediction with a charm quark mass of $ m_c = $ 1.3 GeV is shown in the left plot, and that with $ m_c = $ 1.5 GeV is shown on the right. The dark shade represents the uncertainty from the nPDF parametrization, whereas the lighter shade represents the uncertainty from scale variations on the FONLL calculation.
png pdf	Figure 5-b: Cross section of $ \rm D^{0} $ meson production in data (black boxes) compared with the theoretical predictions calculated in the FONLL framework using the EPPS21 nuclear PDF parametrization. The prediction with a charm quark mass of $ m_c = $ 1.3 GeV is shown in the left plot, and that with $ m_c = $ 1.5 GeV is shown on the right. The dark shade represents the uncertainty from the nPDF parametrization, whereas the lighter shade represents the uncertainty from scale variations on the FONLL calculation.
png pdf	Figure 6: Cross section of $ \rm D^{0} $ meson production in data (black boxes) compared with the theoretical prediction provided by FONLL with nNNPDF3.0 nuclear PDF parametrization. The dark shade represents the uncertainty from the nPDF parametrization, whereas the lighter shade represents the uncertainty from scale variations on the FONLL calculation.
png pdf	Figure 7: Ratio of $ \rm D^{0} $ meson production cross sections relative to FONLL predictions using the CTEQ18 proton PDF parametrization, which serves as the theoretical baseline for the absence of nuclear modification effects. The data (black boxes) are compared with FONLL predictions using EPPS21 (red band) and nNNPDF3.0 (purple band) for nuclear PDFs. Only the uncertainties on the nuclear PDFs are shown.
png pdf	Figure 8: Cross section of $ \rm D^{0} $ meson production in data (black boxes) compared with predictions based on the color glass condensate (CGC) formalism (red dotted line). The framework used for the predictions is presented in Ref. [22].

Tables
png pdf	Table 1: Summary of directional terminology used in this note.

Summary

This note presents a measurement of the cross section for inclusive (prompt and nonprompt) photonuclear production of $ \rm D^{0} $ mesons as a function of rapidity $ y $ in the $ \rm D^{0} $ $ p_{\mathrm{T}} $ range of 2 $ < p_{\rm T} < $ 5 GeV in ultraperipheral heavy-ion collisions (UPCs). This is the first measurement focusing on the low $ p_{\mathrm{T}} $ region, so far unexplored by previous analyses. The presented $ \rm D^{0} $ meson yields are corrected for detector acceptance and efficiency. The measured cross sections are compared to predictions computed in perturbative quantum chromodynamics (pQCD) using the fixed-order-next-to-leading-logarithm (FONLL) formalism with different parametrizations of parton distribution functions (PDFs). A rapidity-dependent suppression is observed as compared to predictions using proton PDFs, which is consistent with a suppression of low-$ x $ gluons. When compared to predictions that rely on the EPPS21 and nNNPDF3.0 nuclear PDF sets, the measured cross sections are slightly lower than the predicted ones, but still compatible within the uncertainties. Additionally, the data is compared with predictions calculated in the color-glass condensate formalism, which overestimates the measured cross section at low-$ x $. The measured data can be used to constrain the description of cold nuclear matter structure on the low $ x $ and low $ Q^2 $ values in the clean environment provided by photonuclear collisions.

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