The cross-sections of exclusive (coherent) photoproduction $J/\psi$ and $\psi(\mathrm{2S})$ mesons in ultra-peripheral PbPb collisions at a nucleon-nucleon centre-of-mass energy of $5.02 \mathrm{TeV}$ are measured using a data sample corresponding to an integrated luminosity of $228\pm10 \mathrm{\mu b}^{-1}$, collected by the LHCb experiment in 2018. The differential cross-sections are measured separately as a function of transverse momentum and rapidity in the nucleus-nucleus centre-of-mass frame for $J/\psi$ and $\psi(\mathrm{2S})$ mesons. The integrated cross-sections are measured to be $\sigma^\mathrm{coh}_{J/\psi} = 5.965 \pm 0.059 \pm 0.232 \pm 0.262 \mathrm{mb}$ and $\sigma^\mathrm{coh}_{\psi(\mathrm{2S})} = 0.923 \pm 0.086 \pm 0.028 \pm 0.040 \mathrm{mb}$, where the first listed uncertainty is statistical, the second systematic and the third due to the luminosity determination. The cross-section ratio is measured to be $\sigma^\mathrm{coh}_{\psi(\mathrm{2S})}/\sigma^\mathrm{coh}_{J/\psi} = 0.155 \pm 0.014 \pm 0.003$, where the first uncertainty is statistical and the second is systematic. These results are compatible with theoretical predictions.
Schematic diagrams for charmonium production in (a) coherent and (b) incoherent UPC heavy-ion collisions. For incoherent production, the pomeron is emitted from a single nucleon, indicated by the blue dot, and the nucleus typically dissociates, indicated by $A^*$. |
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Dimuon mass distribution for signal candidates in the rapidity range $2.0<y^{*}<4.5$. The data are overlaid with the result of the fit. |
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The $\ln( p_{\mathrm{T}} ^{*2})$ distribution of dimuon candidates in the $2.0<y^{*}<4.5$ range for (left) $ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu}$ candidates and (right) $\psi {(2S)}$ candidates. The data are overlaid with the result of the fit. |
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Differential cross-section as a function $y^*$ for coherent (left) $ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} $ and (right) $\psi {(2S)}$ photoproduction, compared to theoretical predictions. |
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Differential cross-section as a function of $ p_{\mathrm{T}} ^*$ within the rapidity range $2<y^*<4.5$ for coherent (left) $ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} $ and (right) $\psi {(2S)}$ photoproduction compared to theoretical predictions. |
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Differential cross-section ratio of $\psi {(2S)}$ to $ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu}$ coherent photoproduction as a function of $y^*$, compared to theoretical predictions. |
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Animated gif made out of all figures. |
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Total and coherent $ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu}$ and $\psi {(2S)}$ yields from the invariant mass and transverse momentum fits in different rapidity intervals. |
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Total and coherent $ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu}$ yields in different $ p_{\mathrm{T}} ^*$ intervals within the rapidity range $2.0<y^*<4.5$. |
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Total and coherent $\psi {(2S)}$ yields in different $ p_{\mathrm{T}} ^*$ intervals within the rapidity range $2.0<y^*<4.5$. |
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Summary of the systematic uncertainties. |
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The differential cross-section for coherent $ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu}$ production as a function of $y^*$. |
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The differential cross-section of coherent $\psi {(2S)}$ production as a function of $y^*$. |
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The differential cross-section ratio between $\psi {(2S)}$ and $ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu}$ coherent production as a function of $y^*$. The uncertainty due to luminosity determination cancels in the ratio. |
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The double differential cross-section of coherent $ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu}$ production as a function of $ p_{\mathrm{T}} ^*$ in rapidity range $2.0<y^*<4.5$. |
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The double differential cross-section of coherent $\psi {(2S)}$ production as a function of $ p_{\mathrm{T}} ^*$ in rapidity range $2.0<y^*<4.5$. |
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Created on 27 April 2024.