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| CMS-BPH-23-007 ; CERN-EP-2025-163 | ||
| Observation of $ \Upsilon\mathrm{(1S)} $ + Z associated production and measurement of the effective double-parton scattering cross section in proton-proton collisions at $ \sqrt{s}= $ 13 TeV | ||
| CMS Collaboration | ||
| 2 February 2026 | ||
| Submitted to Physical Review Letters | ||
| Abstract: The observation of associated production of an $ \Upsilon\mathrm{(1S)} $ meson with a Z boson and a measurement of the ratio of its fiducial cross section to the fiducial cross section of the Z boson are presented. Both the $ \Upsilon\mathrm{(1S)} $ meson and the Z boson are identified via decays into a pair of opposite-sign muons. The analysis is based on proton-proton (pp) collision data at $ \sqrt{s}= $ 13 TeV, collected with the CMS detector in 2016--2018 and corresponding to an integrated luminosity of 138 fb$ ^{-1} $. Using the production of the Z boson decaying into four muons as a normalization channel, the ratio of the fiducial cross sections $ \sigma(\mathrm{p}\mathrm{p}\to\mathrm{Z}+\Upsilon\mathrm{(1S)}) \mathcal{B}(\mathrm{Z}\to\mu^{+}\mu^{-}) \mathcal{B}(\Upsilon\mathrm{(1S)}\to\mu^{+}\mu^{-}) $ to $ \sigma(\mathrm{p}\mathrm{p}\to\mathrm{Z})\mathcal{B}(\mathrm{Z}\to4\mu) $ is measured to be $ \mathcal{R}_{\mathrm{Z}+ \Upsilon\mathrm{(1S)}}=( $ 21.1 $ \pm $ 5.5 (stat) $ \pm $ 0.6 (syst) $) \times 10^{-3} $, where stat and syst denote the statistical and systematic uncertainties, respectively. The result is used to extract the effective double-parton scattering cross section $ \sigma_\text{eff} = $ 13.0 $ ^{+7.7}_{-3.4} $ mb. In addition, for the first time, $ \sigma_\text{eff} $ is measured in bins of the transverse momentum of the $ \Upsilon\mathrm{(1S)} $ meson or of the Z boson. | ||
| Links: e-print arXiv:2602.0277 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; CADI line (restricted) ; | ||
| Figures | |
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Figure 1:
Dimuon invariant mass distributions for the $ \Upsilon\mathrm{(nS)}$ (left) and Z (right) candidates. In each panel, the data are represented by points, with vertical bars indicating the statistical uncertainties. The solid black curve is the overall fit to data. The red, yellow, and green curves correspond to the signal yields for the three $ \Upsilon\mathrm{(nS)} $ states, while the dash-dotted curve represents the background. |
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Figure 1-a:
Dimuon invariant mass distributions for the $ \Upsilon\mathrm{(nS)}$ (left) and Z (right) candidates. In each panel, the data are represented by points, with vertical bars indicating the statistical uncertainties. The solid black curve is the overall fit to data. The red, yellow, and green curves correspond to the signal yields for the three $ \Upsilon\mathrm{(nS)} $ states, while the dash-dotted curve represents the background. |
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Figure 1-b:
Dimuon invariant mass distributions for the $ \Upsilon\mathrm{(nS)}$ (left) and Z (right) candidates. In each panel, the data are represented by points, with vertical bars indicating the statistical uncertainties. The solid black curve is the overall fit to data. The red, yellow, and green curves correspond to the signal yields for the three $ \Upsilon\mathrm{(nS)} $ states, while the dash-dotted curve represents the background. |
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Figure 2:
Distributions of the $ \Delta y $ (left) and $ \Delta\phi $ (right) separations between the $ \Upsilon\mathrm{(1S)} $meson and the Z boson in data (black points with vertical bars indicating the statistical uncertainties), and the extracted DPS (green histogram), SPS (blue histogram), and SPS+DPS (red histogram) contributions from the template fit to data. The distributions and corresponding uncertainties are obtained after the application of $\textit{sPlot} $ weights from an unbinned template fit to the $ \Delta y $ and $ \Delta\phi $ variables. |
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Figure 2-a:
Distributions of the $ \Delta y $ (left) and $ \Delta\phi $ (right) separations between the $ \Upsilon\mathrm{(1S)} $meson and the Z boson in data (black points with vertical bars indicating the statistical uncertainties), and the extracted DPS (green histogram), SPS (blue histogram), and SPS+DPS (red histogram) contributions from the template fit to data. The distributions and corresponding uncertainties are obtained after the application of $\textit{sPlot} $ weights from an unbinned template fit to the $ \Delta y $ and $ \Delta\phi $ variables. |
png pdf |
Figure 2-b:
Distributions of the $ \Delta y $ (left) and $ \Delta\phi $ (right) separations between the $ \Upsilon\mathrm{(1S)} $meson and the Z boson in data (black points with vertical bars indicating the statistical uncertainties), and the extracted DPS (green histogram), SPS (blue histogram), and SPS+DPS (red histogram) contributions from the template fit to data. The distributions and corresponding uncertainties are obtained after the application of $\textit{sPlot} $ weights from an unbinned template fit to the $ \Delta y $ and $ \Delta\phi $ variables. |
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Figure 3:
The $ m_{4\mu} $ distribution in the normalization channel with vertical bars on the data points indicating the statistical uncertainties. The result of the full fit is shown with the solid black line, while the signal and background components are shown with the dashed red and dash-dotted blue lines, respectively. |
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Figure 4:
Measurements of $ \sigma_\text{eff} $ in bins of the $ \Upsilon\mathrm{(1S)} $meson (left) and Z boson (right) $ p_{\mathrm{T}} $. The data are represented by the points with the black vertical bars showing the total uncertainties, which are dominated by the statistical uncertainty. The horizontal bars represent the bin widths, and the $ x $ axis position of each marker corresponds to the weighted average of the $ p_{\mathrm{T}} $ distribution within the respective bin. The red line indicates the nominal value found for the inclusive $ \sigma_\text{eff} $, and the pink shaded region shows its total uncertainties. |
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Figure 4-a:
Measurements of $ \sigma_\text{eff} $ in bins of the $ \Upsilon\mathrm{(1S)} $meson (left) and Z boson (right) $ p_{\mathrm{T}} $. The data are represented by the points with the black vertical bars showing the total uncertainties, which are dominated by the statistical uncertainty. The horizontal bars represent the bin widths, and the $ x $ axis position of each marker corresponds to the weighted average of the $ p_{\mathrm{T}} $ distribution within the respective bin. The red line indicates the nominal value found for the inclusive $ \sigma_\text{eff} $, and the pink shaded region shows its total uncertainties. |
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Figure 4-b:
Measurements of $ \sigma_\text{eff} $ in bins of the $ \Upsilon\mathrm{(1S)} $meson (left) and Z boson (right) $ p_{\mathrm{T}} $. The data are represented by the points with the black vertical bars showing the total uncertainties, which are dominated by the statistical uncertainty. The horizontal bars represent the bin widths, and the $ x $ axis position of each marker corresponds to the weighted average of the $ p_{\mathrm{T}} $ distribution within the respective bin. The red line indicates the nominal value found for the inclusive $ \sigma_\text{eff} $, and the pink shaded region shows its total uncertainties. |
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Figure 5:
A summary of $ \sigma_\text{eff} $ results measured using quarkonia (blue) and harder objects (black). The present result is shown in red. Arrows indicate that only the lower limits on $ \sigma_\text{eff} $ are available. The error bar without the central value corresponds to a range of $ \sigma_\text{eff} $ extractions using different MC models from a CMS 4-jet analysis [67]. For the results marked with an asterisk, more recent theoretical reinterpretations based on improved calculations of the corresponding SPS cross section are plotted. |
| Tables | |
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Table 1:
Summary of selection criteria applied to the signal $ \mathrm{Z}(\to\mu^{+}\mu^{-}) + \Upsilon{\textrm{(1\mathrm{S})}}(\to\mu^{+}\mu^{-}) $ and normalization $ \mathrm{Z}\to4\mu $ channels. The ``$ \text{---} $'' sign means that the selection is not applied. |
| Summary |
| In summary, the first study of the associated production of an $ \Upsilon\mathrm{(1S)} $ meson and a $ \mathrm{Z} $ boson in proton-proton collisions at $\sqrt{s}= 13 \mathrm{TeV}$ has been reported. Events with an $ \Upsilon $ meson and a $ \mathrm{Z} $ boson, each decaying into a pair of opposite-sign muons, have been reconstructed. After all selection requirements, $ 34.6 \pm 9.0$ events of $ \mathrm{Z} + \Upsilon\mathrm{(1S)}$ production are observed. The statistical significance of the signal, relative to the background-only expectation, exceeds five standard deviations. The ratio of the fiducial cross section of $\mathrm{Z} + \Upsilon\mathrm{(1S)} \to \mu^-\mu^+\mu^-\mu^+$ production to the fiducial cross section of $\mathrm{Z} \to \mu^-\mu^+\mu^-\mu^+$ production is measured to be $ \mathcal{R}_{\mathrm{Z}+ \Upsilon\mathrm{(1S)}}=( $ 21.1 $ \pm $ 5.5 (stat) $ \pm $ 0.6 (syst) $) \times 10^{-3} $. The derived DPS cross section ratio, $ \mathcal{R}^{\mathrm{DPS}}_{\mathrm{Z}+ \Upsilon\mathrm{(1S)}}=( $ 20.2 $ \pm $ 5.4 (stat) $ \pm $ 0.6 (syst) $) \times 10^{-3} $, can be expressed in terms of the effective DPS cross section $ \sigma_\text{eff} = $ 13.0 $ ^{+7.7}_{-3.4} $ mb, which is closely related to the transverse distribution of partons in the proton. Such a value is consistent with $ \sigma_\text{eff}$ values obtained from double-quarkonium measurements in $ \mathrm{pp} $ and $ \mathrm{p\bar{p}} $ collisions [5,6,7,20,23,27]. For the first time, a binned measurement of the effective DPS cross section is performed, in bins of $ \Upsilon\mathrm{(1S)} $ $p_T$ or $ \mathrm{Z} $ $p_T$. Such measurements provide extra information that can be used in studies of double-parton distribution functions [48]. |
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