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CMS-HIN-23-013 ; CERN-EP-2024-167
Observation of double $ \mathrm{J}/\psi $ meson production in pPb collisions at $ \sqrt{\smash[b]{s_{_{\mathrm{NN}}}}} $ = 8.16 TeV
Submitted to Phys. Rev. D
Abstract: The first observation of the concurrent production of two $ \mathrm{J}/\psi $ mesons in proton-nucleus collisions is presented. The analysis is based on a proton-lead (pPb) data sample recorded at a nucleon-nucleon center-of-mass energy of 8.16 TeV by the CMS experiment at the CERN LHC and corresponding to an integrated luminosity of 174.6 nb$^{-1}$. The two $ \mathrm{J}/\psi $ mesons are reconstructed in their $ \mu^{+}\mu^{-} $ decay channels with transverse momenta $ p_{\mathrm{T}} > $ 6.5 GeV and rapidity $ |y| < $ 2.4. Events where one of the $ \mathrm{J}/\psi $ mesons is reconstructed in the dielectron channel are also considered in the search. The $ \mathrm{\mathrm{p} Pb}\to{\mathrm{J}/\psi} {\mathrm{J}/\psi} +\mathrm{X} $ process is observed with a significance of 5.3 standard deviations. The measured inclusive fiducial cross section, using the four-muon channel alone, is $ \sigma(\mathrm{\mathrm{p} Pb}\to{\mathrm{J}/\psi} {\mathrm{J}/\psi} +\mathrm{X})= $ 22.0 $ \pm $ 8.9 (stat) $ \pm $ 1.5 (syst) nb. A fit of the data to the expected rapidity separation for pairs of $ \mathrm{J}/\psi $ mesons produced in single (SPS) and double (DPS) parton scatterings yields $ \sigma^{\mathrm{\mathrm{p} Pb}\to{\mathrm{J}/\psi} {\mathrm{J}/\psi} +\mathrm{X}}_\text{SPS}= $ 16.5 $ \pm $ 10.8 (stat) $ \pm $ 0.1 (syst) nb and $ {\sigma^{\mathrm{\mathrm{p} Pb}\to{\mathrm{J}/\psi} \mathrm{J}/\psi} +\mathrm{X}}_\text{DPS}= $ 5.4 $ \pm $ 6.2 (stat) $ \pm $ 0.4 (syst) nb, respectively. This latter result can be transformed into a lower bound on the effective DPS cross section, closely related to the squared average interparton transverse separation in the collision, of $ \sigma_\text{eff} > $ 1.0 mb at 95% confidence level.
Figures & Tables Summary References CMS Publications
Figures

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Figure 1:
Representative diagrams of SPS (left) and DPS (center and right) contributions to the production of two $ \mathrm{J}/\psi $ mesons in pPb collisions.

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Figure 2:
Dimuon invariant mass distributions of the pPb\ events passing all selection criteria, shown as a 2D map (left panel, with the numbers indicating the counts per bin and the red circle indicating the double $ \mathrm{J}/\psi $ meson signal region) and its 1D projections for each pair (center and right panels). In the 1D projections, the measured data are represented by the points (with vertical bars showing their Poisson statistical uncertainties), the solid curve shows the overall signal$ + $background fit to the data, and the red hashed area shows the signal yields for the $ \mathrm{J}/\psi $ $ \mathrm{J}/\psi $ +$ \mathrm{X} $ process.

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Figure 2-a:
Dimuon invariant mass distributions of the pPb\ events passing all selection criteria, shown as a 2D map (left panel, with the numbers indicating the counts per bin and the red circle indicating the double $ \mathrm{J}/\psi $ meson signal region) and its 1D projections for each pair (center and right panels). In the 1D projections, the measured data are represented by the points (with vertical bars showing their Poisson statistical uncertainties), the solid curve shows the overall signal$ + $background fit to the data, and the red hashed area shows the signal yields for the $ \mathrm{J}/\psi $ $ \mathrm{J}/\psi $ +$ \mathrm{X} $ process.

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Figure 2-b:
Dimuon invariant mass distributions of the pPb\ events passing all selection criteria, shown as a 2D map (left panel, with the numbers indicating the counts per bin and the red circle indicating the double $ \mathrm{J}/\psi $ meson signal region) and its 1D projections for each pair (center and right panels). In the 1D projections, the measured data are represented by the points (with vertical bars showing their Poisson statistical uncertainties), the solid curve shows the overall signal$ + $background fit to the data, and the red hashed area shows the signal yields for the $ \mathrm{J}/\psi $ $ \mathrm{J}/\psi $ +$ \mathrm{X} $ process.

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Figure 3:
Dilepton invariant mass distributions of the pPb\ events passing all selection criteria, shown as a 2D map (left panel, with the numbers indicating the counts per bin and the red circle indicating the double $ \mathrm{J}/\psi $ meson signal region) and its 1D projections for each pair (center and right panels). In the 1D projections, the measured data are represented by the points (vertical bars showing their Poisson statistical uncertainties), the solid curve shows the overall signal$ + $background fit to the data, and the red hashed area shows the signal yields for the $ \mathrm{J}/\psi $ $ \mathrm{J}/\psi $ +$ \mathrm{X} $ process.

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Figure 3-a:
Dilepton invariant mass distributions of the pPb\ events passing all selection criteria, shown as a 2D map (left panel, with the numbers indicating the counts per bin and the red circle indicating the double $ \mathrm{J}/\psi $ meson signal region) and its 1D projections for each pair (center and right panels). In the 1D projections, the measured data are represented by the points (vertical bars showing their Poisson statistical uncertainties), the solid curve shows the overall signal$ + $background fit to the data, and the red hashed area shows the signal yields for the $ \mathrm{J}/\psi $ $ \mathrm{J}/\psi $ +$ \mathrm{X} $ process.

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Figure 3-b:
Dilepton invariant mass distributions of the pPb\ events passing all selection criteria, shown as a 2D map (left panel, with the numbers indicating the counts per bin and the red circle indicating the double $ \mathrm{J}/\psi $ meson signal region) and its 1D projections for each pair (center and right panels). In the 1D projections, the measured data are represented by the points (vertical bars showing their Poisson statistical uncertainties), the solid curve shows the overall signal$ + $background fit to the data, and the red hashed area shows the signal yields for the $ \mathrm{J}/\psi $ $ \mathrm{J}/\psi $ +$ \mathrm{X} $ process.

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Figure 4:
Distribution of rapidity separation (left) between the two $ \mathrm{J}/\psi $ mesons in data (black markers), fitted template of the expected DPS contribution (blue histogram) normalized to the data outside of the SPS-dominated region (as described in the text), and fitted SPS$ + $DPS distribution (red histogram). Distribution of azimuthal separation (right) between the two $ \mathrm{J}/\psi $ mesons measured in data (black markers) and expected DPS contribution (blue histogram), and SPS$ + $DPS sum (red histogram), with the relative normalization derived from the $ \Delta y $ distribution (left panel), as explained in the text.

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Figure 4-a:
Distribution of rapidity separation (left) between the two $ \mathrm{J}/\psi $ mesons in data (black markers), fitted template of the expected DPS contribution (blue histogram) normalized to the data outside of the SPS-dominated region (as described in the text), and fitted SPS$ + $DPS distribution (red histogram). Distribution of azimuthal separation (right) between the two $ \mathrm{J}/\psi $ mesons measured in data (black markers) and expected DPS contribution (blue histogram), and SPS$ + $DPS sum (red histogram), with the relative normalization derived from the $ \Delta y $ distribution (left panel), as explained in the text.

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Figure 4-b:
Distribution of rapidity separation (left) between the two $ \mathrm{J}/\psi $ mesons in data (black markers), fitted template of the expected DPS contribution (blue histogram) normalized to the data outside of the SPS-dominated region (as described in the text), and fitted SPS$ + $DPS distribution (red histogram). Distribution of azimuthal separation (right) between the two $ \mathrm{J}/\psi $ mesons measured in data (black markers) and expected DPS contribution (blue histogram), and SPS$ + $DPS sum (red histogram), with the relative normalization derived from the $ \Delta y $ distribution (left panel), as explained in the text.

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Figure 5:
Comparison of the effective DPS cross sections $ \sigma_\text{eff} $ extracted in this work (red arrow lower limit) with the same parameter derived in pp measurements of double- and triple-quarkonium production [37,14,12,79,15,80,77,16] (blue circles), as well as in final states with jets [81,82,85], $ \gamma+ $jets [83,84,86], $ \mathrm{W}+ $jets [19,20], and double W bosons [26] (black squares). The asterisk shown in a few legend entries indicates that the result has been obtained by more recent phenomenological analyses of the experimental data.
Tables

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Table 1:
Definition of the fiducial phase space for the $ \mathrm{\mathrm{p} Pb}\to{\mathrm{J}/\psi} {\mathrm{J}/\psi} +\mathrm{X} $ cross section measurement in pPb collisions at $ \sqrt{\smash[b]{s_{_{\mathrm{NN}}}}}= $ 8.16 TeV.

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Table 2:
Signal and background yields obtained from the likelihood fit procedure in the four-muon and dimuon-dielectron analyses over the $ m_{\mu\mu,\mathrm{e}\mathrm{e}}=$ 2.6-3.6 GeV mass range.

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Table 3:
Relative contributions to the systematic uncertainty in the $ \sigma(\mathrm{\mathrm{p} Pb}\to{\mathrm{J}/\psi} {\mathrm{J}/\psi} +\mathrm{X}) $ measurement. The last row lists the sum in quadrature of all components.

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Table 4:
Predictions for single and double $ \mathrm{J}/\psi $ meson production cross sections in SPS processes in pPb\ collisions at $ \sqrt{\smash[b]{s_{_{\mathrm{NN}}}}} = $ 8.16 TeV within the fiducial phase space of Table 1, obtained with the HELAC-ONIA code using the CT14NLO proton PDF and the reweighted EPPS16 lead nPDF, as explained in the text. The quoted uncertainty of the single (double) $ \mathrm{J}/\psi $ meson cross section includes nPDF and normalization (nPDF and scale) uncertainties, added in quadrature.
Summary
The first observation of the concurrent production of two $ \mathrm{J}/\psi $ mesons in proton-lead (pPb) collisions has been reported. Events with two $ \mathrm{J}/\psi $ mesons, each decaying into two muons, have been reconstructed in pPb\ collisions at a nucleon-nucleon center-of-mass-energy of $ \sqrt{\smash[b]{s_{_{\mathrm{NN}}}}} = $ 8.16 TeV. The data sample corresponds to an integrated luminosity of 174.6 nb$^{-1}$ collected by the CMS experiment at the CERN LHC. After all selection requirements, 8.5 $ \pm $ 3.4 events are found that confirm the production of the $ {\mathrm{J}/\psi} {\mathrm{J}/\psi} +\mathrm{X} $ final state. Events where the least energetic $ \mathrm{J}/\psi $ meson is reconstructed in the dielectron channel are also considered in the search, leading to a signal yield of 5.7 $ \pm $ 4.0 events. The statistical significance of the signal, relative to the background-only expectation, corresponds to 5.3 standard deviations (4.9 standard deviations in the four-muon channel alone). The fiducial cross section for $ {\mathrm{J}/\psi} {\mathrm{J}/\psi} +\mathrm{X} $ production is $ \sigma(\mathrm{\mathrm{p} Pb}\to{\mathrm{J}/\psi} {\mathrm{J}/\psi} +\mathrm{X})= $ 22.0 $ \pm $ 8.9 (stat) $ \pm $ 1.5 (syst) nb. This result is compared with the theoretical expectations for $ {\mathrm{J}/\psi} {\mathrm{J}/\psi} +\mathrm{X} $ production via the sum of contributions from single- and double-parton scatterings (SPS and DPS, respectively). Under the simplest assumption of factorization of multiple hard scattering probabilities in terms of SPS cross sections, the measured $ {\mathrm{J}/\psi} {\mathrm{J}/\psi} +\mathrm{X} $ cross section is consistent with the production of SPS$ + $DPS processes with fiducial cross sections $ \sigma^{\mathrm{\mathrm{p} Pb}\to{\mathrm{J}/\psi} {\mathrm{J}/\psi} +\mathrm{X}}_\text{SPS}= $ 16.5 $ \pm $ 10.8 (stat) $ \pm $ 0.1 (syst) nb and $ \sigma^{\mathrm{\mathrm{p} Pb}\to{\mathrm{J}/\psi} {\mathrm{J}/\psi} +\mathrm{X}}_\text{DPS}= $ 5.4 $ \pm $ 6.2 (stat) $ \pm $ 0.4 (syst) nb. The derived DPS cross section can be transformed into a lower bound on the effective DPS cross section parameter ($ \sigma_\text{eff} $) that is closely related to the squared average interparton transverse separation in the collision. A limit of $ \sigma_\text{eff} > $ 1.0 mb at 95% confidence level is set, which is consistent with $ \sigma_\text{eff} $ values obtained from double and triple quarkonium measurements in pp collisions. The present analysis supports the interest of exploiting the production of multiple heavy and/or high-$ p_{\mathrm{T}} $ particles in pPb collisions at the LHC as a novel means to study the dynamics of multiple independent hard scatterings.
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