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| CMS-PAS-HIN-23-013 | ||
| Observation of double-J/$\psi$ meson production in pPb collisions at 8.16 TeV | ||
| CMS Collaboration | ||
| 27 February 2024 | ||
| Abstract: The first observation of the concurrent production of two J/$\psi$ mesons in proton-nucleus collisions is presented. The analysis is based on a data sample recorded at a nucleon-nucleon center-of-mass energy of 8.16 TeV by the CMS experiment at the CERN LHC corresponding to an integrated luminosity of 174.6 nb$^{-1} $. The two J/$\psi$ mesons are reconstructed in their $ \mu^+\mu^- $ decay channel for transverse momenta $ p_{\mathrm{T}} > $ 6.5 GeV and rapidity $ |y| < $ 2.4. Events where the second J/$\psi$ meson is reconstructed in the dielectron channel are also considered in the search. The $ \mathrm{pPb}\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{pPb}\to\mathrm{J}/\psi\mathrm{J}/\psi+\mathrm{X})= $ 22.0 $ \pm $ 8.9 $ $ (stat) $ \pm 1.5 $ (syst) nb. This result is compared to theoretical perturbative quantum chromodynamics predictions at next-to-leading-order accuracy, including nuclear parton distribution functions, for the production of two J/$\psi$ mesons in single- (SPS) and double- (DPS) parton scatterings. A fit of the data to the expected SPS and DPS kinematic distributions of the two J/$\psi$ mesons yields, $ \sigma^{\mathrm{pPb}\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{pPb}\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 an upper limit on the effective DPS cross section, which characterizes the area occupied by the interacting partons in the transverse space, of $ \sigma_\text{eff} > $ 1.0 mb at 95% confidence level. | ||
| Links: CDS record (PDF) ; CADI line (restricted) ; | ||
| Figures | |
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Figure 1:
Representative diagrams of DPS (left and center) and SPS (right) contributions to the production of two J/$\psi $ mesons in $ \mathrm{pPb} $ collisions. |
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Figure 2:
Invariant mass distributions for the leading (left) and subleading (right) J/$\psi $ mesons in the events passing all selection criteria. The data are represented by the points with the vertical bars showing the (Poisson) statistical uncertainties. The solid curve shows the overall fit to the data, and the red shaded area the Gaussian signal yields for the J/$\psi $ J/$\psi $ +$ \mathrm{X} $ process extracted from the 2D fit projection over the background (green dashed area). |
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Figure 3:
Invariant mass distributions for the $ {\mathrm{J}/\psi} \to\mu^{+}\mu^{-} $ (left) and $ {\mathrm{J}/\psi} \to\mathrm{e}^+\mathrm{e}^- $ (right) mesons in the events passing all selection criteria in the cross-check analysis. The data are represented by the points with the vertical bars showing the (Poisson) statistical uncertainties. The solid curve shows the overall fit to the data, and the red shaded area the Gaussian signal yields for the J/$\psi $ J/$\psi $ +$ \mathrm{X} $ process extracted from the 2D fit projection over the background (green dashed area). |
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Figure 4:
Left: Distribution of rapidity separations between the two 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 (see text), and fitted SPS$ \,+\, $DPS distribution (red histogram). Right: Distribution of azimuthal separations between the two J/$\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) to the same parameter derived in pp measurements of double- and triple-quarkonium production [36,78,12,72,15,73,70,16] (blue circles), as well as also in final states with jets [74,75], $ \gamma$+jets [76,77], W+jets [19,20], and double W bosons [26] (black squares). For the experimental results marked with a star, more recent theoretical reinterpretations based on more accurate calculations of the corresponding SPS cross section are plotted. |
| Tables | |
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Table 1:
Definition of the fiducial phase space for the $ \sigma(\mathrm{pPb}\to{\mathrm{J}/\psi} {\mathrm{J}/\psi} +\mathrm{X}) $ cross section measurement in $ \mathrm{pPb} $ collisions at $ \sqrt{\smash[b]{s_{_{\mathrm{NN}}}}}= $ 8.16 TeV. |
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Table 2:
Percent contributions to the systematic uncertainty of the $ \sigma(\mathrm{pPb}\to{\mathrm{J}/\psi} {\mathrm{J}/\psi} +\mathrm{X}) $ measurement from various sources. The last row lists the sum in quadrature of all components. |
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Table 3:
Predictions for single- and double- J/$\psi $ meson production cross sections in SPS processes, within the fiducial criteria of Table 1, in $ \mathrm{pPb} $\ collisions at $ \sqrt{\smash[b]{s_{_{\mathrm{NN}}}}} = $ 8.16 TeV obtained, as explained in the text, with the HELAC-ONIA code using the CT14nlo proton PDF and the reweighted EPPS16 lead nPDF. The quoted uncertainty of the single- (double-) J/$\psi $ cross section includes nPDF and normalization (nPDF and scale) uncertainties added in quadrature. |
| Summary |
| The first observation of the concurrent production of two J/$\psi $ mesons in proton-lead ($ \mathrm{pPb} $) collisions at a nucleon-nucleon center-of-mass-energy of $ \sqrt{\smash[b]{s_{_{\mathrm{NN}}}}} = $ 8.16 TeV has been reported. Events with two J/$\psi $ mesons, each decaying into two muons have been reconstructed. 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 second 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 cross section for $ {\mathrm{J}/\psi} {\mathrm{J}/\psi} +\mathrm{X} $ production, measured in the four-muon channel within a fiducial region defined in Table 1, is $ \sigma(\mathrm{pPb}\to{\mathrm{J}/\psi} {\mathrm{J}/\psi} +\mathrm{X})= $ 22.0 $ \pm $ 8.9 (stat) $ \pm $ 1.5 (syst) nb. This result is compared to the theoretical expectations for $ {\mathrm{J}/\psi} {\mathrm{J}/\psi} +\mathrm{X} $ production via the sum of contributions from single- (SPS), and double- (DPS) parton scatterings. Under the most economical 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{pPb}\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{pPb}\to{\mathrm{J}/\psi} {\mathrm{J}/\psi} +\mathrm{X}}_\text{DPS}= $ 5.4 $ \pm $ 6.2 (stat) $ \pm $ 0.4 (syst) nb, respectively. The derived DPS cross section can be transformed into a lower value of the effective DPS cross section parameter of $ \sigma_\text{eff} > $ 1.0 mb at 95% confidence level, closely related to the squared average interparton transverse separation in the collision. Such a value is consistent with $ \sigma_\text{eff} $ values obtained from double- and triple-quarkonium measurements in pp collisions. The present analysis confirms the interest of exploiting the production of multiple heavy and/or high-$ p_{\mathrm{T}} $ particles in $ \mathrm{pPb} $ collisions at the LHC as a novel means to study the dynamics of multiple independent hard scatterings. |
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