CMS-BPH-21-004

CMS-BPH-21-004 ; CERN-EP-2021-215
Observation of triple J/$\psi$ meson production in proton-proton collisions at $\sqrt{s} = $ 13 TeV
CMS Collaboration
9 November 2021
NP 19 (2023) 338
Abstract: The first observation of the simultaneous production of three J/$\psi$ mesons in proton-proton (pp) collisions is presented, based on a data sample collected by the CMS experiment at a center-of-mass energy of 13 TeV and corresponding to an integrated luminosity of 133 fb$^{-1}$. The ${\mathrm{p}}{\mathrm{p}}\to\mathrm{J}/\psi\,\mathrm{J}/\psi\,\mathrm{J}/\psi\,\mathrm{X}$ process is observed with a significance above five standard deviations in final states with three $\mu\mu$ pairs. The measured inclusive fiducial cross section of $\sigma(\mathrm{pp}\to\mathrm{J}/\psi\,\mathrm{J}/\psi\,\mathrm{J}/\psi \,\mathrm{X})= $ 272 $^{+141}_{-104}$ (stat) $\pm$ 17 (syst) fb is compared to theoretical expectations for the production of three J/$\psi$ mesons in single- (SPS), double- (DPS), and triple- (TPS) parton scatterings. Assuming factorization of multiple hard-scattering probabilities in terms of SPS cross sections, the measured process is found to be dominated by DPS and TPS contributions, and an effective DPS cross section, related to the transverse distribution of partons in the proton, of ${\sigma_\mathrm{eff,DPS}} = $ 2.7 $^{+1.4}_{-1.0}$ (exp) $^{+1.5}_{-1.0}$ (theo) mb is determined.
Links: e-print arXiv:2111.05370 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; HepData record ; Physics Briefing ; CADI line (restricted) ;

Figures & Tables	Summary	References	CMS Publications

Figures
png pdf	Figure 1: Leading-order diagrams for inclusive triple-J/$\psi$ production in pp collisions via SPS (upper), DPS (middle), and TPS (lower) processes. The leftmost diagrams show triple prompt-J/$\psi$ processes. The remaining diagrams show (left to right) final states with increasing contributions of nonprompt J/$\psi$ mesons from beauty hadron decays. The symbols $\sigma ^{i\mathrm {p}j\mathrm {np}}_\mathrm {NPS}$ identify the number ($i$ and $j$) of prompt (p) and nonprompt (np) contributions to the cross section of each diagram.
png pdf	Figure 2: Invariant mass distributions for the three ${\mu^{+} \mu^{-}}$ pairs, ordered (left to right) by decreasing pair ${p_{\mathrm {T}}}$, in the selected events. The data are represented by the points with the vertical bars showing the (Poisson) statistical uncertainties. The solid (dotted) curve shows the overall fit to the data (in the extended mass range), and the red shaded area the fitted signal yield.
png pdf	Figure 2-a: Invariant mass distribution for the ${\mu^{+} \mu^{-}}$ pair with highest ${p_{\mathrm {T}}}$, in the selected events. The data are represented by the points with the vertical bars showing the (Poisson) statistical uncertainties. The solid (dotted) curve shows the overall fit to the data (in the extended mass range), and the red shaded area the fitted signal yield.
png pdf	Figure 2-b: Invariant mass distribution for the ${\mu^{+} \mu^{-}}$ pair with intermediate ${p_{\mathrm {T}}}$, in the selected events. The data are represented by the points with the vertical bars showing the (Poisson) statistical uncertainties. The solid (dotted) curve shows the overall fit to the data (in the extended mass range), and the red shaded area the fitted signal yield.
png pdf	Figure 2-c: Invariant mass distribution for the ${\mu^{+} \mu^{-}}$ pair with lowest ${p_{\mathrm {T}}}$, in the selected events. The data are represented by the points with the vertical bars showing the (Poisson) statistical uncertainties. The solid (dotted) curve shows the overall fit to the data (in the extended mass range), and the red shaded area the fitted signal yield.
png pdf	Figure 3: Comparison of the $ {\sigma _\mathrm {eff,DPS}} $ parameter extracted here (upper red circle) to those derived in midrapidity measurements of double-quarkonium and EW boson plus quarkonium production [23,65,26,64,25,66,67] (blue circles), as well as in final states with jets [60,61,38,20], $\gamma $+jets [62,63], W+jets [14,15], and same-sign W bosons [19] (black squares and triangles). The arrows indicate lower limits at 95% confidence level.

Tables
png pdf	Table 1: Dimuon invariant mass, proper decay-length, transverse momentum, rapidity, and azimuthal angle of each of the three J/$\psi$ candidates measured in the six triple-J/$\psi$ events passing our selection criteria.
png pdf	Table 2: Definition of the fiducial phase space for the triple-J/$\psi$ cross section measurement.
png pdf	Table 3: Relative contributions to the systematic uncertainty of the $\sigma ({{\mathrm{p}} {\mathrm{p}}} \to {\mathrm{J}/\psi} \,{\mathrm{J}/\psi} \,{\mathrm{J}/\psi} \,\mathrm{X})$ measurement. The last row gives the sum in quadrature of all components.
png pdf	Table 4: Predictions for single-, double-, and triple-J/$\psi$ production cross sections in SPS processes, which pass the fiducial criteria listed in Table 2, derived from the HELAC-Onia (ho) and MadGraph 5\_aMC@NLO (mg5nlo) matrix element calculators, complemented with the PYTHIA 8 (py8) parton shower, as described in the text.
png pdf	Table 5: Central values of the predictions for triple-J/$\psi$ production cross sections (in fb) and yields from SPS, DPS (for $ {\sigma _\mathrm {eff,DPS}} = $ 2.7 mb), and TPS (for $ {\sigma _\mathrm {eff,TPS}} = 0.82\, {\sigma _\mathrm {eff,DPS}} = $ 2.2 mb) processes, and their total sum. The values are given in columns for combinations of $n$ prompt and $(3-n)$ nonprompt J/$\psi$ mesons, the last column giving their corresponding sums. The DPS and TPS results are derived via Eqs. (3) and (4) from the SPS cross sections listed in Table 4 with $ {\sigma _\mathrm {eff,DPS}} $ chosen so that the sum of contributions yields a total cross section equal to the experimental value of $\sigma ^{3{\mathrm{J}/\psi}}_\text {tot}$. The expected yield, $N^{3{\mathrm{J}/\psi}}_\mathrm {NPS}$, is given for an effective integrated luminosity of $\epsilon \, {\mathcal {L}} _\mathrm {int} = $ 87 fb$^{-1} $ for each contributing process. The last column lists the total cross sections and yields for SPS, DPS, TPS and their sums.

Summary

In summary, the first observation of the simultaneous production of three J/$\psi$ mesons in proton-proton (pp) collisions has been reported. Events with three reconstructed J/$\psi$ mesons, each decaying into two muons, are selected from a data sample collected at $\sqrt{s} = $ 13 TeV corresponding to an integrated luminosity of 133 fb$^{-1}$. After all selection requirements, five events are found consistent with triple-J/$\psi$ production, plus one event assigned to combinatorial background, through a three-dimensional unbinned extended maximum likelihood fit of the measured dimuon invariant mass distributions. The statistical significance of the signal, including prompt as well as nonprompt (i.e., coming from beauty hadron decays) contributions, relative to the background-only expectation, is above five standard deviations. The cross section for inclusive triple-J/$\psi$ production, within the fiducial region defined in Table 2, is $\sigma(\mathrm{pp}\to\mathrm{J}/\psi\,\mathrm{J}/\psi\,\mathrm{J}/\psi \,\mathrm{X})= $ 272 $^{+141}_{-104}$ (stat) $\pm$ 17 (syst) fb. This result is compared to the theoretical expectations for triple-J/$\psi$ production via a sum of contributions from single- (SPS), double- (DPS), and triple- (TPS) parton scatterings. Under the simplest assumption of factorization of multiple hard-scattering probabilities in terms of SPS cross sections, the measured triple-J/$\psi$ cross section is consistent with the production via DPS ($\approx$74%), TPS ($\approx$20%), and SPS ($\approx$6%) processes for a value of the effective DPS cross section parameter, closely related to the transverse distribution of partons in the proton, of ${\sigma_\mathrm{eff,DPS}} = $ 2.7 $^{+1.4}_{-1.0}$ (exp) $^{+1.5}_{-1.0}$ (theo) mb. Within its large uncertainty, this parameter is consistent with those extracted from double-quarkonium measurements, but significantly smaller than the ${\sigma_\mathrm{eff,DPS}}$ values derived from DPS studies based on high-$ p_{\mathrm{T}}$ jets and/or electroweak bosons. This work presents a novel approach to study multiple hard-scatterings in pp collisions exploiting, for the first time, the simultaneous production of three heavy particles.

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