CMSBPH21003 ; CERNEP2023109  
Observation of new structure in the J/$\psi$J/$\psi$ mass spectrum in protonproton collisions at $ \sqrt{s} = $ 13 TeV  
CMS Collaboration  
12 June 2023  
Phys. Rev. Lett. 132 (2024) 111901  
Abstract: A search is reported for nearthreshold structures in the J/$\psi$J/$\psi$ invariant mass spectrum produced in protonproton collisions at $ \sqrt{s} = $ 13 TeV from data collected by the CMS experiment, corresponding to an integrated luminosity of 135 fb$^{1}$. A new structure is observed with a significance above 5 standard deviations at a mass of 6552 $ \pm $ 10 (stat) $ \pm $ 12 (syst) MeV. Another structure with even higher significance is found at a mass of 6927 $ \pm $ 9 (stat) $ \pm $ 4 (syst) MeV, which is consistent with the X(6900) resonance reported by the LHCb experiment and confirmed by the ATLAS experiment. Evidence for another new structure, with a local significance of 4.1 standard deviations, is found at a mass of 7287 $ ^{+20}_{18} $ (stat) $\pm$ 5 (syst) MeV. The masses and significances are obtained in a model without considering possible quantum mechanical interference between the resonances. Incorporating this interference provides a better description of the mass spectrum between the resonances and shifts the measured masses by up to 150 MeV.  
Links: eprint arXiv:2306.07164 [hepex] (PDF) ; CDS record ; inSPIRE record ; HepData record ; Physics Briefing ; CADI line (restricted) ; 
Figures  
png pdf 
Figure 1:
The J/$\psi$J/$\psi$ invariant mass spectrum in the range up to 9 GeV, with fits consisting of three signal functions ($ \mathrm{BW_1} $, $ \mathrm{BW_2} $ and $ \mathrm{BW_3} $) and a background model (see text). The left plot shows the fit without interference. The right plot shows the fit that includes interference, where ``Interfering BWs'' refers to the total contribution of all the interfering amplitudes, and their crossterms. For clarity, only the sum of the three background components (NRSPS+DPS+$ \mathrm{BW_0} $) is shown on the plots. The lower portion of the plots shows the pulls, i.e., the number of standard deviations (statistical uncertainties only) that the binned data differ from the fit. 
png pdf 
Figure 1a:
The J/$\psi$J/$\psi$ invariant mass spectrum in the range up to 9 GeV, with fits consisting of three signal functions ($ \mathrm{BW_1} $, $ \mathrm{BW_2} $ and $ \mathrm{BW_3} $) and a background model (see text). The plot shows the fit without interference. For clarity, only the sum of the three background components (NRSPS+DPS+$ \mathrm{BW_0} $) is shown. The lower portion of the plot shows the pulls, i.e., the number of standard deviations (statistical uncertainties only) that the binned data differ from the fit. 
png pdf 
Figure 1b:
The J/$\psi$J/$\psi$ invariant mass spectrum in the range up to 9 GeV, with fits consisting of three signal functions ($ \mathrm{BW_1} $, $ \mathrm{BW_2} $ and $ \mathrm{BW_3} $) and a background model (see text). The right shows the fit that includes interference, where ``Interfering BWs'' refers to the total contribution of all the interfering amplitudes, and their crossterms. For clarity, only the sum of the three background components (NRSPS+DPS+$ \mathrm{BW_0} $) is shown. The lower portion of the plot shows the pulls, i.e., the number of standard deviations (statistical uncertainties only) that the binned data differ from the fit. 
png pdf 
Figure A1:
The J/$\psi$J/$\psi$ invariant mass spectrum with the nointerference fit (upper) and the interference fit (lower) in the full fit range (see text for model details). The ``Interference BWs'' curve is the total contribution of all the interference amplitudes and their cross terms. The lower portion of the plots shows the pulls, i.e.,, the number of standard deviations (statistical uncertainties only) that the binned data differ from the fit. 
png pdf 
Figure A1a:
The J/$\psi$J/$\psi$ invariant mass spectrum with the nointerference interference fit in the full fit range (see text for model details). The lower portion of the plots shows the pulls, i.e.,, the number of standard deviations (statistical uncertainties only) that the binned data differ from the fit. 
png pdf 
Figure A1b:
The J/$\psi$J/$\psi$ invariant mass spectrum with the nointerference interference fit in the full fit range (see text for model details). The ``Interference BWs'' curve is the total contribution of all the interference amplitudes and their cross terms. The lower portion of the plots shows the pulls, i.e.,, the number of standard deviations (statistical uncertainties only) that the binned data differ from the fit. 
png pdf 
Figure B1:
The J/$\psi$J/$\psi$ invariant mass spectrum with fits using LHCb models [14]: Model I (nointerference) (upper) and Model II (interference) (lower). Model details are given in the text. The ``Interfering components'' is the total contribution of $ \mathrm{BW_{1}} $, NRSPS, and their interference. The lower portion of the plots shows the pulls between the binned data and the fit. 
png pdf 
Figure B1a:
The J/$\psi$J/$\psi$ invariant mass spectrum with fits using LHCb models [14] for Model I (nointerference). Model details are given in the text. The lower portion of the plot shows the pulls between the binned data and the fit. 
png pdf 
Figure B1b:
The J/$\psi$J/$\psi$ invariant mass spectrum with fits using LHCb models [14] Model II (interference). Model details are given in the text. The ``Interfering components'' is the total contribution of $ \mathrm{BW_{1}} $, NRSPS, and their interference. The lower portion of the plot shows the pulls between the binned data and the fit. 
Tables  
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Table 1:
Summary of the fit results for the J/$\psi$J/$\psi$ invariant mass distribution. The mass $ m $ and natural width $ \Gamma $ for both the nointerference model and the interference model, and the signal yields $ N $ for the nointerference model, are given for the three signal structures. The dual uncertainties are the statistical followed by the systematic components, and single uncertainties are only statistical. 
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Table 2:
Dominant contributions to the systematic uncertainties in masses and widths, in MeV, for the two fits. The ``Total uncertainty'' is the quadratic sum of all individual components, including the unlisted nondominant contributions. 
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Table B1:
Summary of results for the two LHCb fit models from LHCb and CMS: the mass $ m $ and natural width $ \Gamma $, in MeV, are given for the principal structures (the BreitWigner function at the threshold is not included in Model I), and LHCb values not reported in Ref. [14] are marked as ``$ \text{} $''. Single uncertainties are statistical only. 
Summary 
In summary, the J/$\psi$J/$\psi$ invariant mass spectrum has been presented. The data were collected with the CMS detector from pp collisions at $ \sqrt{s} = $ 13 TeV and correspond to an integrated luminosity of 135 fb$ ^{1} $. Two new structures, tentatively named X(6600) and X(7300), are found with masses of 6552 $ \pm $ 10 (stat) $ \pm $ 12 (syst) MeV and 7287 $ ^{+20}_{18} $ (stat) $ \pm $ 5 (syst) MeV, and the X(6900) structure observed by LHCb is confirmed with a mass of 6927 $ \pm $ 9 (stat) $ \pm $ 4 (syst) MeV. The local significances of these peaks are, for increasing mass, 6.5, 9.4, and 4.1 standard deviations. These results are from a fit that does not include interference effects. Adding interference terms between the three signals results in better agreement to the data in the regions between the resonances and results in shifts of the resonance parameters. Numeric results are provided in the HEPData record [61]. As this Letter was being finalized the ATLAS Collaboration released a paper [62] confirming the X(6900) structure in the J/$\psi$J/$\psi$ spectrum, and also reported nearthreshold excesses, including a possible feature in the $\psi$(2S) J/$\psi$ channel. 
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