CMS-PAS-BPH-22-004

CMS-PAS-BPH-22-004
Observation of X(6900) and evidence for X(7100) in the $ \mathrm{J}/\psi\ \psi(2S) \rightarrow \mu^{+} \mu^{-} \mu^{+} \mu^{-} $ mass spectrumin 314 fb$ ^{-1} $ of pp collisions at CMS
CMS Collaboration
7 April 2025

Abstract: A search is performed for structures near threshold in the $ \mathrm{J}/\psi\, \psi(2S) \rightarrow \mu^{+} \mu^{-} \mu^{+} \mu^{-} $ channel using a sample of proton-proton collisions at $ \sqrt{s} = $ 13.0 TeV and 13.6 TeV recorded by the CMS detector at the CERN LHC. The data correspond to an integrated luminosity of about 314 fb$ ^{-1} $. A threshold structure is observed near 6900 MeV with a significance in excess of five standard deviations ($ > $5$ \sigma $), and there is also evidence (4.0$ \sigma $) for a second peak, in a model where the structures mutually interfere. The mass and width of the two peaks are measured to be 6876 $ ^{+46\,+110}_{-29\,-110} $, 253 $ ^{+290\,+120}_{-100\,-120} $ MeV and 7169 $ ^{+26\,+74}_{-52\,-70} $, 154 $ ^{+110\,+140}_{-82\,-160} $ MeV, which are consistent to within one standard deviation of the X(6900) and X(7100) structures previously observed in the $ \mathrm{J}/\psi\, \mathrm{J}/\psi $ spectrum.
Links: CDS record (PDF) ; CADI line (restricted) ;

Figures & Tables	Summary	References	CMS Publications

Figures
png pdf	Figure 1: The $ \mathrm{J}/\psi\, \psi(2S) $ spectrum under 9 GeV (to make structures more visible) for the Run 2 dataset (upper left), the Run 3 dataset (upper right), and the combined Run 2 + Run 3 dataset (bottom). The number of events below 15 GeV for these three datasets are 139, 415, and 554, respectively.
png pdf	Figure 1-a: The $ \mathrm{J}/\psi\, \psi(2S) $ spectrum under 9 GeV (to make structures more visible) for the Run 2 dataset (upper left), the Run 3 dataset (upper right), and the combined Run 2 + Run 3 dataset (bottom). The number of events below 15 GeV for these three datasets are 139, 415, and 554, respectively.
png pdf	Figure 1-b: The $ \mathrm{J}/\psi\, \psi(2S) $ spectrum under 9 GeV (to make structures more visible) for the Run 2 dataset (upper left), the Run 3 dataset (upper right), and the combined Run 2 + Run 3 dataset (bottom). The number of events below 15 GeV for these three datasets are 139, 415, and 554, respectively.
png pdf	Figure 1-c: The $ \mathrm{J}/\psi\, \psi(2S) $ spectrum under 9 GeV (to make structures more visible) for the Run 2 dataset (upper left), the Run 3 dataset (upper right), and the combined Run 2 + Run 3 dataset (bottom). The number of events below 15 GeV for these three datasets are 139, 415, and 554, respectively.
png pdf	Figure 2: The $ \mathrm{J}/\psi\, \psi(2S) $ spectrum with the $ f_{i23} $ fit of interfering X(6900) and X(7100) components plus backgrounds (fit extends up to 15 GeV).

Tables
png pdf	Table 1: Fit results of the $ \mathrm{J}/\psi\, \psi(2S) $, or $ \mathrm{J}/\psi\mathrm{J}/\psi $, mass spectra. The role of interference, if any, is noted where $ \mathrm{BW}_i $ represents X(6600), X(6900), and X(7100) for $ i=1,2, $ 3. Masses and widths are in MeVns. The top row of numbers is for fit without any constraints from $ \mathrm{J}/\psi\mathrm{J}/\psi $ and that the second row of numbers is the $ \mathrm{J}/\psi\mathrm{J}/\psi $ result for comparison.
png pdf	Table 2: Dominant contributions to the systematic uncertainties on the X(6900) and X(7100) masses and widths, in MeV, for fit $ f_{i23} $. The "Total" is the sum in quadrature of all components, including unlisted non-dominant contributions.

Summary

In summary, using 314 fb$^{-1}$ of collisions recorded in 2016-2018 and 2022-2024, CMS observes an excess in $ \mathrm{J}/\psi\, \psi(2S) $ near threshold well above five standard deviations (5 $ \sigma $), consistent with $ X(6900){\to}\mathrm{J}/\psi\, \psi(2S) $. There is evidence of an $ X(7100){\to}\mathrm{J}/\psi\, \psi(2S) $ signal with 4.0 $ \sigma $ local significance. These excesses are consistent with similar ones observed in the $ \mathrm{J}/\psi\mathrm{J}/\psi $ mass spectrum identified by CMS [1], LHCb [11] and ATLAS [12]. Recently CMS reported an angular analysis of the $ \mathrm{X} {\to} \mathrm{J}/\psi\mathrm{J}/\psi $ decay characteristics and found the $ J^{PC} $ quantum numbers of the $ \mathrm{X} $ family is most likely $ 2^{++} $ [45].

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