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| CMS-PAS-BPH-21-003 | ||
| Observation of new structures in the J/$\psi$J/$\psi$ mass spectrum in pp collisions at $\sqrt{s} = $ 13 TeV | ||
| CMS Collaboration | ||
| July 2022 | ||
| Abstract: A search is reported for low-mass structures in the J/$\psi$J/$\psi$ mass spectrum produced by proton-proton collisions at $\sqrt{s} = $ 13 TeV. The data sample corresponds to an integrated luminosity of 135 fb$^{-1}$ collected by the CMS experiment at the LHC. Modelling signals with relativistic Breit-Wigner shapes, and under the assumption of the absence of interference between signal components, and between signal and background, three structures are identified. Two structures are observed with local significances well above 5 standard deviations at masses of 6927 $\pm$ 9 (stat) $\pm$ 5 (syst) MeV and 6552 $\pm$ 10 (stat) $\pm$ 12 (syst) MeV. The first one is consistent with the previously observed X(6900). Evidence for a third structure is found at a mass of 7287 $\pm$ 19 (stat) $\pm$ 5 (syst) MeV with a local significance of 4.1 standard deviations. | ||
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These preliminary results are superseded in this paper, Accepted by PRL. The superseded preliminary plots can be found here. |
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| Figures & Tables | Summary | Additional Figures & Tables | References | CMS Publications |
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| Figures | |
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Figure 1:
The CMS J/$\psi$J/$\psi$ mass spectrum with a fit consisting of three signal BW functions and a background model (described in the text). The left plot shows the fit over the full mass range, and on the right is the same fit expanded by only displaying masses below 9 GeV. The lower portion of the plots shows the "pull quantities,'' i.e. the number of standard deviations that the binned data differs from the fit. |
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Figure 1-a:
The CMS J/$\psi$J/$\psi$ mass spectrum with a fit consisting of three signal BW functions and a background model (described in the text). The plot shows the fit over the full mass range. The lower portion of the plot shows the "pull quantities,'' i.e. the number of standard deviations that the binned data differs from the fit. |
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Figure 1-b:
The CMS J/$\psi$J/$\psi$ mass spectrum with a fit consisting of three signal BW functions and a background model (described in the text). The plot shows the fit over the full mass rang, only displaying masses below 9 GeV. The lower portion of the plot shows the "pull quantities,'' i.e. the number of standard deviations that the binned data differs from the fit. |
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Figure 2:
The J/$\psi$J/$\psi$ mass spectrum after the final selection. The data are fit using LHCb models [15]: Model I (non-interference) on the left and Model II (interference) on the right (see text for model details). |
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Figure 2-a:
The J/$\psi$J/$\psi$ mass spectrum after the final selection. The data are fit using LHCb Model I (non-interference) [15] (see text for model details). |
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Figure 2-b:
The J/$\psi$J/$\psi$ mass spectrum after the final selection. The data are fit using LHCb Model II (interference) [15] (see text for model details). |
| Tables | |
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Table 1:
Summary of the fit results of the CMS J/$\psi$J/$\psi$ distribution: the mass $m$ and natural width $\Gamma $, in MeV, and the signal yields $N$ are given for the three signal structures. The systematic uncertainties are discussed in Sec. 5. |
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Table 2:
Systematic uncertainties on masses and widths, in MeV. |
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Table 3:
Summary of results for the two LHCb fit models for LHCb and CMS: the mass $m$ and natural width $\Gamma $, in MeV, are given for the principal structures (the BW at threshold in Model I is not included), and LHCb values not reported in Ref. [15] are marked as `unrep.' Single uncertainties are statistical only. |
| Summary |
| In summary, the J/$\psi$J/$\psi$ mass spectrum from 135 fb$^{-1}$ of pp collisions recorded with the CMS detector has been presented. Observation of the X(6900) structure is confirmed, and two new ones, X(6600) and X(7300), are reported with respective masses 6552 $\pm$ 10 (stat) $\pm$ 12 (syst) MeV and 7287 $\pm$ 19 (stat) $\pm$ 5 (syst) MeV. The local (statistical) significance of these peaks, determined by the log-likelihood difference between the full non-interference fit and the version where a given Breit-Wigner lineshape is removed, are, for increasing mass, 6.5, 9.4, and 4.1 standard deviations. The resonance parameters of these states can be modified by interference--whose relevance is currently an open question--and this ambiguity motivates labeling these states with only rough mass values at this time. |
| Additional Figures | |
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Additional Figure 1:
The dimuon-dimuon mass distribution after the final selection ($ \mathrm{J}/\psi\mathrm{J}/\psi $ mass window requirement and mass constraint excluded), with the $ (\mu^{+}\mu^{-})_1 $ pair being the higher $ p_{\mathrm{T}} $ pair (top). The horizontal [$ m_{(\mu^{+}\mu^{-})_1} $] and vertical [$ m_{(\mu^{+}\mu^{-})_2} $] projections are respectively shown in bottom left figure and bottom right figure, which include the results of a fit of the 2D distribution for the various signal and background components. |
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Additional Figure 1-a:
Horizontal [$ m_{(\mu^{+}\mu^{-})_1} $] projection, which includes the results of a fit of the 2D distribution for the various signal and background components. |
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Additional Figure 1-b:
Vertical [$ m_{(\mu^{+}\mu^{-})_2} $] projection, which includes the results of a fit of the 2D distribution for the various signal and background components. |
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Additional Figure 1-c:
The dimuon-dimuon mass distribution after the final selection ($ \mathrm{J}/\psi\mathrm{J}/\psi $ mass window requirement and mass constraint excluded), with the $ (\mu^{+}\mu^{-})_1 $ pair being the higher $ p_{\mathrm{T}} $ pair (top). The horizontal [$ m_{(\mu^{+}\mu^{-})_1} $] and vertical [$ m_{(\mu^{+}\mu^{-})_2} $] projections are respectively shown in bottom left figure and bottom right figure, which include the results of a fit of the 2D distribution for the various signal and background components. |
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Additional Figure 2:
The $ \mathrm{J}/\psi \mathrm{J}/\psi $ mass spectrum with a fit consisting of three signal BW functions and background composed of NRSPS and NRDPS components, and a threshold enhancement, BW0. Left figure shows the fit over the full mass range, and right figure is the same fit expanded by only displaying masses below 9 GeV. |
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Additional Figure 2-a:
The $ \mathrm{J}/\psi \mathrm{J}/\psi $ mass spectrum with a fit consisting of three signal BW functions and background composed of NRSPS and NRDPS components, and a threshold enhancement, BW0. Fit over the full mass range. |
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Additional Figure 2-b:
The $ \mathrm{J}/\psi \mathrm{J}/\psi $ mass spectrum with a fit consisting of three signal BW functions and background composed of NRSPS and NRDPS components, and a threshold enhancement, BW0. Same fit expanded by only displaying masses below 9 GeV. |
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Additional Figure 3:
The $ \mathrm{J}/\psi \mathrm{J}/\psi $ mass spectrum with a fit consisting of three interfering signal BW functions and non-interfering background, composed of NRSPS and NRDPS components, and a threshold enhancement. The dashed ``interfering BWs" line refers to $ |r_1\exp(i\phi_1)\text{BW}_1+\text{BW}_2+r_3\exp(i\phi_3)\text{BW}_3|^2 $, where $ r_{1,3} $ and $ \phi_{1,3} $ are the relative magnitude and phase of $ \text{BW}_{1,3} $, with respect to $ \text{BW}_2 $. The three $ \text{BW}_{i} $ curves shown correspond to the squares of the BWs weighted to the squares of the corresponding coefficients in the amplitude. The continuous ``Background" line refers to the sum of the three background components. Left figure shows the fit over the full mass range, and right figure is the same fit expanded by only displaying masses below 9 GeV. |
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Additional Figure 3-a:
The $ \mathrm{J}/\psi \mathrm{J}/\psi $ mass spectrum with a fit consisting of three interfering signal BW functions and non-interfering background, composed of NRSPS and NRDPS components, and a threshold enhancement. The dashed ``interfering BWs" line refers to $ |r_1\exp(i\phi_1)\text{BW}_1+\text{BW}_2+r_3\exp(i\phi_3)\text{BW}_3|^2 $, where $ r_{1,3} $ and $ \phi_{1,3} $ are the relative magnitude and phase of $ \text{BW}_{1,3} $, with respect to $ \text{BW}_2 $. The three $ \text{BW}_{i} $ curves shown correspond to the squares of the BWs weighted to the squares of the corresponding coefficients in the amplitude. The continuous ``Background" line refers to the sum of the three background components. Fit over the full mass range. |
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Additional Figure 3-b:
The $ \mathrm{J}/\psi \mathrm{J}/\psi $ mass spectrum with a fit consisting of three interfering signal BW functions and non-interfering background, composed of NRSPS and NRDPS components, and a threshold enhancement. The dashed ``interfering BWs" line refers to $ |r_1\exp(i\phi_1)\text{BW}_1+\text{BW}_2+r_3\exp(i\phi_3)\text{BW}_3|^2 $, where $ r_{1,3} $ and $ \phi_{1,3} $ are the relative magnitude and phase of $ \text{BW}_{1,3} $, with respect to $ \text{BW}_2 $. The three $ \text{BW}_{i} $ curves shown correspond to the squares of the BWs weighted to the squares of the corresponding coefficients in the amplitude. The continuous ``Background" line refers to the sum of the three background components. Same fit expanded by only displaying masses below 9 GeV. |
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Additional Figure 4:
Event display showing a candidate for a X(6600) state decaying to a pair of $ \mathrm{J}/\psi $ mesons, followed by the decays of each $ \mathrm{J}/\psi $ to two muons. The mass of the muon pair represented by the red lines (blue lines) is 3.12 GeV (3.05 GeV) and the mass of the double-$ \mathrm{J}/\psi $ candidate is 6.52 GeV. |
| Additional Tables | |
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Additional Table 1:
The fit results of the interference model for the CMS $ \mathrm{J}/\psi \mathrm{J}/\psi $ distribution: the mass $ m $ and natural width $ \Gamma $, in MeV, are given for the three signal structures. |
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Additional Table 2:
Dominant contributions to the systematic uncertainties on masses and widths, in MeV, for the fit accounting for interference between the three resonances. The full uncertainty includes contributions from non-dominant uncertainties not listed here, and is a sum of individual components in quadrature. |
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Compact Muon Solenoid LHC, CERN |
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