New Structures in the $J/\psi J/\psi$ Mass Spectrum in Proton-Proton Collisions at $\sqrt{s}=13\mathrm{TeV}$

A search is reported for near-threshold structures in the $J/\psi J/\psi$ invariant mass spectrum produced in proton-proton collisions at $\sqrt{s}=13\mathrm{TeV}$ from data collected by the CMS experiment, corresponding to an integrated luminosity of $135{\mathrm{fb}}^{-1}$. Three structures are found, and a model with quantum interference among these structures provides a good description of the data. A new structure is observed with a local significance above 5 standard deviations at a mass of ${6638}_{-38}^{+43}{(\mathrm{stat})}_{-31}^{+16}(\mathrm{syst})\mathrm{MeV}$. Another structure with even higher significance is found at a mass of ${6847}_{-28}^{+44}{(\mathrm{stat})}_{-20}^{+48}(\mathrm{syst})\mathrm{MeV}$, 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 4.7 standard deviations, is found at a mass of ${7134}_{-25}^{+48}{(\mathrm{stat})}_{-15}^{+41}(\mathrm{syst})\mathrm{MeV}$. Results are also reported for a model without interference, which does not fit the data as well and shows mass shifts up to 150 MeV relative to the model with interference.

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). Left: the fit without interference. Right: the fit that includes interference, where “Interfering BWs” refers to the total contribution of all the interfering amplitudes and their cross terms. For clarity, only the sum of the three background components ($\mathrm{NRSPS}+\mathrm{DPS}+{\mathrm{BW}}_0$) 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.