(Bottom right) Two-dimensional $(M^{(1)}_{\mu\mu},M^{(2)}_{\mu\mu})$ distribution of di-$ { J \mskip -3mu/\mskip -2mu\psi } $ candidates and its projections on (bottom left) $M^{(1)}_{\mu\mu}$ and (top) $M^{(2)}_{\mu\mu}$. Four components are present as each projection consists of signal and background $ { J \mskip -3mu/\mskip -2mu\psi } $ candidates. The labels $ { J \mskip -3mu/\mskip -2mu\psi } _{1,2}$ and bkg${}_{1,2}$ represent the signal and background contributions, respectively, in the $M^{(1),(2)}_{\mu\mu}$ distribution.

Invariant mass spectrum of $ { J \mskip -3mu/\mskip -2mu\psi } $-pair candidates passing the $ p^{{\rm di}{\text -} J \mskip -3mu/\mskip -2mu\psi }_{\rm T} >5.2\text{ Ge V /}c $ requirement with reconstructed $ { J \mskip -3mu/\mskip -2mu\psi } $ masses in the (black) signal and (blue) background regions, respectively.

Invariant mass spectra of weighted di-$ { J \mskip -3mu/\mskip -2mu\psi } $ candidates with $ p^{{\rm di}{\text -} J \mskip -3mu/\mskip -2mu\psi }_{\rm T} >5.2\text{ Ge V /}c $ and overlaid projections of the $ p^{{\rm di}{\text -} J \mskip -3mu/\mskip -2mu\psi }_{\rm T} $-threshold fit using (a) the NRSPS plus DPS model, (b) model I, and (c) model II.

Invariant mass spectra of weighted di-$ { J \mskip -3mu/\mskip -2mu\psi } $ candidates in bins of $ p^{{\rm di}{\text -} J \mskip -3mu/\mskip -2mu\psi }_{\rm T} $ and overlaid projections of the $ p^{{\rm di}{\text -} J \mskip -3mu/\mskip -2mu\psi }_{\rm T} $-binned fit with model I.

Invariant mass spectra of $ { J \mskip -3mu/\mskip -2mu\psi } $-pair candidates in the six $ p^{{\rm di}{\text -} J \mskip -3mu/\mskip -2mu\psi }_{\rm T} $ regions with boundaries $\left\{ 0, 5, 6, 8, 9.5, 12, 50 \right\}\text{ Ge V /}c $ with reconstructed $ { J \mskip -3mu/\mskip -2mu\psi } $ masses in the (black) signal and (blue) background regions, respectively.

Invariant mass spectra of weighted di-$ { J \mskip -3mu/\mskip -2mu\psi } $ candidates with $ p^{{\rm di}{\text -} J \mskip -3mu/\mskip -2mu\psi }_{\rm T} >5.2\text{ Ge V /}c $ and overlaid projections of the $ p^{{\rm di}{\text -} J \mskip -3mu/\mskip -2mu\psi }_{\rm T} $-threshold fit with (a) the threshold structure described as a single BW function, and (b) assuming a single BW interfering with the SPS continuum.

Invariant mass spectra of weighted di-$ { J \mskip -3mu/\mskip -2mu\psi } $ candidates with $ p^{{\rm di}{\text -} J \mskip -3mu/\mskip -2mu\psi }_{\rm T} >5.2\text{ Ge V /}c $ and overlaid projections of the $ p^{{\rm di}{\text -} J \mskip -3mu/\mskip -2mu\psi }_{\rm T} $-threshold fit with an additional BW function introduced to describe the $7.2\text{ Ge V /}c^2 $ structure, based on the model that contains two BW lineshapes for the threshold structure and a BW shape for the $X(6900)$ structure on top of the NRSPS plus DPS continuum.

Invariant mass spectra of weighted di-$/ { J \mskip -3mu/\mskip -2mu\psi } $ candidates in bins of $ p^{{\rm di}{\text -} J \mskip -3mu/\mskip -2mu\psi }_{\rm T} $ and overlaid projections of the $ p^{{\rm di}{\text -} J \mskip -3mu/\mskip -2mu\psi }_{\rm T} $-binned fit assuming that the threshold structure interferes with the SPS continuum.

Dependence of the experimental efficiency on $ M_{{\rm di}{\text -} J \mskip -3mu/\mskip -2mu\psi } $. The error bars include the statistical uncertainties only.

Animated gif made out of all figures.

Global significance evaluated under the various assumptions described in the text.

Systematic uncertainties on the mass ($m$) and natural width ($\Gamma$) of the $X(6900)$ structure.