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Observation of structure in the $J/\psi$-pair mass spectrum

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Abstract

Using proton-proton collision data at centre-of-mass energies of $\sqrt{s} = 7$, $8$ and $13\mathrm{ TeV}$ recorded by the LHCb experiment at the Large Hadron Collider, corresponding to an integrated luminosity of $9\mathrm{ fb}^{-1}$, the invariant mass spectrum of $J/\psi$ pairs is studied. A narrow structure around $6.9\mathrm{ GeV/}c^2$ matching the lineshape of a resonance and a broad structure just above twice the $J/\psi$ mass are observed. The deviation of the data from nonresonant $J/\psi$-pair production is above five standard deviations in the mass region between $6.2$ and $7.4\mathrm{ GeV/}c^2$, covering predicted masses of states composed of four charm quarks. The mass and natural width of the narrow $X(6900)$ structure are measured assuming a Breit--Wigner lineshape.

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(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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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

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

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

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Created on 19 February 2021.