Observation of a family of all-charm tetraquarks

Three structures, X(6600), X(6900), and X(7100), have emerged from the J$\mathrm{/}\psi$J$\mathrm{/}\psi$ (J$\mathrm{/}\psi$ $\to$ ${\mu }^{+}{\mu }^{-}$) mass spectrum. These are candidates of all-charm tetraquarks, an exotic form of hadronic matter. A clearer picture of these states is obtained using proton-proton collision data collected by the CMS detector that corresponds to 315 fb${}^{-1}$, which yields 3.6 times more J$\mathrm{/}\psi$J$\mathrm{/}\psi$ pairs than previous studies by CMS. All three structures, and their mutual interference, have statistical significances above five standard deviations. The presence of interference implies that the structures have common quantum numbers. Their squared masses align linearly with a resonance index and have natural widths that systematically decrease as the index increases. These features are consistent with radial excitations of tetraquarks composed of two aligned spin-1 diquarks without orbital excitation, and disfavor other interpretations. The J$\mathrm{/}\psi$$\psi$(2S) $\to$ ${\mu }^{+}{\mu }^{-}{\mu }^{+}{\mu }^{-}$ decay mode is also explored and the X(6900) and X(7100) states are found with significances exceeding 8 and 4 standard deviations, respectively.