Observation of a family of all-charm tetraquarks at CMS

Using $315~\mathrm{fb}^{-1}$ of proton-proton collision data recorded by the CMS detector, which provides 3.6 times more $J/\psi J/\psi$ pairs than the Run 2 study, the CMS experiment reports the observation of three structures in the $J/\psi J/\psi$ to four-muon channel, named $X(6600)$, $X(6900)$, and $X(7100)$, each with a significance well above 5$\sigma$, confirming and extending the previous Run 2 results. Strong interference effects among these states, also exceeding 5$\sigma$, indicate that they share the same quantum numbers. A complementary search in the $J/\psi\psi(2S)$ channel identifies two structures consistent with $X(6900)$ and $X(7100)$. Assuming common quantum numbers for the three states, as suggested by the interference pattern, a spin–parity analysis favors a $J^{PC}=2^{++}$ assignment, and hadrons with such high spins are rather rare. Together, these results support the interpretation of these states as a family of all-charm tetraquarks, favoring a diquark–antidiquark configuration with both diquarks in a spin-1 state and providing new insights into the dynamics of exotic resonances.