About the $J/\psi$ $J/\psi$ peak of LHCb: fully-charmed tetraquark?

Since several decades, a particular attention has been devoted to hadrons with anomalous properties [1], [2], [3], [4], [5]. On the experimental side, the search has been very difficult and disappointing. For instance, in the early 60's, there has been some claims for baryon resonances with strangeness , the so-called Z baryons, while the established baryons have either or . But the Z spectrum was based on somewhat hazardous analyses of -nucleon scattering data without spin observables, and it faded away. It reappeared under the name of pentaquark, but it was never firmly established.