In the particle physics' youth, researchers started discovering a dizzying variety of ever heavier, unstable particles. Quarks brought some order to this chaos. The zoo of particles, along with the familiar proton and neutron, were built from combinations of the six quarks or their antiparticles. The system neatly explained the spin and charge of these particles, and helped make sense of particles discovered as accelerators reached even higher energies.
All of the particles we knew of were built using either two or three quarks. But there was nothing in our theories that prevented larger assemblages having even more quarks. Discovery, however, lagged well behind the initial proposal, which came in the 1970s. A buzz of excitement about a five quark particle came and subsided after other accelerator teams couldn't reproduce the result.
Just two years ago, however, two different teams announced evidence for a tetraquark particle, which picked up the name Zc(3900). And now, an experiment at the Large Hadron Collider (LHC) has come up with evidence of a five-quark behemoth.
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