The particles known as quarks were suggested as a way of making sense of a large collection of particles that kept popping out of our atom smashers. While some particles, like electrons and neutrinos, are fundamental, others are composed of two or three quarks and a few gluons to hold them together. The familiar proton and neutron, for example, are composed of collections of three quarks.

But as far as we knew, three has been the upper limit for quarks in a single particle, so that's all our theories bothered to deal with. Over the past several years, however, evidence has been piling up that four- and even five-quark particles can be produced in particle accelerators. That data has led to a little theoretical confusion, as it wasn't clear whether these were single particles with all those quarks or a composite object composed of a combination of two familiar particles.

Further Reading LHC makes clear identification of a weird particle made of four quarks

Now, researchers who worked on Fermilab's Tevatron have gone through old data and discovered that it too had produced evidence of a four-quark particle. This is the first four-quark particle to have each of its component quarks come in different flavors. And the particle's mass suggests that it's likely to be a single unit rather than a composite particle.

To find the particle, the team behind the D0 detector reasoned that the particle would decay into a pair of two-quark particles, a pion and a B-meson. The B-meson, in turn, would decay into two muons and two particles called kaons. So the researchers searched for this combination of particles in the full collection of data from D0.

The challenge here is that these particles could all have been produced separately in the original collision. To show this wasn't the case, the team had to trace back the particles' trajectories and show that they originated from a point that was outside of the location of the collision. This implies that a heavier particle, going by X, was produced in the collision and traveled a bit before decaying.

The search turned up a rather large difference from the expected background at a mass of 5.568 Giga-electronVolts, earning the particle the name X(5568). Based on its decay, the new species appears to contain the following quarks: up, anti-down, anti-strange, and bottom. The deviation from background predictions is 5.1 sigma, clearly allowing the team to claim a discovery of something new.

Given the previous results from other accelerators, a new four-quark particle isn't that much of a shock. But the authors are able to speak a bit about the big unresolved issue: the nature of this new particle. If it were simply composed of some loosely bound two-quark particles, you'd expect the mass to be roughly the same as the sum of their masses. It's not; instead, it's about 0.2GeV heavier.

Where could this extra mass come from? When quarks are held together by gluons, the binding energy contributes to the mass (remember, mass and energy are equivalent). This implies that there are some extra gluons involved in holding together the quad-quark, which would not happen if it were simply a loose composite of two separate particles.

The evidence is gradually piling up that our understanding of quark-gluon interactions is going to have to be updated to account for putting together large collections of particles, which should keep some theorists busy for a while.

The arXiv. Abstract number: 1602.07588v2 (About the arXiv).