The particles called quarks were proposed as a way to understand a large collection of particles that kept emerging from our atomic crushers. While some particles, such as electrons and neutrinos, are fundamental, others are composed of two or three quarks and a few gluons to hold them together. For example, the well-known proton and neutron are composed of sets of three quarks.
But as far as we knew, three was the upper limit for quarks in a single particle, so that was all our theories. In recent years, however, evidence has been accumulating that four- and even five-quark particles can be produced in particle accelerators. That data has led to a bit of theoretical confusion, as it wasn’t clear whether these were single particles containing all those quarks or a composite object composed of a combination of two known particles.
Now, researchers working on Fermilab’s Tevatron have sifted through old data and found that it, too, had provided evidence for a four-quark particle. This is the first particle with four quarks where each of the constituent quarks comes in different flavours. And the particle’s mass suggests it is likely 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 would in turn decay into two muons and two particles called kaons. So the researchers looked for this combination of particles in D0’s full set of data.
The challenge here is that these particles could all have been produced separately in the original collision. To demonstrate that this was not the case, the team had to track the trajectories of the particles and show that they originated from a point outside the collision site. This implies that a heavier particle, with X, was produced during the collision and traveled a bit before breaking up.
The search turned up quite a difference from the expected background at a mass of 5.568 Giga-electron Volts, giving 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, which clearly allows the team to claim a discovery of something new.
Given previous results from other accelerators, a new four-quark particle isn’t much of a shock. But the authors can say something about the big unsolved problem: the nature of this new particle. If it were simply composed of a few loosely bound two-quark particles, you would expect its mass to be about the same as the sum of their masses. It’s not; instead, it is about 0.2 GeV heavier.
Where could this extra mass come from? When quarks are held together by gluons, the binding energy adds to the mass (remember, mass and energy are equivalent). This implies that there are some extra gluons involved in holding the quad quark together, which wouldn’t happen if it were simply a loose assembly of two separate particles.
Evidence is gradually accumulating that our understanding of quark-gluon interactions will need to be updated to account for the assembly of large collections of particles, which should take some theorists some time.
The arXiv. Digest Number: 1602.07588v2 (About the arXiv).