Scientific Method / Science & Exploration

Two accelerators find signs of a particle that nobody can explain

There seem to be four quarks involved, but nobody's sure how they're linked.

The Belle detector at Japan's KEK facility.

Two different accelerators have found evidence for a particle that appears to contain four quarks, according to papers published in Physical Review Letters. Although particles with two and three quarks are common, this would be the first time that something containing four quarks has been spotted. Depending on the precise nature of the interactions among the quarks, this could be a discovery that keeps the theoreticians very busy.

With the discovery of the Higgs boson, the predicted collection of fundamental particles was complete. But one family of these fundamental particles—the quarks—combine with gluons to make more complex particles called hadrons and mesons. Hadrons include the proton and neutron, and they are formed by combinations of three quarks. Mesons, which are unstable, are comprised of pairs of quarks.

Having only two quarks would seem to make mesons fairly simple when actually they're anything but. There are three families of quarks, each with a particle and antiparticle, and mesons can consist of any combination of these. They also create some of the more amusing nomenclature in physics, with mesons involving a strange quark being referred to as strangeonium, and those with a bottom quark as bottomonium.

Earlier collider experiments had suggested that the presence of a meson with two bottom quarks might be associated with a heavier particle of unknown properties. So, two different teams, one working at a collider in Japan, the other at one in Beijing, decided to look at whether the same was true with charmonium. (Both colliders are electron-positron colliders, which have many advantages, despite their relatively low energies.)

So, the teams looked at events that included a J/ψ meson, which is a single particle (with two names, since two groups announced its discovery simultaneously) that is composed of a charm quark and a charm antiquark. To do this, they scanned the data sets generated by two different detectors: BES III in Beijing, and Belle at the KEK facility in Japan. The researchers pulled out those events that included a J/ψ and a pair of π particles (another type of meson), with the J/ψ being spotted due to its decay either into an electron and positron, or a muon and antimuon. With those in hand, they searched for indications that the J/ψ was the product of the decay of a heavier particle. (They do this by looking at what's called the "structure of the mass spectrum").

Both teams found something at 3.9GeV, which they're terming Zc(3900) due to its apparent mass of 3900MeV. But its presence is coupled to the appearance of charged π particles, which suggests that the new particle itself is charged. This means that it is probably comprised of four quarks. And, as mentioned above, particles with four quarks have not been previously detected.

The results have a statistical significance well above that required to count for discovery in particle physics, and the fact that there seems to be a similar heavy particle for bottom quarks suggests that this may be a common feature for all the quarks. There's also nothing in particular that rules out four-quark particles but we've gotten pretty deep into the era of particle physics without ever detecting one, so the results are surprising.

Most of the debate, however, seems to focus on how exactly could four quarks combine. One option is that they combine in the same way that two quarks combine with gluons to make mesons and three combine with gluons to form hadrons. The alternative would be what some reports are calling a "meson molecule," where a pair of two-quark mesons are held together by an attractive force. The problem with the latter option, as noted by Nature News, is that the molecule should be less stable than its constituent mesons. But the detectors see no sign of it splitting apart before it decays.

Given the clear method for spotting the Zc(3900) laid out by these papers, it should be easy for anybody to comb through their data and look for similar events, which may shed some more light on the particle's properties. And, in the mean time, it's a safe bet that theorists will be looking carefully at various forms of four-quark particles (molecule and otherwise) to see what sort of predictions they could make about the particle's behavior.

Physical Review Letters, 2013. DOI: 10.1103/PhysRevLett.110.252002, 10.1103/PhysRevLett.110.252001  (About DOIs).

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