A new particle that is a bound state of four different flavours of quarks has been discovered by physicists working on the D0 experiment at Fermilab. Called X(5568), the particle has a mass of about 5568 MeV/c2, and appears to contain "up" and "bottom" quarks as well as "down" and "strange" antiquarks.

Although other tetraquarks have previously been identified, X(5568) is the first in which all of the quarks have different flavours, which could affect our understanding of how quarks interact with each other. The discovery is also notable because X(5568) is produced at a much higher rate in proton–antiproton collisions than had been expected.

The particle was discovered by sifting through data acquired by D0 – an experiment that ran at Fermilab's Tevatron proton–antiproton collider from 2002 to 2011. The statistical significance of the discovery is 5.1σ, which puts it just above the 5σ required for a discovery in particle physics.

Possible configurations

Most known hadrons are either mesons, which contain a quark and an antiquark, or baryons, which comprise three quarks. A proton, for example, contains two up quarks and one down quark, while a BS meson contains a bottom quark and a strange antiquark.

The theory of the strong force – quantum chromodynamics (QCD) – allows for other types of baryons with four or more quarks. But doing calculations using QCD is extremely difficult, so it is not clear what tetraquark configurations are possible.

A tetraquark could comprise four quarks that are tightly bound to each other or it could be made of two mesons more loosely bound in a molecule-like structure. Indeed, understanding what goes on inside tetraquarks and pentaquarks would provide very important information about QCD itself.

In 2008 physicists working on the BELLE experiment in Japan discovered a tetraquark with a mass of 4430 MeV/c2. The discovery was backed up in 2014 by the LHCb experiment at CERN, which was able to detect the particle with a statistical significance of greater than 13σ. Then in 2015 LHCb physicists discovered that five quarks can be bound together to form pentaquarks.

Charming cores

Until the discovery of X(5568), all known tetraquarks and pentaquarks contained a charm quark/antiquark pair. This led some physicists to speculate that charmonium – a bound state of a charm quark and antiquark – creates a "core" around which tetraquarks and pentaquarks can form. The fact that X(5568) does not contain any quark/antiquark pairs of the same flavour, suggests that charmonium does not hold the key to understanding the formation of tetraquarks.

Another interesting aspect of the X(5568) discovery, according to Tim Gershon of the University of Warwick, is the rate at which it is being produced in the proton–antiproton collisions. The particle appears to be produced at a rate that is several orders of magnitude higher than expected, and Gershon believes that it is very important that the result be confirmed by other experiments.

Gershon, who works on LHCb and was not involved with D0, told physicsworld.com that the LHCb collaboration is now looking through its collision data for evidence of X(5568). He adds that this latest discovery shows that the study of exotic particles such as tetraquarks and pentaquarks will be a rich area for the future for experiments like LHCb.

X(5568) is described in a preprint on arXiv.