Scientists on the DZero collaboration at the US Department of Energy’s Fermilab (CSA CSM) have discovered a new particle—the latest member to be added to the exotic species of particle known as tetraquarks.
Quarks are point-like particles that typically come in packages of two or three, the most familiar of which are the proton and neutron (each is made of three quarks). There are six types (or “flavors”) of quark to choose from: up, down, strange, charm, bottom and top. Each of these also has an antimatter counterpart.
An ordinary meson is composed of a quark and an antiquark, whereas an ordinary baryon is made from three quarks. Physicists have long conjectured that exotic particles containing an additional quark-antiquark pair could exist, and a handful of candidates have been seen. Such particles, called tetraquarks, or their close relatives, the pentaquarks, would be exotic states forming a new particle species paralleling the ordinary mesons and baryons.
DZero is one of two experiments at Fermilab’s Tevatron collider. It searched for new exotic states decaying into a Bs meson and a pi meson. Both of these well-known mesons travel finite distances before decaying via the weak nuclear interaction. The Bs meson is composed of a quark and an antiquark of bottom and strange types, and the pi meson has an up and down quark and antiquark.
Although the Tevatron was retired in 2011, the experiments continue to analyze billions of previously recorded events from its collisions. As is the case with many discoveries, the tetraquark observation came as a surprise when DZero scientists first saw hints in July 2015 of the new particle, called X(5568), named for its mass—5568 megaelectronvolts. “At first, we didn’t believe it was a new particle,” says DZero spokeman Dmitri Denisov. “Only after we performed multiple cross-checks did we start to believe that the signal we saw could not be explained by backgrounds or known processes, but was evidence of a new particle.”
And the X(5568) is not just any new tetraquark. The fact that it decays via the strong interaction into a Bs and pi mesons tells scientists that it contains four distinct flavors of quarks and antiquarks—bottom, strange, up and down. Several other previously observed particles are good candidates to be tetraquark or pentaquark states, but all of these have a quark and antiquark of the same flavor, and thus their character as an exotic particle is less certain.
“The next question will be to understand how the four quarks are put together,” says DZero spokesman Paul Grannis. “They could all be scrunched together in one tight ball, or they might be one pair of tightly bound quarks that revolves at some distance from the other pair.” Four-quark states are rare, and although there’s nothing in nature that forbids the formation of a tetraquark, scientists don’t understand them nearly as well as they do two- and three-quark states.
This latest discovery comes on the heels of the first observation of a pentaquark—a five-quark particle—announced last year by the LHCb experiment at the Large Hadron Collider. Scientists will sharpen their picture of the quark quartet by making measurements of properties such as the ways X(5568) decays or how much it spins on its axis. Measuring the properties of the new X(5568) particle and other tetraquark candidates—their masses, lifetimes, spins and parities, as well as the probabilities for them to decay into various final particle combinations—will give valuable new information about how the strong force binds quarks (and antiquarks) into observable particles. And perhaps the emerging tetraquark species will become an established class in the future, showing themselves to be as numerous as their two- and three-quark siblings.
“The discovery of a unique member of the tetraquark family with four different quark flavors will help theorists develop models that will allow for a deeper understanding of these particles,” says Fermilab Director Nigel Lockyer.