Physicists on the DZero international collaboration at Fermilab, the U.S. Department of Energy’s laboratory specializing in high-energy particle physics, have discovered a new subatomic particle, X(5568), whose quark content appears to be qualitatively different from normal.

Quarks are point-like elementary 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.

The newly-discovered tetraquark particle X(5568), named for its mass – 5568 megaelectronvolts, contains four distinct flavors – bottom, strange, up and down.

“It is exciting to discover a new and unusual particle that will help us understand the strong interaction- one of the four known fundamental interactions in physics,” said team member Prof. Iain Bertram, of Lancaster University, UK.

“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,” said Fermilab Director Nigel Lockyer.

The discovery will be published in the journal Physical Review Letters, but have been published on arXiv.org ahead of time.

X(5568) decays via the strong interaction into a B s and pi mesons, according to the DZero physicists. The B s meson decays into a J/psi and a phi meson, and these in turn decay into two muons and two kaons, respectively.

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 scientists said that the detailed internal structure of X(5568) is not yet understood.

“The next question will be to understand how the four quarks are put together,” said DZero co-spokesperson Paul Grannis.

“They could be a relatively tightly bound combination of all four quarks and antiquarks, or they could be structures in which two tightly bound quark-antiquark states revolve around each other.”

Measuring the properties of the new 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.

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