Fermilab Scientists Discover Four-Flavor ‘Tetraquark’

A team of researchers analyzing the data gathered during the final run of Fermilab’s Tevatron collider announced the discovery of a new particle Friday. The particle, whose detailed internal structure is still not fully understood, is believed to be part of an exotic family called “tetraquarks.”

Quarks, the fundamental particles that make up protons and neutrons, come in six different “flavors” — up, down, strange, top, bottom and charm. Each quark has an anti-matter equivalent known as antiquark. Quarks usually come in packages of two or three. Both protons and neutrons — contained within the nucleus of an atom — are made up of three quarks bound together.

The 1964 model that first proposed the existence of quarks also postulates the existence of tetraquarks, composed of four quarks, and pentaquarks, composed of five quarks.

Last August, scientists at CERN’s Large Hadron Collider announced the discovery of a pentaquark, in which quarks were arranged in a pattern never before seen in over 50 years of experimental searches. Prior to that, in 2014, experiments at LHC had also provided strong evidence of the existence of a tetraquark.

The discovery announced Thursday was made by physicists from the DZero collaboration — one of the two experiments conducted at Fermilab’s Tevatron Collider before it was shut down in 2011.

“At first, we didn’t believe it was a new particle,” DZero co-spokesman Dmitri Denisov told Symmetry magazine. “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.”

Moreover, after studying the decay of the particle, dubbed X(5568), scientists found that it contains four distinct flavors of quarks and antiquarks — bottom, strange, up and down. While several other previously observed particles are good candidates to be in a tetraquark state, all of them have a quark and antiquark of the same flavor, thereby making them less “exotic.”

“The next question will be to understand how the four quarks are put together,” DZero co-spokesman Paul Grannis told Symmetry magazine. “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.”