After a long and glorious 24-year run filled with amazing scientific discoveries, the Tevatron collider was finally put to rest in 2011.
Despite the collider's retirement, however, data from its last proceedings is still yielding potentially remarkable results.
One such finding is the discovery of a new elementary particle believed to be part of an exotic species known as tetraquarks.
From its prefix, a tetraquark is a particle composed of four valence quarks. Tetraquarks are typically "glued" together by a strong nuclear force.
Physicists involved in the DZero collaboration at Fermilab - a government laboratory where the Tevatron is located - were in disbelief when they detected the new particle.
"Only after we performed multiple cross-checks did we start to believe that the signal we saw could not be explained by background or known processes, but was evidence of a new particle," Dmitri Denisov, spokesman of the DZero project, told Symmetry magazine.
The Flavors Of A Quark
Along with leptons, quarks are the building blocks of matter at the subatomic level. Quarks were first proposed in 1964 by physicists George Zweig and Murray Gell-Mann.
These particles are known as one of the tiniest components of matter, and usually combine in groups of two or three to form subatomic particles. Both the proton and the neutron in an atomic nucleus are each made up of a combination of three quarks.
There are six types of quarks, which are strangely called flavors: up, down, strange, charm, bottom and top, in hundreds of combinations.
Named X(5568), the new tetraquark is strange even among its family members. Previously discovered tetraquarks contained two quarks of the same flavor, but X(5568) is made up of entirely different flavors: up, down, strange and bottom.
The new particle was found after the DZero team picked out a unique decay signature from Tevatron data gathered in 2002 to 2011 that involved billions of recorded collision events. The signal they found in the data matched the "five-sigma" threshold, the standard for discovery in particle physics.
An Unbelievable Discovery
For quite a long time, scientists had no evidence that there are other configurations for quarks aside from two or three, but Gell-Mann predicted that there could be particles made up of four or even five quarks.
Everything changed in 2003, when physicists in Japan found the first supporting evidence for tetraquarks. Other candidates for tetraquarks have been identified since then, one of them detected at the Large Hadron Collider (LHC) about two years ago.
Scientists said tetraquarks are related to the exotic pentaquark, a particle with five quarks detected by the LHC in 2015.
What's Next?
Paul Grannis, who is also a member of the DZero team, said the next thing they will look into is how the four quarks are put together.
He said the quarks might be divided into two pairs: one tightly bound pair that revolves at a distance from the other; or the quarks could all be clumped together in one tight ball.
In the meantime, further investigations could provide scientists insight about how the strong force works to bind subatomic particles together. Denisov said while they do understand the features of the strong force, they do not understand everything, such as how the strong force acts on large distances.
"And on a fundamental level we still don't have a very good model of how quarks interact when there are quite a few of them joined together," said Denisov.
The team's findings are submitted to the journal Physical Review Letters.
