Scientists have found an incredible new particle that's part of the exotic family of tetraquarks. Tetraquarks are made of four quarks, which makes them quite unique. Most particles in nature are made by either three quarks (like protons and neutrons) or just two.
The particle, called X(5568), was discovered thanks to Fermilab’s Tevatron particle accelerator. It has a mass of almost six protons and it is the first tetraquark to be made by four different types of quarks. A paper with the results is available on arXiv.
The particle was found by the DZero collaboration thanks to the retired Fermilab’s Tevatron collider. The experiment was closed down in 2011, but scientists have continued studying the data from the billions of collisions produced over its operational years.
"At first, we didn't believe it was a new particle," said DZero co-spokesperson Dmitri Denisov in a statement. "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."
There are six types, or flavors, of quarks: Up and Down (which make up protons and neutrons), charm and strange, and top and bottom. Each of them has its own antimatter equivalent, and the combinations of these 12 fundamental particles give rise to hundreds of new particles.
These combinations, called baryons, usually come in two forms: hadrons (with three quarks) or mesons (a quark and an antiquark). A few candidates for tetraquarks have been observed in the last few years, and last summer CERN even discovered a pentaquark, a five-quark particle. Quarks cannot be found in a singular state.
Tetraquarks are not found in nature and the first candidate was only put forward eight years ago. While their existence doesn’t violate the laws of physics, scientists are yet to understand them completely. X(5568) particles are made by up, down, strange, and bottom quarks, which has made scientists even more puzzled.
"The next question will be to understand how the four quarks are put together," says DZero co-spokesperson 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."
Tetraquarks are significantly heavier than hadrons and mesons, so they require more energy to be produced. CERN and future particle colliders might be able to see many more tetraquarks, and maybe the tetraquark family will become as numerous as the other baryon families.