Three cousins join family of four-quark particles

An exotic particle now has three new cousins, making for a happy family of four.

Scientists with the LHCb experiment, located at the Large Hadron Collider near Geneva, announced the discovery of the particle’s new cousins in two papers published online at arXiv.org on June 25. The particles are each made up of four quarks, elementary particles known for their role as the building blocks of protons and neutrons.

The previously known particle, X(4140), and its cousins —  X(4274), X(4500) and X(4700) — are composed of two charm quarks and two strange quarks. Each particle’s quarks are arranged in configurations of increasingly higher energy, making each particle heftier than the last — thanks to the equivalence of mass and energy expressed by the equation E=mc².

It’s most likely that the particles are tetraquarks, particles composed of four quarks tightly bound together, says physicist Tomasz Skwarnicki of Syracuse University in New York, who led the analysis. For decades, scientists thought quarks grouped up only into pairs or triplets, but recent research has unearthed tetraquarks (SN: 7/27/13, p. 9) and even pentaquarks (SN: 8/8/15, p. 8). A competing explanation, that the particles are molecule-like pairings made up of two quark partners each, has been ruled out, Skwarnicki says.

Even among tetraquarks, the particles are unusual because they are made up solely of heavy, exotic types of quarks. The lightest quarks, known as up and down quarks, make up protons and neutrons. Heavier quarks — like the charm and strange quarks that make up the new family — are not found in everyday materials.