This is the first known particle with four of the same kind of quark
In a never-before-seen particle, four quarks of a feather flock together. Physicists think have detected the first conglomerate of four quarks incorporating more than two of the same kind. This tetraquark contains four quarks of the charm variety: two charm quarks and their antimatter counterparts, called anticharm quarks, researchers report online at arXiv.org on June 30. Quarks — fundamental building blocks of matter — typically make up three-quark particles, like protons and neutrons, or quark-antiquark pairs, like pions and kaons. Physicists have observed some more exotic quark quartets (SN: 4/11/14) and even quintets (SN: 7/14/15). But the new four-quark particle, dubbed X(6900), is the first four-quark particle with all of the same type. Since charm quarks and their anticharm counterparts are among the heaviest types of quarks, it is also the first tetraquark to include more than two heavy quarks. “It’s a pretty exciting finding,” says physicist Matthew Shepherd of Indiana University Bloomington,...