XYZ particle

In particle physics, XYZ particles (also referred to as XYZ states) are recently-discovered heavy mesons whose properties do not appear to fit the standard picture of charmonium and bottomonium states. They are therefore types of exotic mesons. The term arises from the names given to some of the first such particles discovered: X(3872), Y(4260) and Z_c(3900), although the symbols X and Y have since been deprecated by the Particle Data Group.