> After almost two decades of searching, a team of physicists in China says it has observed strong evidence of a mysterious particle called a glueball, which is made entirely of force-carrying particles.
> ... The discovery of glueballs would provide direct evidence that gluons can interact with themselves, researchers say. This is a key prediction of quantum chromodynamics
I had to google it: "the study of the strong interaction between quarks mediated by gluons"