as a glueball–meson mixing state or pure glueball is not well established. Other candidates Gluon jets at the LEP experiment show a 40% excess over theoretical expectations of electromagnetically neutral clusters which suggests that electromagnetically neutral particles expected in gluon-rich environments such as glueballs are likely to be present. Many of these candidates have been the subject of active investigation for at least eighteen years. The GlueX experiment has been specifically designed to produce more definitive experimental evidence of glueballs. See also Exotic meson GlueX Gluon Yang–Mills theory References Exotic matter Gluons Hypothetical composite particles Quantum chromodynamics Unsolved problems in physics