After 50 years of searching, there is the strongest evidence yet for a glueball – a particle made of pure binding energy of the strong nuclear force.

Matter consists of quarks. The building blocks of atomic nuclei, protons and neutrons, are composed of these fundamental particles – held together by gluons. These act as glue (English glue = Klebstoff) and hold the quarks together. However, the theory of the strong nuclear force predicts something else: gluons should also be able to bind to each other – forming so-called “gluon balls”, particles without any quark content, essentially matter made of pure cohesion.

Physicists have been searching for this state for decades. Now, the Beijing Spectrometer(BESIII) collaboration reports in a preprint on arXiv and a presentation at the International Conference on High Energy Physics. The particle X(2370) is most likely predominantly composed of a gluon ball.