Glueballs are made of gluons, which bind quarks to form protons and neutrons. Credit: Sefa kart/Getty 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. Researchers at the Beijing Spectrometer III (BESIII) Collaboration, an international particle-physics experiment, presented their results at the International Conference on High Energy Physics in Natal, Brazil, last week. They say that a particle known as X (2370) — which was discovered in 2011 — is mostly made up of glueballs. These are clusters of gluons, the elementary particles that bind quarks to form protons and neutrons and hold them inside the nucleus of an atom. There is no single smoking gun that proves that this particle is made of glueballs, says Bruce Yabsley, a particle physicist at the University of Sydney in Australia, who has reviewed the results from BESIII.…