Scientists have discovered an exotic atom formed from two specific types of meson, which consist of only a quark and an anti-quark, instead of the three quarks found in constituents of normal matter.

Mesons are unstable particles produced in high-energy collisions and can be used to test the quantum theory of the strong force, which holds quarks together. To generate dimesonic atoms, physicists working on the DIRAC experiment, based at CERN, the European particle-physics laboratory near Geneva, fired a proton beam at thin sheets of platinum or nickel. The team detected roughly 350 atoms consisting of a π meson and a K meson, the first statistically significant detection of such an exotic atom.

Phys. Rev. Lett. 117, 112001 (2016)