Theta meson
The theta meson is a hypothetical form of quarkonium formed by a top quark and top antiquark. As a P-odd and C-odd state, it is analogous to the [phi meson|], [J/psi meson|] and [upsilon meson|] mesons. Due to the top quark's short lifetime, the theta meson is expected to not be observed in nature.
In 2025, new research from the CMS and ATLAS collaborations demonstrated the existence of theta mesons at greater than 5-sigma significance.