The theta meson () is a hypothetical form of quarkonium (i.e. a flavourless meson) formed by a top quark () and top antiquark (). As a P-odd and C-odd state, it is analogous to the (), () and () 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.