Theta meson

Not to be confused with the θ of the τ–θ puzzle, which is now identified as kaon, or with the Θ and Θ+
c, formerly thought to be pentaquarks.

The theta meson (θ) is a hypothetical form of quarkonium (i.e. a flavourless meson) formed by a top quark and top antiquark. It is the equivalent of the phi (strange quark, strange antiquark), psi (charm quark, charm antiquark) and upsilon (bottom quark, bottom antiquark) mesons. Due to the top quark's shortlifetime, the theta meson is not expected to be observed in nature.