T meson

T mesons are hypothetical mesons composed of a top quark and one additional subatomic particle. The second subatomic particle can be either an up antiquark, down antiquark, strange antiquark, charm antiquark or bottom antiquark.
Because of the top quark's short lifetime, T mesons are not expected to be found in nature.
The combination of a top quark and top antiquark is not an open T meson, but rather a closed top-quark meson called toponium. Each T meson has an antiparticle that is composed of a top antiquark and an up, down, strange, charm quark or bottom quark respectively.