Topness (formerly also called Truth) quantum number is the number of top quarks (t) minus the number of top anti-quarks (
) that are present in a particle:
Top quarks have a topness of +1 and anti-top quarks have a topness of −1.
Topness is preserved under strong and electromagnetic interaction, but not under weak interaction. However top quark is extremely unstable, with a half life under 10−23 s, which is the required time for the strong interaction to take place. For that reason top is never found to form any meson or barion: by the time it can interact strongly it has already decayed to another flavor of quark (usually to a bottom quark).
See also