# Topness

 Flavour in particle physics Flavour quantum numbers: Isospin: I or I3 Charm: C Strangeness: S Topness: T Bottomness: B′ Related quantum numbers: Baryon number: B Lepton number: L Weak isospin: T or T3 Electric charge: Q X-charge: X Combinations: Hypercharge: Y Y = (B + S + C + B′ + T) Y = 2 (Q − I3) Weak hypercharge: YW YW = 2 (Q − T3) X + 2YW = 5 (B − L) Flavour mixing This box:

Topness (also called truth), a flavour quantum number, represents the difference between the number of top quarks (t) and number of top antiquarks (t) that are present in a particle:

$T = n_\text{t} - n_\bar{\text{t}}$

By convention, top quarks have a topness of +1 and top antiquarks have a topness of −1.The term "topness" is rarely used; most physicists simply refer to "the number of top quarks" and "the number of top antiquarks".

## Conservation

Like all flavour quantum numbers, topness is preserved under strong and electromagnetic interactions, but not under weak interaction. However the 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 the top quark doesn't hadronize, that is it never forms any meson or baryon, so the topness of a meson or a baryon is every time equal at zero. By the time it can interact strongly it has already decayed to another flavour of quark (usually to a bottom quark).