In physics, bottomness (symbol B′) or beauty is a flavour quantum number reflecting the difference between the number of bottom antiquarks (n
b
) and the number of bottom quarks (n
b
) that are present in a particle:
Bottom quarks have (by convention) a bottomness of −1 while bottom antiquarks have a bottomness of +1. The convention is that the flavour quantum number sign for the quark is the same as the sign of the electric charge (symbol Q) of that quark (in this case, Q = − 1⁄3).
As with other flavour-related quantum numbers, bottomness is preserved under strong and electromagnetic interactions, but not under weak interactions. For first-order weak reactions, it holds that
This term is rarely used. Most physicists simply refer to "the number of bottom quarks" and "the number of bottom antiquarks".