In physics, bottomness (symbol B′; using a prime as plain B is used already for baryon number) 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:

${\displaystyle B^{\prime }=-(n_{b}-n_{\bar {b}})}$

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 ${\displaystyle \Delta B^{\prime }=\pm 1}$.

This term is rarely used. Most physicists simply refer to "the number of bottom quarks" and "the number of bottom antiquarks".

• Anchordoqui, L.; Halzen, F. (2009). "Lessons in Particle Physics". arXiv:0906.1271 [physics.ed-ph].

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