Strangeness ($S$)

An additive flavor symmetry quantum number: a strange quark has $S=-1$, its antiquark has $S=+1$, and up and down quarks have $S=0$. The strong interaction conserves net strangeness. Together with baryon number, it determines flavor hypercharge $Y=B+S$.

Strong and electromagnetic interactions conserve net strangeness; a decay changing it requires the weak interaction. A strange baryon decay into a less-strange baryon and a pion is an example. The weak interaction does not impose a parity selection rule, so a final orbital partial wave cannot by itself determine the parent's parity. Qualitative lifetime classes do not provide an exact lifetime without an amplitude calculation or measurement.