Strangeness Number

(S), an additive quantum number characterizing hadrons, that is, strongly interacting elementary particles. All hadrons have definite integral values of S (zero, positive, or negative). The strangeness number of an antiparticle is opposite in sign to that of the corresponding particle. Hadrons with S ≠ 0 are called strange particles. The strangeness number of particles that do not participate in strong interactions—that is, of photons and leptons—is taken as 0. The greatest value of the strangeness number that has been observed is ǀSǀ = 3 (for Ω-and Ω̄-hyperons). In particle-transformation processes due to strong or electromagnetic interactions, particularly in decay processes, the total strangeness of the particles is conserved. In weak-interaction processes, conservation of strangeness does not hold; in all cases studied, ǀΔSǀ = 1.

The strangeness number was originally introduced to deal with the circumstance that single kaons or hyperons are not produced when pions collide with nucléons (protons or neutrons). Only associated production of kaons and hyperons was observed—that is, the particles were produced in pairs. The particles in such a pair were assigned strangeness numbers of opposite signs, and it was assumed that strangeness is conserved in strong-interaction processes.

For all hadrons that have been studied, the quantity S exhibits a definite relation to the electric charge Q, the baryon number B, and the projection of the isotopic spin I₃. This relation is given by the empirical Gell-Mann-Nishijima formula Q = I₃ + (S + B)/2. The quantity S + B is known as the hypercharge.