strangeness

In the late 1940s, cosmic-ray experiments turned up particles that behaved very strangely indeed. They were produced copiously and quickly, as if by the strong force — yet once made, they lived a surprisingly long time before decaying, far too long for the strong force to be doing the decaying. They were also always produced in pairs, never alone. To explain this odd habit, physicists invented a new quantum number and frankly named it strangeness.

Strangeness is a counting label, like electric charge or baryon number, that each particle carries. We now know it simply counts strange quarks: each strange quark contributes -1 unit of strangeness, each strange antiquark +1. The key rule is that the strong and electromagnetic forces conserve strangeness — they cannot change the net number of strange quarks — while the weak force does not. This explains everything odd about these particles. They are made by the strong force, which must produce strange quarks in matched quark-antiquark pairs (so total strangeness stays zero, hence the pairing). But a lone strange particle can only decay by the weak force, which is slow, so it lives a comparatively long time.

Strangeness was a vital stepping stone. It was the extra axis, alongside isospin, on which the eightfold way's neat patterns were drawn, and the patterns made no sense without it. Eventually the bookkeeping was understood at a deeper level: strangeness is conserved by the strong and electromagnetic forces precisely because they cannot change one quark flavor into another, whereas the weak force can turn a strange quark into a lighter up quark, which is exactly why strange particles decay at all. Strangeness was the first of several flavor quantum numbers, soon joined by charm, bottomness, and topness as heavier quarks were discovered.