Strangeness

Strangeness is a property used in particle physics to describe certain particles, encoded as a quantum number associated with the presence of strange quarks within hadrons. The concept was introduced in the 1950s to explain why some particles produced in high-energy collisions seemed to appear and decay in ways that were not easily reconciled with existing patterns, notably their relatively long lifetimes compared with other strongly produced particles. Murray Gell-Mann and Kazuhiko Nishijima independently proposed a quantum number, called strangeness, and a classification scheme that led to the Eightfold Way or SU(3) flavor symmetry.

In the quark model, strange quarks carry strangeness S = -1, while anti-strange quarks carry S = +1; up, down, and other non-strange quarks have S = 0. The strangeness of a hadron is the sum of the strangeness of its constituent quarks. Strangeness is conserved in strong and electromagnetic interactions, but can be changed in weak interactions, typically by ΔS = ±1 in a single weak decay. Strange particles are often produced in pairs to conserve strangeness in strong processes and then decay by the weaker interaction with relatively long lifetimes.

Examples of strange hadrons include kaons (K mesons) with S = ±1 and hyperons such as the Lambda (S = -1) and Sigma (S = -1). The strangeness framework helped organize experimental results and deepen understanding of quark content and hadron spectroscopy.

Beyond physics, strangeness in everyday language denotes the quality of being unusual or unfamiliar, sometimes provoking curiosity.