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Strangeness
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In particle physics, strangeness, denoted as S, is a property of particles, expressed as a quantum number for describing decay of particles in strong and electro-magnetic reactions, which occur in a short period of time. The strangeness of a particle is defined as:
where 
represents the number of strange anti-quarks (
) and 
represents the number of strange quarks.
The derivation of the phrase "strange" or "strangeness" precedes the discovery of the quark, and was adopted after its discovery in order to preserve the continuity of the phrase; strangeness of anti-particles being referred to as +1, and particles as -1 as per the original definition. For all the quark flavor quantum numbers (strangeness, charm, topness and bottomness) the convention is that the flavor charge and the electric charge of a quark have the same sign. With this, any flavor carried by a charged meson has the same sign as its charge.
Strangeness was introduced, by Murray Gell-Mann and Kazuhiko Nishijima, originally to explain the fact that certain particles, such as the kaons or certain hyperons were created easily in particle collisions, yet decayed much more slowly than expected for their large masses and large production cross sections. Noting that collisions seemed to always produce pairs of these particles, it was postulated that a new conserved quality, dubbed "strangeness", was preserved during their creation, but not conserved in their decay.
In our modern understanding, strangeness is conserved during the strong and the electromagnetic interactions, but not during the weak interactions. Consequently the lightest particles containing a strange quark cannot decay by the strong interaction, and must instead decay via the much slower weak interaction. In most cases these decays change the value of the strangeness by one unit. However, this doesn't necessarily hold in second-order, weak reactions, where there are mixes of 
and 
mesons.
- Griffiths, David J. (1987). Introduction to Elementary Particles. Wiley, John & Sons, Inc. ISBN 0-471-60386-4.
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