Strangeness

From Wikipedia, the free encyclopedia

Flavour in particle physics
Flavour quantum numbers: Lepton number: L Baryon number: B Strangeness: S Charm: C Bottomness: B' Topness: T Isospin: I_z or I Weak isospin: T_z Electric charge: Q Combinations: Hypercharge: Y Y=2(Q-I_z) Y=B+S+C+B'+T Weak hypercharge: Y_W Y_W=2(Q-T_z) Y_W=B−L Related topics: CPT symmetry CKM matrix CP symmetry Chirality

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:

$S = N_{\overline{s}} - N_s$

$N_{\overline{s}}$ representing the number (

N

) of strange anti-quarks, denoted as s-bar ( ${\overline{s}}$ )

$N_s \$ representing 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.

[edit] Strangeness conservation

The strangeness was introduced, by Murray Gell-Mann, originally to explain the fact that certain particles, such as the kaons or certain hyperons were always created in particle-antiparticle pairs. In these reactions, it was assumed that a certain unknown law, dubbed "strangeness," was preserved during their creation.

Strangeness is conserved during the strong and the electromagnetic interaction, but not during the weak interactions. Consequently the lightest particles containing a strange quark cannot decay by the strong interaction, and their otherwise anomalously long strange lifetimes via the weak interaction led to their name. In most cases the absolute value of the variation in strangeness is 1. However, this doesn't necessarily hold in second-order, weak reactions, where there are mixes of $K 0$ and $\overline{K}^0$ mesons.