Strangeness
Template:Short description Template:About Template:See also Template:Flavour quantum numbers In particle physics, strangeness (symbol S)<ref>Template:Cite book</ref><ref>Template:Cite journal</ref> is a property of particles, expressed as a quantum number, for describing decay of particles in strong and electromagnetic interactions that occur in a short period of time. The strangeness of a particle is defined as: <math display="block">S = -(n_\text{s} - n_{\bar{\text{s}}})</math> where nTemplate:SubatomicParticle represents the number of strange quarks (Template:SubatomicParticle) and nTemplate:SubatomicParticle represents the number of strange antiquarks (Template:SubatomicParticle). Evaluation of strangeness production has become an important tool in search, discovery, observation and interpretation of quark–gluon plasma (QGP).<ref>Template:Cite journal</ref> Strangeness is an excited state of matter and its decay is governed by CKM mixing.
The terms strange and strangeness predate the discovery of the quark, and were adopted after its discovery in order to preserve the continuity of the phrase: strangeness of particles as −1 and anti-particles as +1, per the original definition. For all the quark flavour quantum numbers (strangeness, charm, topness and bottomness) the convention is that the flavour charge and the electric charge of a quark have the same sign. With this, any flavour carried by a charged meson has the same sign as its charge.
ConservationEdit
Strangeness was introduced by Murray Gell-Mann,<ref>Template:Cite journal</ref> Abraham Pais,<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> Tadao Nakano and Kazuhiko Nishijima<ref>Template:Cite journal</ref> to explain the fact that certain particles, such as the kaons or the hyperons Template:Subatomic particle and Template:Subatomic particle, 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 quantity, dubbed "strangeness", was preserved during their creation, but not conserved in their decay.<ref name=":0">Template:Cite book</ref>
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. This doesn't necessarily hold in second-order weak reactions, however, where there are mixes of Template:SubatomicParticle and Template:SubatomicParticle mesons. All in all, the amount of strangeness can change in a weak interaction reaction by +1, 0 or −1 (depending on the reaction).
For example, the interaction of a K− meson with a proton is represented as: <math display="block">K^-+p \rightarrow \Xi^0+K^0</math> <math display="block">(-1) + (0) \rightarrow (-2) + (1)</math>
Here strangeness is conserved and the interaction proceeds via the strong nuclear force.<ref name=":1">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>
Nonetheless, in reactions like the decay of the positive kaon: <math display="block">K^+ \rightarrow \pi^+ + \pi^0</math> <math display="block">+1 \rightarrow (0) + (0)</math>
Since both pions have a strangeness of 0, this violates conservation of strangeness, meaning the reaction must go via the weak force.<ref name=":1" />
See alsoEdit
ReferencesEdit
<references />