View source for Strangeness

{{Short description|Property of elementary particles}}
{{About||the 2022 Italian comedy film|Strangeness (film)}}
{{see also|Strangeness and quark–gluon plasma}}
{{Flavour_quantum_numbers}}
In [[particle physics]], '''strangeness''' (symbol '''''S''''')<ref>{{Cite book|last=Jacob|first=Maurice|url=https://www.worldscientific.com/worldscibooks/10.1142/1653|title=The Quark Structure of Matter|publisher=World Scientific|year=1992|isbn=978-981-02-0962-9| series=World Scientific Lecture Notes in Physics|volume=50|language=en|doi=10.1142/1653}}</ref><ref>{{Cite journal| last1=Tanabashi|first1=M.|last2=Hagiwara|first2=K.|last3=Hikasa|first3=K.|last4=Nakamura|first4=K.|last5=Sumino|first5=Y.|last6=Takahashi|first6=F.|last7=Tanaka|first7=J.|last8=Agashe|first8=K.|last9=Aielli|first9=G.|last10=Amsler|first10=C.|last11=Antonelli|first11=M.|date=2018-08-17|title=Review of Particle Physics|journal=Physical Review D|language=en|volume=98|issue=3|page=030001 |doi=10.1103/PhysRevD.98.030001|pmid=10020536|bibcode=2018PhRvD..98c0001T|issn=2470-0010|quote=pages 1188 (Mesons), 1716 ff (Baryons)|doi-access=free|hdl=10044/1/68623|hdl-access=free}}</ref> is a [[Physical property|property]] of [[Particle|particles]], expressed as a [[quantum number]], for describing [[Particle decay|decay]] of particles in [[Strong interaction|strong]] and [[Electromagnetic interaction|electromagnetic interactions]] which 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 ''n''<sub>{{SubatomicParticle|Strange quark}}</sub> represents the number of [[strange quark]]s ({{SubatomicParticle|Strange quark}}) and ''n''<sub>{{SubatomicParticle|Strange antiquark}}</sub> represents the number of [[strange antiquark]]s ({{SubatomicParticle|Strange antiquark}}). Evaluation of [[strangeness production]] has become an important tool in search, discovery, observation and interpretation of [[quark–gluon plasma]] (QGP).<ref>{{Cite journal|last1=Margetis|first1=Spyridon|last2=Safarík|first2=Karel|last3=Villalobos Baillie|first3=Orlando|year=2000|title=Strangeness Production in Heavy-Ion Collisions|journal=[[Annual Review of Nuclear and Particle Science]]|language=en|volume=50|issue=1|pages=299–342|doi=10.1146/annurev.nucl.50.1.299|doi-access=free| bibcode=2000ARNPS..50..299S|issn=0163-8998}}</ref> Strangeness is an excited state of matter and its decay is governed by [[Cabibbo–Kobayashi–Maskawa matrix|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 (quantum number)|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.

==Conservation==
Strangeness was introduced by [[Murray Gell-Mann]],<ref>{{Cite journal|last=Gell-Mann|first=M.|date=1953-11-01|title=Isotopic Spin and New Unstable Particles|url=https://link.aps.org/doi/10.1103/PhysRev.92.833|journal=Physical Review|language=en|volume=92|issue=3|pages=833–834|doi=10.1103/PhysRev.92.833|bibcode=1953PhRv...92..833G|issn=0031-899X}}</ref> [[Abraham Pais]],<ref>{{Cite journal|last=Pais|first=A.|date=1952-06-01|title=Some Remarks on the V -Particles|url=https://link.aps.org/doi/10.1103/PhysRev.86.663|journal=Physical Review|language=en|volume=86|issue=5|pages=663–672|doi=10.1103/PhysRev.86.663|bibcode=1952PhRv...86..663P|issn=0031-899X}}</ref><ref>{{Cite journal|last=Pais|first=A.|date=October 1953|title=On the Baryon–meson–photon System|journal=Progress of Theoretical Physics|language=en|volume=10|issue=4|pages=457–469|doi=10.1143/PTP.10.457|bibcode=1953PThPh..10..457P|issn=0033-068X|doi-access=free}}</ref> [[Tadao Nakano]] and [[Kazuhiko Nishijima]]<ref>{{Cite journal|last1=Nakano|first1=Tadao|last2=Nishijima|first2=Kazuhiko|date=November 1953|title=Charge Independence for V -particles|journal=Progress of Theoretical Physics|language=en|volume=10|issue=5|pages=581–582|doi=10.1143/PTP.10.581|bibcode=1953PThPh..10..581N|issn=0033-068X|doi-access=free}}</ref> to explain the fact that certain particles, such as the [[kaon]]s or the [[hyperon]]s {{Subatomic particle|Sigma}} and {{Subatomic particle|Lambda}}, were created easily in particle collisions, yet decayed much more slowly than expected for their large masses and large production [[cross section (physics)|cross section]]s.  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">{{Cite book|last=Griffiths, David J. (David Jeffery), 1942–|title=Introduction to elementary particles|date=1987|publisher=Wiley|isbn=0-471-60386-4|location=New York|oclc=19468842}}</ref>

In our modern understanding, strangeness is conserved during the [[strong interaction|strong]] and the [[electromagnetic interaction]]s, but not during the [[weak interaction]]s.  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 {{SubatomicParticle|Kaon0}} and {{SubatomicParticle|Antikaon0}} 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<sup>−</sup> 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">{{Cite web| url=https://www.nobelprize.org/prizes/physics/1968/alvarez/lecture/|title=The Nobel Prize in Physics 1968| website=NobelPrize.org| language=en-US|access-date=2020-03-15}}</ref>

However, 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 also==
*[[Strangeness and quark–gluon plasma]]
*[[Strange particle]]s

==References==

<references />

[[Category:Physical quantities]]
[[Category:Quarks]]
[[Category:Flavour (particle physics)]]
[[Category:Strange quark]]