View source for Kaon

{{Short description|Quantum particle}}
{{Other uses}}
{{Distinguish|Kōan}}
{{Infobox Particle
| name = Kaon
| image = 
| caption = 
| num_types = 4
| composition = {{SubatomicParticle|Kaon+}}: {{SubatomicParticle|Up quark}}{{SubatomicParticle|Strange antiquark}}<br>
{{SubatomicParticle|Kaon0}}: {{SubatomicParticle|Down quark}}{{SubatomicParticle|Strange antiquark}}<br>
{{SubatomicParticle|Kaon-}}: {{SubatomicParticle|Strange quark}}{{SubatomicParticle|Up antiquark}}
| statistics = [[Bosonic]]
| group = [[Meson]]s
| interaction = [[Strong interaction|Strong]], [[Weak interaction|weak]], [[Electromagnetic interaction|electromagnetic]], [[Gravity|gravitational]]
| antiparticle = {{SubatomicParticle|Kaon+}}: {{SubatomicParticle|Kaon-}}<br>
{{SubatomicParticle|Kaon0}}: {{SubatomicParticle|antiKaon0}}<br>
{{SubatomicParticle|Kaon-}}: {{SubatomicParticle|Kaon+}}
| theorized = 
| discovered = 1947 ([[Clifford Butler]] and [[George Rochester]] at [[Department of Physics and Astronomy, University of Manchester]])
| symbol = {{SubatomicParticle|Kaon+}}, {{SubatomicParticle|Kaon0}}, {{SubatomicParticle|Kaon-}}
| mass = {{SubatomicParticle|Kaon+-}}: {{val|493.677|0.016|ul=MeV/c2}}<br />{{SubatomicParticle|Kaon0}}: {{val|497.611|0.013|u=MeV/c2}}
| mean_lifetime = {{SubatomicParticle|Kaon+-}}: {{val|1.2380|0.0020|e=-8|ul=s}}<br />{{SubatomicParticle|K-short}}: {{val|8.954|0.004|e=-11|u=s}}<br />{{SubatomicParticle|K-long}}: {{val|5.116|0.021|e=-8|u=s}}
| decay_particle =
| electric_charge = {{SubatomicParticle|Kaon+-}}: ±1&nbsp;[[Elementary charge|''e'']]<br />{{SubatomicParticle|Kaon0}}: 0&nbsp;''e''
| spin = 0&nbsp;[[reduced Planck constant|''ħ'']]
| strangeness = {{SubatomicParticle|Kaon+}}, {{SubatomicParticle|Kaon0}}: +1<br>
{{SubatomicParticle|Kaon-}}, {{SubatomicParticle|antiKaon0}}: −1
| charm = 
| bottomness = 
| topness = 
| isospin = {{SubatomicParticle|Kaon+}}, {{SubatomicParticle|antiKaon0}}: +{{sfrac|1|2}}<br>
{{SubatomicParticle|Kaon0}}, {{SubatomicParticle|Kaon-}}: −{{sfrac|1|2}}
| hypercharge = 
| parity = −1
| c_parity = 
}}[[File:Kaon-Decay.svg|thumb|300px| The decay of a kaon&nbsp;({{SubatomicParticle|Kaon+}}) into three [[pion]]s&nbsp;(2&nbsp;{{SubatomicParticle|Pion+}},&nbsp;1&nbsp;{{SubatomicParticle|Pion-}}) is a process that involves both [[weak interaction|weak]] and [[strong interaction]]s.

''Weak interactions'' : The [[strange antiquark]]&nbsp;({{SubatomicParticle|Strange antiquark}}) of the kaon transmutes into an [[up antiquark]]&nbsp;({{SubatomicParticle|Up antiquark}}) by the emission of a [[W and Z bosons|{{SubatomicParticle|W boson+}} boson]]; the {{SubatomicParticle|W boson+}} boson subsequently decays into a [[down antiquark]]&nbsp;

({{SubatomicParticle|Down antiquark}}) and an [[up quark]]&nbsp;({{SubatomicParticle|Up quark}}).

''Strong interactions'': An up quark&nbsp;({{SubatomicParticle|up quark}}) emits a [[gluon]]&nbsp;({{SubatomicParticle|gluon}}) which decays into a down quark&nbsp;({{SubatomicParticle|down quark}}) and a down antiquark&nbsp;({{SubatomicParticle|Down antiquark}}).]] In [[particle physics]], a '''kaon''', also called a '''K meson''' and denoted {{SubatomicParticle|Kaon}},{{efn|Until the 1960s the positively charged kaon was formerly called τ<sup>+</sup> or θ<sup>+</sup>, as it was believed to be two different particles. See the [[#Parity violation|§&nbsp;Parity violation]].}} is any of a group of four [[meson]]s distinguished by a [[quantum number]] called [[Strangeness (particle physics)|strangeness]]. In the [[quark model]] they are understood to be [[bound states]] of a [[strange quark]] (or antiquark) and an [[up quark|up]] or [[down quark|down]] antiquark (or quark).

Kaons have proved to be a copious source of information on the nature of [[fundamental interactions]] since their discovery by [[George Rochester]] and [[Clifford Butler]] at the [[Department of Physics and Astronomy, University of Manchester]] in [[cosmic ray]]s in 1947. They were essential in establishing the foundations of the [[Standard Model]] of particle physics, such as the [[quark model]] of [[hadron]]s and the theory of [[Cabibbo–Kobayashi–Maskawa matrix|quark mixing]] (the latter was acknowledged by a [[Nobel Prize in Physics]] in 2008). Kaons have played a distinguished role in our understanding of fundamental [[Conservation law (physics)|conservation law]]s: [[CP violation]], a phenomenon generating the observed matter–antimatter asymmetry of the universe, was discovered in the kaon system in 1964 (which was acknowledged by a Nobel Prize in 1980). Moreover, direct CP violation was discovered in the kaon decays in the early 2000s by the [[NA48 experiment]] at [[CERN]] and the KTeV experiment at [[Fermilab]].

== Basic properties ==
The four kaons are:
# {{SubatomicParticle|Kaon-}}, negatively charged (containing a [[strange quark]] and an [[up quark|up antiquark]]) has mass {{val|493.677|0.013|u=MeV}} and [[mean lifetime]] {{val|1.2380|0.0020|e=-8|u=s}}.
# {{SubatomicParticle|Kaon+}} ([[antiparticle]] of above) positively charged (containing an [[up quark]] and a [[Strange quark|strange antiquark]]) must  (by [[CPT invariance]]) have mass and lifetime equal to that of {{SubatomicParticle|Kaon-}}. Experimentally, the mass difference is {{val|0.032|0.090|u=MeV}}, consistent with zero; the difference in lifetimes is {{val|0.11|0.09|e=-8|u=s}}, also consistent with zero.
# {{SubatomicParticle|Kaon0}}, neutrally charged (containing a [[down quark]] and a [[strange quark|strange antiquark]]) has mass {{val|497.648|0.022|u=MeV}}. It has mean squared [[charge radius]] of {{val|-0.076|0.01|ul=fm2}}.
# {{SubatomicParticle|AntiKaon0}}, neutrally charged (antiparticle of above) (containing a [[strange quark]] and a [[down quark|down antiquark]]) has the same mass.

As the [[quark model]] shows, assignments that the kaons form two [[Doublet state|doublets]] of [[isospin]]; that is, they belong to the [[fundamental representation]] of [[SU(2)]] called the '''2'''. One doublet of strangeness +1 contains the {{SubatomicParticle|Kaon+}} and the {{SubatomicParticle|Kaon0}}. The antiparticles form the other doublet (of strangeness &minus;1).

{| class="wikitable sortable" style="text-align: center;"
|+ Properties of kaons
|-
! class=unsortable|Particle<br/>name
! Particle <br>symbol
! Antiparticle <br>symbol
! class=unsortable | Quark <br>content
! [[Rest mass]]<br/>([[electron volt|MeV]]/[[speed of light|c]]<sup>2</sup>)
! [[Isospin|I]]<sup>[[G parity|G]]</sup>
! [[Total angular momentum|J]]<sup>[[Parity (physics)|P]][[C parity|C]]</sup>
! [[strangeness|S]]
! [[charm (quantum number)|C]]
! [[bottomness|B']]
! [[Mean lifetime]] ([[second|s]])
! class=unsortable | Commonly decays to <br>(>5% of decays)
|-
| Kaon<ref name=PDGKaon>{{cite web |author1=Zyla, P.A. |display-authors=etal |year=2020 |url=http://pdg.lbl.gov/2021/listings/rpp2021-list-K-plus-minus.pdf |title=Particle listings – {{SubatomicParticle|Kaon+-}} }}</ref>
| style="text-align:center;" | {{SubatomicParticle|link=yes|Kaon+}}
| style="text-align:center;" | {{SubatomicParticle|link=yes|Kaon-}}
| style="text-align:center;" | {{SubatomicParticle|link=yes|Up quark}}{{SubatomicParticle|link=yes|Strange antiquark}}
| style="text-align:left;" | {{val|493.677|0.016}}
| style="text-align:center;" | {{frac|1|2}}
| style="text-align:center;" | 0<sup>&minus;</sup>
| style="text-align:center;" | 1
| style="text-align:center;" | 0
| style="text-align:center;" | 0
| style="text-align:right;" | {{val|1.2380|0.0020|e=-8}}
| style="text-align:left;" | {{math|{{SubatomicParticle|link=yes|Antimuon}} + {{SubatomicParticle|link=yes|Muon neutrino}}}} or {{math|{{SubatomicParticle|link=yes|Pion+}} + {{SubatomicParticle|link=yes|Pion0}}}} or {{math|{{SubatomicParticle|link=yes|pion+}} + {{SubatomicParticle|link=yes|pion+}} + {{SubatomicParticle|link=yes|pion-}}}} or {{math|{{SubatomicParticle|link=yes|pion0}} + {{SubatomicParticle|link=yes|positron}} + {{SubatomicParticle|link=yes|Electron neutrino}}}}
|-
| Kaon<ref name=PDGKaon0>{{cite web |author1=Zyla, P.A. |display-authors=etal |year=2020 |url=http://pdg.lbl.gov/2021/listings/rpp2021-list-K-zero.pdf |title=Particle listings – {{SubatomicParticle|Kaon0}} }}</ref>
| style="text-align:center;" | {{SubatomicParticle|link=yes|Kaon0}}
| style="text-align:center;" | {{SubatomicParticle|link=yes|Antikaon0}}
| style="text-align:center;" | {{SubatomicParticle|link=yes|Down quark}}{{SubatomicParticle|link=yes|Strange antiquark}}
| style="text-align:left;" | {{val|497.611|0.013}}
| style="text-align:center;" | {{frac|1|2}}
| style="text-align:center;" | 0<sup>&minus;</sup>
| style="text-align:center;" | 1
| style="text-align:center;" | 0
| style="text-align:center;" | 0
| style="text-align:center;" | <sup>{{ref|strongforce|[§]}}</sup>
| style="text-align:center;" | <sup>{{ref|strongforce|[§]}}</sup>
|-
| K-Short<ref name=PDGK-short>{{cite web |author1=Zyla, P.A. |display-authors=etal |year=2020 |url=http://pdg.lbl.gov/2021/listings/rpp2021-list-K-zero-S.pdf |title=Particle listings – {{SubatomicParticle|K-short0}} }}</ref>
| style="text-align:center;" | {{SubatomicParticle|link=yes|K-short0}}
| style="text-align:center;" | Self
| style="text-align:center;" | <math>\mathrm{\tfrac{d\bar{s} - s\bar{d}}{\sqrt{2}}}\,</math><sup>{{ref|Kaon|[†]}}</sup><ref name="cambridge">{{cite web|author=M.A. Thomson|website=Cambridge HEP group|access-date=2024-06-02|url=https://www.hep.phy.cam.ac.uk/~thomson/MPP/partIIIparticles/Handout_12_2011.pdf|title=The CKM Matrix and CP Violation}}</ref><ref name="southampton">{{cite web|website=University of Southampton|access-date=2024-06-02|url=https://www.soton.ac.uk/~ab1u06//teaching/phys3002/course/20_PCCP.pdf|title=Parity, Charge Conjugation and CP}}</ref>
| style="text-align:center;" | {{val|497.611|0.013}}<sup>{{ref|Kaonmass|[‡]}}</sup>
| style="text-align:center;" | {{frac|1|2}}
| style="text-align:center;" | 0<sup>&minus;</sup>
| style="text-align:center;" | [[List of mesons#Notes on neutral kaons|<sup>[*]</sup>]]
| style="text-align:center;" | 0
| style="text-align:center;" | 0
| style="text-align:right;" | {{val|8.954|0.004|e=-11}}
| style="text-align:left;" | {{math|{{SubatomicParticle|link=yes|pion+}} + {{SubatomicParticle|link=yes|pion-}}}} or {{math|{{SubatomicParticle|link=yes|pion0}} + {{SubatomicParticle|link=yes|pion0}}}}
|-
| K-Long<ref name=PDGK-long>{{cite web |author1=Zyla, P.A. |display-authors=etal |year=2020 |url=http://pdg.lbl.gov/2021/listings/rpp2021-list-K-zero-L.pdf |title=Particle listings – {{SubatomicParticle|k-long0}} }}</ref>
| style="text-align:center;" | {{SubatomicParticle|link=yes|K-long0}}
| style="text-align:center;" | Self
| style="text-align:center;" | <math>\mathrm{\tfrac{d\bar{s} + s\bar{d}}{\sqrt{2}}}\,</math><sup>{{ref|Kaon|[†]}}</sup><ref name="cambridge"></ref><ref name="southampton"></ref>
| style="text-align:left;" | {{val|497.611|0.013}}<sup>{{ref|Kaonmass|[‡]}}</sup>
| style="text-align:center;" | {{frac|1|2}}
| style="text-align:center;" | 0<sup>&minus;</sup>
| style="text-align:center;" | [[List of mesons#Notes on neutral kaons|<sup>[*]</sup>]]
| style="text-align:center;" | 0
| style="text-align:center;" | 0
| style="text-align:right;" | {{val|5.116|0.021|e=-8}}
| style="text-align:left;" | {{math|{{SubatomicParticle|link=yes|Pion+-}} + {{SubatomicParticle|link=yes|electron-+}} + {{SubatomicParticle|link=yes|Electron neutrino}}}} or {{math|{{SubatomicParticle|link=yes|Pion+-}} + {{SubatomicParticle|link=yes|muon-+}} + {{SubatomicParticle|link=yes|Muon neutrino}}}} or {{math|{{SubatomicParticle|link=yes|Pion0}} + {{SubatomicParticle|link=yes|Pion0}} + {{SubatomicParticle|link=yes|Pion0}}}} or {{math|{{SubatomicParticle|link=yes|Pion+}} + {{SubatomicParticle|link=yes|Pion0}} + {{SubatomicParticle|link=yes|Pion-}}}}
|}
[[File:Quark structure kaon plus.svg|thumb|150x150px|Quark structure of the kaon (K<sup>+</sup>).]]
<sup>[*]</sup> See [[List of mesons#Notes on neutral kaons|Notes on neutral kaons]] in the article [[List of mesons]], and [[#Neutral kaon mixing|neutral kaon mixing]], below.<br>
<sup>[§]</sup>{{note|strongforce}}[[Strong force|Strong]] [[eigenstate]]. No definite lifetime (see [[#Neutral kaon mixing|neutral kaon mixing]]).<br>
<sup>[†]</sup>{{note|Kaon}}[[Weak force|Weak]] [[eigenstate]]. Makeup is missing small [[CP violation|CP–violating]] term (see [[#Neutral kaon mixing|neutral kaon mixing]]).<br>
<sup>[‡]</sup>{{note|Kaonmass}} The mass of the {{SubatomicParticle|K-long0}} and {{SubatomicParticle|K-short0}} are given as that of the {{SubatomicParticle|Kaon0}}. However, it is known that a relatively minute difference between the masses of the {{SubatomicParticle|K-long0}} and {{SubatomicParticle|K-short0}} on the order of {{val|3.5|e=-6|u=eV/c2}} exists.<ref name=PDGK-long/>

Although the {{SubatomicParticle|Kaon0}} and its antiparticle {{SubatomicParticle|AntiKaon0}} are usually produced via the [[strong force]], they decay [[weak force|weakly]]. Thus, once created the two are better thought of as superpositions of two weak [[eigenstate]]s which have vastly different lifetimes:
* The '''long'''-lived neutral kaon is called the {{SubatomicParticle|K-long}} ("K-long"), decays primarily into three [[pion]]s, and has a mean lifetime of {{val|5.18|e=-8|u=s}}.
* The '''short'''-lived neutral kaon is called the {{SubatomicParticle|K-short}} ("K-short"), decays primarily into two pions, and has a mean lifetime {{val|8.958|e=-11|u=s}}.[[File:Quark structure antikaon.svg|thumb|150x150px|Quark structure of the antikaon (K<sup>−</sup>).]]

(''See discussion of [[#Neutral kaon mixing|neutral kaon mixing]] below.'')

An experimental observation made in 1964 that K-longs rarely decay into two pions was the discovery of [[CP violation]] (see below).

Main decay modes for {{SubatomicParticle|Kaon+}}:
: [[File:Quark structure neutral kaon.svg|thumb|150x150px|Quark structure of the neutral kaon (K<sup>0</sup>).]]
 {| class="wikitable sortable"
 ! Results
 ! Mode
 ! [[Branching ratio]]
 |- style="height: 2em;"
 | {{math|{{SubatomicParticle|Antimuon}} {{SubatomicParticle|Muon neutrino}}}}
 | leptonic
 | style="text-align:right;" | {{val|63.55|0.11|s=%}}
 |- style="height: 2em;"
 | {{math|{{SubatomicParticle|Pion+}} {{SubatomicParticle|Pion0}}}}
 | hadronic
 | style="text-align:right;" | {{val|20.66|0.08|s=%}}
 |- style="height: 2em;"
 | {{math|{{SubatomicParticle|Pion+}} {{SubatomicParticle|Pion+}} {{SubatomicParticle|Pion-}}}}
 | hadronic
 | style="text-align:right;" | {{val|5.59|0.04|s=%}}
 |- style="height: 2em;"
 | {{math|{{SubatomicParticle|Pion+}} {{SubatomicParticle|Pion0}} {{SubatomicParticle|Pion0}}}}
 | hadronic
 | style="text-align:right;" | {{val|1.761|0.022|s=%}}
 |- style="height: 2em;"
 | {{math|{{SubatomicParticle|Pion0}} {{SubatomicParticle|Positron}} {{SubatomicParticle|Electron neutrino}}}}
 | semileptonic
 | style="text-align:right;" | {{val|5.07|0.04|s=%}}
 |- style="height: 2em;"
 | {{math|{{SubatomicParticle|Pion0}} {{SubatomicParticle|Muon+}} {{SubatomicParticle|Muon neutrino}}}}
 | semileptonic
 | style="text-align:right;" | {{val|3.353|0.034|s=%}}
 |}

Decay modes for the {{SubatomicParticle|Kaon-}} are charge conjugates of the ones above.

== Parity violation ==
Two different decays were found for charged strange mesons into [[pion]]s:
: {| border=0
|- style="height: 2em;"
| {{SubatomicParticle|Theta+}}    || → || {{SubatomicParticle|Pion+}} + {{SubatomicParticle|Pion0}}
|- style="height: 2em;"
| {{SubatomicParticle|Antitauon}} || → || {{SubatomicParticle|Pion+}} + {{SubatomicParticle|Pion+}} + {{SubatomicParticle|Pion-}}
|}
The intrinsic [[parity (physics)|parity]] of the pion is P&nbsp;=&nbsp;−1 (since the pion is a bound state of a quark and an antiquark, which have opposite parities, with zero angular momentum), and parity is a multiplicative quantum number. Therefore, assuming the parent particle has zero spin, the two-pion and the three-pion final states have different parities (P&nbsp;=&nbsp;+1 and P&nbsp;=&nbsp;−1, respectively). It was thought that the initial states should also have different parities, and hence be two distinct particles. However, with increasingly precise measurements, no difference was found between the masses and lifetimes of each, respectively, indicating that they are the same particle. This was known as the '''τ–θ puzzle'''. It was resolved only by the discovery of [[parity (physics)#Parity violation|parity violation]] in [[weak interaction]]s (most importantly, by the [[Wu experiment]]). Since the mesons decay through weak interactions, parity is not conserved, and the two decays are actually decays of the same particle,<ref name="Lee 1956">{{cite journal |last1=Lee |first1=T. D. |author-link1=Tsung-Dao Lee |last2=Yang |first2=C. N. |author-link2=Chen Ning Yang |title=Question of Parity Conservation in Weak Interactions |journal=[[Physical Review]] |date=1 October 1956 |volume=104 |number=1 |doi=10.1103/PhysRev.104.254 |page=254 |quote=One way out of the difficulty is to assume that parity is not strictly conserved, so that {{SubatomicParticle|Theta+}} and {{SubatomicParticle|Antitauon}} are two different decay modes of the same particle, which necessarily has a single mass value and a single lifetime.|bibcode = 1956PhRv..104..254L |doi-access=free }}</ref> now called the {{SubatomicParticle|Kaon+}}.

== History ==
{{See also|Strangeness}}
<blockquote> The discovery of hadrons with the internal quantum number "strangeness" marks the beginning of a most exciting epoch in particle physics that even now, fifty years later, has not yet found its conclusion ... by and large experiments have driven the development, and that major discoveries came unexpectedly or even against expectations expressed by theorists. &nbsp;—&nbsp;Bigi & Sanda (2016)<ref>{{cite book |author1=Bigi, I.I. |author2=Sanda, A.I. |date=2016-10-06 |title=CP Violation |edition=5th |publisher=Cambridge University Press |series=Cambridge Monographs on Particle Physics, Nuclear Physics, and Cosmology |volume=28 |isbn=978-0-521-44349-4}}</ref></blockquote>

While looking for the hypothetical [[Meson#History|nuclear meson]], [[Louis Leprince-Ringuet]] found evidence for the existence of a positively charged heavier particle in 1944.<ref name=":0">{{cite journal |last1=Degrange |first1=Bernard |last2=Fontaine |first2=Gérard |last3=Fleury |first3=Patrick |year=2013 |title=Tracking Louis Leprince-Ringuet's contributions to cosmic-ray physics |journal=Physics Today |lang=en |volume=66 |issue=6 |pages=8 |doi=10.1063/PT.3.1989 |bibcode=2013PhT....66f...8D |issn=0031-9228 |url=http://physicstoday.scitation.org/doi/10.1063/PT.3.1989}}</ref><ref name=":1">{{cite conference |last=Ravel |first=Olivier |year=2012 |chapter=Early cosmic ray research in France |editor-last=Ormes |editor-first=Jonathan F. |title=Centenary Symposium 2012: Discovery of cosmic rays |location=Denver, Colorado |series=AIP Conference Proceedings |volume=1516 |pages=67–71 |publisher=[[American Institute of Physics]] |doi=10.1063/1.4792542 |bibcode=2013AIPC.1516...67R |isbn=978-0-7354-1137-1 |chapter-url=https://hal.archives-ouvertes.fr/hal-00841758}}</ref>

In 1947, [[George Rochester|G.D. Rochester]] and [[Clifford Charles Butler|C.C. Butler]] of the [[University of Manchester]] published two [[cloud chamber]] photographs of [[cosmic ray]]-induced events, one showing what appeared to be a neutral particle decaying into two charged pions, and one which appeared to be a charged particle decaying into a charged pion and something neutral. The estimated mass of the new particles was very rough, about half a proton's mass. More examples of these "V-particles" were slow in coming.

[[File:The "k track plate" showing three-pion decay mode of a kaon, 15 Jan 1949.png|thumb|The "k track plate" showing the three-pion decay mode of a kaon. The kaon enters from the left, and decays at the point labelled A]]
In 1949, [[Rosemary Fowler|Rosemary Brown]] (later Rosemary Fowler), a research student of [[C. F. Powell|Cecil Powell]] of the [[University of Bristol]], spotted her 'k'&nbsp;track, made by a particle of very similar mass that decayed to three pions.<ref name=":2">{{Cite web |date=2024-07-23 |title=Physicist pioneer, 98, honoured 75 years after discovery |url=https://www.bbc.com/news/articles/cp38dz7gyjxo |access-date=2024-07-23 |website=BBC News |language=en-GB}}</ref><ref name=Brown-Camerini-etal-1949/>{{rp|style=ama|p=82}} 

<Blockquote> I knew at once that it was new and would be very important. We were seeing things that hadn't been seen before - that's what research in particle physics was. It was very exciting.&nbsp;—&nbsp;Fowler (2024)<ref name=":2" /></Blockquote>

This led to the so-called 'tau–theta' problem:<ref name="Sheehy2024">{{Cite journal |last=Sheehy |first=Suzie |date=2024-01-15 |title=How a forgotten physicist's discovery broke the symmetry of the Universe |url=https://www.nature.com/articles/d41586-024-00109-5 |journal=Nature |language=en |volume=625 |issue=7995 |pages=448–449 |access-date=23 July 2024 |doi=10.1038/d41586-024-00109-5|pmid=38225463 |bibcode=2024Natur.625..448S }}</ref> what seemed to be the same particle (now called {{SubatomicParticle|Kaon+}}) decayed in two different modes, Theta to two pions (parity +1), Tau to three pions (parity −1).<ref name="Brown-Camerini-etal-1949">{{cite journal |last1=Brown |first1=R. |last2=Camerini |first2=U. |last3=Fowler |first3=P.H. |last4=Muirhead |first4=H. |last5=Powell |first5=C.F. |last6=Ritson |first6=D.M. |year=1949 |title=Part&nbsp;2: Observations with electron-sensitive plates exposed to cosmic radiation |journal=[[Nature (journal)|Nature]] |volume=163 |issue=4133 |pages=82–87 |doi=10.1038/163082a0 |bibcode=1949Natur.163...82B |s2cid=12974912}}  Note same issue: {{cite journal |author1=Brown |display-authors=etal |year=1949 |title=Part&nbsp;1 |journal=Nature |volume=163 |issue=4133 |pages=47–51|doi=10.1038/163047a0 |s2cid=4097342 }}</ref> The solution to this puzzle turned out to be that weak interactions [[Parity violation|do not conserve parity]].<ref name="Lee 1956" />

The first breakthrough was obtained at [[California Institute of Technology|Caltech]], where a cloud chamber was taken up [[Mount Wilson (California)|Mount Wilson]], for greater cosmic ray exposure. In 1950, 30&nbsp;charged and 4&nbsp;neutral "V-particles" were reported. Inspired by this, numerous mountaintop observations were made over the next several years, and by 1953, the following terminology was being used: "L&nbsp;meson" for either a [[muon]] or charged [[pion]]; "K&nbsp;meson" meant a particle intermediate in mass between the pion and [[nucleon]].

Leprince-Rinquet coined the still-used term "[[hyperon]]" to mean any particle heavier than a nucleon.<ref name=":0"/><ref name=":1"/> The Leprince-Ringuet particle turned out to be the K{{sup|+}} meson.<ref name=":0"/><ref name=":1"/>

The decays were extremely slow; typical lifetimes are of the order of {{val|e=-10|u=seconds}}. However, production in [[pion]]–[[proton]] reactions proceeds much faster, with a time scale of {{val|e=-23|u=s}}. The problem of this mismatch was solved by [[Abraham Pais]] who postulated the new quantum number called "[[Strangeness (particle physics)|strangeness]]" which is conserved in [[strong interaction]]s but violated by the [[weak interaction]]s. [[Strange particle]]s appear copiously due to "associated production" of a strange and an antistrange particle together. It was soon shown that this could not be a [[multiplicative quantum number]], because that would allow reactions which were never seen in the new [[synchrotron]]s which were commissioned in [[Brookhaven National Laboratory]] in 1953 and in the [[Lawrence Berkeley Laboratory]] in 1955.

== CP violation in neutral meson oscillations ==
Initially it was thought that although [[parity (physics)|parity]] was violated, [[CP symmetry|CP (charge parity) symmetry]] was conserved. In order to understand the discovery of [[CP violation]], it is necessary to understand the mixing of neutral kaons; this phenomenon does not require CP violation, but it is the context in which CP violation was first observed.

=== Neutral kaon mixing ===
<!--This section is redirected to from [[Neutral kaon mixing]].-->
[[File:Kaon-box-diagram-with-bar.svg|thumb|Two different neutral K mesons, carrying different strangeness, can turn from one into another through the [[weak interaction]]s, since these interactions do not conserve strangeness. The strange quark in the anti-{{SubatomicParticle|Kaon0}} turns into a down quark by successively absorbing two [[W-boson]]s of opposite charge. The down antiquark in the anti-{{SubatomicParticle|Kaon0}} turns into a strange antiquark by emitting them.]]

Since neutral kaons carry strangeness, they cannot be their own antiparticles. There must be then two different neutral kaons, differing by two units of strangeness. The question was then how to establish the presence of these two mesons. The solution used a phenomenon called '''[[neutral particle oscillations]]''', by which these two kinds of mesons can turn from one into another through the weak interactions, which cause them to decay into pions (see the adjacent figure).

These oscillations were first investigated by [[Murray Gell-Mann]] and [[Abraham Pais]] together. They considered the CP-invariant time evolution of states with opposite strangeness. In matrix notation one can write
:: <math> \psi(t) = U(t)\psi(0) = {\rm e}^{iHt} \begin{pmatrix}a \\ b\end{pmatrix}, \qquad H =\begin{pmatrix}M & \Delta\\ \Delta & M\end{pmatrix} ,</math>
where ''ψ'' is a [[quantum state]] of the system specified by the amplitudes of being in each of the two [[quantum mechanics#Basis states|basis states]] (which are ''a'' and ''b'' at time ''t''&nbsp;=&nbsp;0). The diagonal elements (''M'') of the [[Hamiltonian (quantum mechanics)|Hamiltonian]] are due to [[strong interaction]] physics which conserves strangeness. The two diagonal elements must be equal, since the particle and antiparticle have equal masses in the absence of the weak interactions. The off-diagonal elements, which mix opposite strangeness particles, are due to [[weak interactions]]; [[CP symmetry]] requires them to be real.

The consequence of the matrix ''H'' being real is that the probabilities of the two states will forever oscillate back and forth. However, if any part of the matrix were imaginary, as is forbidden by [[CP symmetry]], then part of the combination will diminish over time. The diminishing part can be either one component (''a'') or the other (''b''), or a mixture of the two.

==== Mixing ====
The eigenstates are obtained by diagonalizing this matrix. This gives new eigenvectors, which we can call '''K<sub>1</sub>''' which is the  difference  of the two states of opposite strangeness, and '''K<sub>2</sub>''', which is the  sum. The two are eigenstates of '''CP''' with opposite eigenvalues; '''K<sub>1</sub>''' has '''CP'''&nbsp;=&nbsp;+1, and '''K<sub>2</sub>''' has '''CP'''&nbsp;=&nbsp;−1 Since the two-pion final state also has '''CP'''&nbsp;=&nbsp;+1, only the '''K<sub>1</sub>''' can decay this way. The '''K<sub>2</sub>''' must decay into three pions. <ref>* {{cite book |last1=Griffiths |first1=D. J. |year=1987 |title=Introduction to Elementary Particle |publisher=[[John Wiley & Sons]] |isbn=0-471-60386-4}}</ref>

Since the mass of '''K<sub>2</sub>''' is just a little larger than the sum of the masses of three pions, this decay proceeds very slowly, about 600 times slower than the decay of '''K<sub>1</sub>''' into two pions. These two different modes of decay were observed by [[Leon Lederman]] and his coworkers in 1956, establishing the existence of the two [[weak interaction|weak]] [[eigenstate]]s (states with definite [[mean lifetime|lifetimes]] under decays via the [[weak force]]) of the neutral kaons.

These two weak eigenstates are called the {{SubatomicParticle|K-long}} (K-long, τ) and {{SubatomicParticle|K-short}} (K-short, θ).  [[CP symmetry]], which was assumed at the time, implies that {{SubatomicParticle|K-short}}&nbsp;=&nbsp;'''K<sub>1</sub>''' and {{SubatomicParticle|K-long}}&nbsp;=&nbsp;'''K<sub>2</sub>'''.

==== Oscillation ====
{{Main|Neutral particle oscillation}}
An initially pure beam of {{SubatomicParticle|Kaon0}} will turn into its antiparticle, {{SubatomicParticle|AntiKaon0}}, while propagating, which will turn back into the original particle, {{SubatomicParticle|Kaon0}}, and so on. This is called particle oscillation. On observing the weak decay ''into leptons'', it was found that a {{SubatomicParticle|Kaon0}} always decayed into a positron, whereas the antiparticle {{SubatomicParticle|AntiKaon0}} decayed into the electron. The earlier analysis yielded a relation between the rate of electron and positron production from sources of pure {{SubatomicParticle|Kaon0}} and its antiparticle {{SubatomicParticle|AntiKaon0}}. Analysis of the time dependence of this [[semileptonic decay]] showed the phenomenon of oscillation, and allowed the extraction of the mass splitting between the {{SubatomicParticle|K-short}} and {{SubatomicParticle|K-long}}. Since this is due to weak interactions it is very small, 10<sup>−15</sup> times the mass of each state, namely {{nobr|{{math| ∆M{{sub|K}} {{=}} M(K{{sub|L}}) &minus; M(K{{sub|S}}) {{=}} 3.484(6)×{{10^|&minus;12}} MeV }}.}}<ref>
{{cite journal
 |last1  = Aoki        |first1  = S.       |last2  = Aoki       |first2  = Y.
 |last3  = Bečirević   |first3  = D.       |last4  = Blum       |first4  = T.
 |last5  = Colangelo   |first5  = G.       |last6  = Collins    |first6  = S.
 |last7  = Della Morte |first7  = M.       |last8  = Dimopoulos |first8  = P.
 |last9  = Dürr        |first9  = S.       |last10 = Fukaya     |first10 = H.
 |last11 = Golterman   |first11 = M.       |last12 = Gottlieb   |first12 = S.
 |last13 = Gupta       |first13 = R.       |last14 = Hashimoto  |first14 = S.
 |last15 = Heller      |first15 = U.M.     |last16 = Herdoiza   |first16 = G.
 |last17 = Horsley     |first17 = R.       |last18 = Jüttner    |first18 = A.
 |last19 = Kaneko      |first19 = T.       |last20 = Lin        |first20 = C.-J.D.
 |last21 = Lunghi      |first21 = E.       |last22 = Mawhinney  |first22 = R.
 |last23 = Nicholson   |first23 = A.       |last24 = Onogi      |first24 = T.
 |last25 = Pena        |first25 = C.       |last26 = Portelli   |first26 = A.
 |last27 = Ramos       |first27 = A.       |last28 = Sharpe     |first28 = S.R.
 |last29 = Simone      |first29 = J.N.     |last30 = Simula     |first30 = S.
 |display-authors = 6
 |year = 2020
 |title = FLAG Review 2019
 |journal = The European Physical Journal C
 |volume = 80 |issue = 2 |page = 113
 |doi = 10.1140/epjc/s10052-019-7354-7 |arxiv = 1902.08191
 |bibcode = 2020EPJC...80..113A
|s2cid = 119401756 }}
</ref>

==== Regeneration ====
A beam of neutral kaons decays in flight so that the short-lived {{SubatomicParticle|K-short}} disappears, leaving a beam of pure long-lived {{SubatomicParticle|K-long}}. If this beam is shot into matter, then the {{SubatomicParticle|Kaon0}} and its antiparticle {{SubatomicParticle|AntiKaon0}} interact differently with the nuclei. The {{SubatomicParticle|Kaon0}} undergoes quasi-[[elastic scattering]] with [[nucleon]]s, whereas its antiparticle can create [[hyperon]]s. [[Quantum coherence]] between the two particles is lost due to the different interactions that the two components separately engage in. The emerging beam then contains different linear superpositions of the {{SubatomicParticle|Kaon0}} and {{SubatomicParticle|AntiKaon0}}. Such a superposition is a mixture of {{SubatomicParticle|K-long}} and {{SubatomicParticle|K-short}}; the {{SubatomicParticle|K-short}} is regenerated by passing a neutral kaon beam through matter.<ref>{{cite journal |last1=Pais |first1=A. |last2=Piccioni |first2=O. |title=Note on the Decay and Absorption of the θ⁰ |journal=Physical Review |date=1 December 1955 |volume=100 |issue=5 |pages=1487–1489 |doi=10.1103/PhysRev.100.1487}}</ref> Regeneration was observed by [[Oreste Piccioni]] and his collaborators at [[Lawrence Berkeley National Laboratory]].<ref>{{cite journal |last1=Good |first1=R. H. |last2=Matsen |first2=R. P. |last3=Muller |first3=F. |last4=Piccioni |first4=O. |last5=Powell |first5=W. M. |last6=White |first6=H. S. |last7=Fowler |first7=W. B. |last8=Birge |first8=R. W. |title=Regeneration of Neutral K Mesons and Their Mass Difference |journal=Physical Review |date=15 November 1961 |volume=124 |issue=4 |pages=1223–1239 |doi=10.1103/PhysRev.124.1223|bibcode=1961PhRv..124.1223G }}</ref> Soon thereafter, Robert Adair and his coworkers reported excess {{SubatomicParticle|K-short}} regeneration, thus opening a new chapter in this history.

=== CP violation ===
While trying to verify Adair's results, J. Christenson, [[James Cronin]], [[Val Fitch]] and [[Rene Turlay]] of [[Princeton University]] found decays of {{SubatomicParticle|K-long}} into two pions ('''CP'''&nbsp;=&nbsp;+1)
in an [[Fitch–Cronin experiment|experiment performed in 1964]] at the [[Alternating Gradient Synchrotron]] at the [[Brookhaven National Laboratory|Brookhaven laboratory]].<ref>{{cite journal|title=Evidence for the 2π Decay of the K<sub>2</sub><sup>0</sup> Meson|first1=J. H.|last1=Christenson|first2=J. W.|last2=Cronin|first3=V. L.|last3=Fitch|first4=R.|last4=Turlay|date=27 July 1964|journal=Physical Review Letters|volume=13|issue=4|pages=138–140|doi=10.1103/physrevlett.13.138|doi-access=free|bibcode=1964PhRvL..13..138C}}</ref> As explained in [[#Mixing|an earlier section]], this required the assumed initial and final states to have different values of '''CP''', and hence immediately suggested [[CP violation]]. Alternative explanations such as nonlinear quantum mechanics and a new unobserved particle ([[hyperphoton]]) were soon ruled out, leaving CP violation as the only possibility. Cronin and Fitch received the [[Nobel Prize in Physics]] for this discovery in 1980.

It turns out that although the {{SubatomicParticle|K-long}} and {{SubatomicParticle|K-short}} are [[weak interaction|weak]] [[eigenstates]] (because they have definite [[mean lifetime|lifetimes]] for decay by way of the weak force), they are not quite '''CP''' eigenstates. Instead, for small ε (and up to normalization),
: {{SubatomicParticle|K-long}} = '''K<sub>2</sub>''' + ε'''K<sub>1</sub>'''
and similarly for {{SubatomicParticle|K-short}}. Thus occasionally the {{SubatomicParticle|K-long}} decays as a '''K<sub>1</sub>''' with '''CP'''&nbsp;=&nbsp;+1, and likewise the {{SubatomicParticle|K-short}} can decay with '''CP'''&nbsp;=&nbsp;−1. This is known as '''indirect CP violation''', CP violation due to mixing of {{SubatomicParticle|Kaon0}} and its antiparticle. There is also a '''direct CP violation''' effect, in which the CP violation occurs during the decay itself. Both are present, because both mixing and decay arise from the same interaction with the [[W boson]] and thus have CP violation predicted by the [[CKM matrix]]. Direct CP violation was discovered in the kaon decays in the early 2000s by the [[NA48]] and [[KTeV]] experiments at CERN and Fermilab.<ref>
{{cite book
 | last       = ANZIVINO
 | first      = GIUSEPPINA
 | arxiv     = hep-ph/0111393
 | title      = Multiparticle Dynamics
 | chapter = Measurement of Direct Cp Violation by Na48
 | date       =2001
 | pages = 7–14
 | doi = 10.1142/9789812778048_0002
 | isbn = 978-981-02-4844-4
 | s2cid = 15184466
}}</ref>

== See also ==
* [[Hadron]]s, [[meson]]s, [[hyperon]]s and [[flavour (particle physics)|flavour]]
* [[Strange quark]] and the [[quark model]]
* [[Parity (physics)]], [[charge conjugation]], [[T-symmetry|time reversal symmetry]], [[CPT invariance]] and [[CP violation]]
* [[Neutrino oscillation]]
* [[Neutral particle oscillation]]

== Footnotes ==
{{Notelist|1}}

== References ==
{{Reflist|40em}}

== Bibliography ==
* {{cite journal |last1=Amsler |first1=C. |last2=Doser |first2=M. |last3=Antonelli |first3=M. |last4=Asner |first4=D. |last5=Babu |first5=K. |last6=Baer |first6=H. |last7=Band |first7=H. |last8=Barnett |first8=R. |last9=Bergren |first9=E. |last10=Bergren |first10=E. |last11=Beringer |first11=J. |last12=Bernardi |first12=G. |last13=Bertl |first13=W. |last14=Bichsel |first14=H. |last15=Biebel |first15=O. |last16=Bloch |first16=P. |last17=Blucher |first17=E. |last18=Blusk |first18=S. |last19=Cahn |first19=R.N. |last20=Carena |first20=M. |last21=Caso |first21=C. |last22=Ceccucci |first22=A. |last23=Chakraborty |first23=D. |last24=Chen |first24=M.-C. |last25=Chivukula |first25=R.S. |last26=Cowan |first26=G. |last27=Dahl |first27=O. |last28=d'Ambrosio |first28=G. |last29=Damour |first29=T. |last30=de Gouvêa |first30=A. |display-authors=6 |collaboration=[[Particle Data Group]] |year=2008 |title=Review of Particle Physics |journal=[[Physics Letters B]] |volume=667 |issue=1 |pages=1–1340 |bibcode=2008PhLB..667....1A |doi=10.1016/j.physletb.2008.07.018 |hdl=1854/LU-685594 |s2cid=227119789 |url=https://www.zora.uzh.ch/id/eprint/11249/2/scalarsV.pdf |access-date=2019-12-13 |archive-date=2020-09-07 |archive-url=https://web.archive.org/web/20200907211544/https://www.zora.uzh.ch/id/eprint/11249/2/scalarsV.pdf |url-status=dead }}
* {{cite journal |last1=Eidelman |first1=S. |last2=Hayes |first2=K.G. |last3=Olive |first3=K.A. |last4=Aguilar-Benitez |first4=M. |last5=Amsler |first5=C. |last6=Asner |first6=D. |last7=Babu |first7=K.S. |last8=Barnett |first8=R.M. |last9=Beringer |first9=J. |last10=Burchat |first10=P.R. |last11=Carone |first11=C.D. |last12=Caso |first12=S. |last13=Conforto |first13=G. |last14=Dahl |first14=O. |last15=d'Ambrosio |first15=G. |last16=Doser |first16=M. |last17=Feng |first17=J. L. |last18=Gherghetta |first18=T. |last19=Gibbons |first19=L. |last20=Goodman |first20=M. |last21=Grab |first21=C. |last22=Groom |first22=D.E. |last23=Gurtu |first23=A. |last24=Hagiwara |first24=K. |last25=Hernández-Rey |first25=J.J. |last26=Hikasa |first26=K. |last27=Honscheid |first27=K. |last28=Jawahery |first28=H. |last29=Kolda |first29=C. |display-authors=6 |collaboration=[[Particle Data Group]] |year=2004 |title=Review of Particle Physics |journal=[[Physics Letters B]] |volume=592 |issue=1 |pages=1 |arxiv=astro-ph/0406663 |bibcode=2004PhLB..592....1P |doi=10.1016/j.physletb.2004.06.001}}
* ''The quark model'', by J.J.J. Kokkedee <!-- Book? --> {{full citation needed|date=November 2019}}
* {{cite book |last=Sozzi |first=M.S. |year=2008 |title=Discrete symmetries and CP violation |publisher=[[Oxford University Press]] |isbn=978-0-19-929666-8}}
* {{cite book |last1=Bigi |first1=I. I. |last2=Sanda |first2=A.I. |year=2000 |title=CP violation |publisher=[[Cambridge University Press]] |isbn=0-521-44349-0}}
* {{cite book |last1=Griffiths |first1=D. J. |year=1987 |title=Introduction to Elementary Particle |publisher=[[John Wiley & Sons]] |isbn=0-471-60386-4}}

== External links ==
{{Wiktionary|kaon}}
* [https://www.feynmanlectures.caltech.edu/III_11.html#Ch11-S5 The neutral K-meson – The Feynman Lectures on Physics]
* {{Commons category-inline}}

{{Particles}}
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[[Category:Mesons]]
[[Category:Strange quark]]
[[Category:Asymmetry]]