View source for Weak interaction

{{short description|Interaction between subatomic particles and one of the four known fundamental interactions}}
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{{Use dmy dates|date=April 2020}}{{Use Oxford spelling|date=August 2016}}
[[File:Beta-minus Decay.svg|thumb|The radioactive [[beta decay]] is due to the weak interaction, which transforms a neutron into a proton, an electron, and an [[electron neutrino|electron antineutrino]].]]

{{Standard model of particle physics}}
{{good article}}

In [[nuclear physics]] and [[particle physics]], the '''weak interaction''', also called the '''weak force''', is one of the four known [[fundamental interaction]]s, with the others being [[electromagnetism]], the [[strong interaction]], and [[gravitation]]. It is the mechanism of interaction between [[subatomic particles]] that is responsible for the [[radioactive decay]] of atoms: The weak interaction participates in [[nuclear fission]] and [[nuclear fusion]]. The theory describing its behaviour and effects is sometimes called '''quantum flavordynamics''' ('''QFD'''); however, the term QFD is rarely used, because the weak force is better understood by [[Electroweak interaction|electroweak theory]] (EWT).<ref name="griffiths">{{cite book |last=Griffiths |first=David |year=2009 |title=Introduction to Elementary Particles |isbn=978-3-527-40601-2 |pages=59–60}}</ref>

The effective range of the weak force is limited to subatomic distances and is less than the diameter of a proton.<ref>{{cite journal |last=Schwinger |first=Julian |date=1957-11-01 |title=A theory of the fundamental interactions |journal=Annals of Physics |language=en |volume=2 |issue=5 |pages=407–434 |doi=10.1016/0003-4916(57)90015-5 |bibcode=1957AnPhy...2..407S |issn=0003-4916  |url=https://dx.doi.org/10.1016/0003-4916%2857%2990015-5}}</ref>

== Background ==

The [[Standard Model]] of [[particle physics]] provides a uniform framework for understanding electromagnetic, weak, and strong interactions. An interaction occurs when two particles (typically, but not necessarily, [[half-integer]] [[Spin (physics)|spin]] [[fermion]]s) exchange integer-spin, force-carrying [[boson]]s. The fermions involved in such exchanges can be either elementary (e.g. [[electron]]s or [[quark]]s) or composite (e.g. [[proton]]s or [[neutron]]s), although at the deepest levels, all weak interactions ultimately are between [[elementary particles]].

In the weak interaction, fermions can exchange three types of force carriers, namely [[W and Z bosons|{{math|W}}{{sup|+}}, {{math|W}}{{sup|−}}, and {{math|Z}}&nbsp;bosons]]. The [[mass]]es of these bosons are far greater than the mass of a proton or neutron, which is consistent with the short range of the weak force.<ref name="hyperphysics">{{cite web |last1=Nave |first1=CR |title=Fundamental Forces - The Weak Force |url=http://hyperphysics.phy-astr.gsu.edu/hbase/Forces/funfor.html |publisher=Georgia State University |access-date=12 July 2023 |archive-url=https://web.archive.org/web/20230402013629/http://hyperphysics.phy-astr.gsu.edu/hbase/hph.html |archive-date=2 April 2023}}</ref> In fact, the force is termed ''weak'' because its [[field strength]] over any set distance is typically several orders of magnitude less than that of the electromagnetic force, which itself is further orders of magnitude less than the strong nuclear force.

The weak interaction is the only fundamental interaction that breaks [[Parity (physics)|parity symmetry]], and similarly, but far more rarely, the only interaction to break [[CP-symmetry|charge–parity symmetry]].

[[Quark]]s, which make up composite particles like neutrons and protons, come in six "flavours"{{snd}} up, down, charm, strange, top and bottom{{snd}} which give those composite particles their properties. The weak interaction is unique in that it allows quarks to swap their flavour for another. The swapping of those properties is mediated by the force carrier bosons. For example, during [[beta-minus decay]], a down quark within a neutron is changed into an up quark, thus converting the neutron to a proton and resulting in the emission of an electron and an electron antineutrino. 

Weak interaction is important in the [[Stellar nucleosynthesis#Hydrogen fusion|fusion of hydrogen into helium]] in a star. This is because it can convert a proton (hydrogen) into a neutron to form deuterium which is important for the continuation of nuclear fusion to form helium. The accumulation of neutrons facilitates the buildup of heavy nuclei in a star.<ref name="hyperphysics"/>

Most fermions decay by a weak interaction over time. Such decay makes [[radiocarbon dating]] possible, as [[carbon-14]] decays through the weak interaction to [[nitrogen-14]]. It can also create [[radioluminescence]], commonly used in [[Tritium radioluminescence|tritium luminescence]], and in the related field of [[betavoltaics]]<ref>{{cite press release |title=The Nobel Prize in Physics 1979 |website=NobelPrize.org |publisher=Nobel Media |url=http://nobelprize.org/nobel_prizes/physics/laureates/1979/press.html |access-date=22 March 2011}}</ref> (but ''not'' similar to [[Radium dial|radium luminescence]]).

The [[Electroweak interaction|electroweak force]] is believed to have separated into the electromagnetic and weak forces during the [[quark epoch]] of the [[Chronology of the universe#Early universe|early universe]].

== History ==
In 1933, [[Enrico Fermi]] proposed the first theory of the weak interaction, known as [[Fermi's interaction]]. He suggested that [[beta decay]] could be explained by a four-[[fermion]] interaction, involving a contact force with no range.<ref name="Fermi's theory">{{cite journal | last=Fermi | first=Enrico | year=1934 | title=Versuch einer Theorie der β-Strahlen. I | language=de | trans-title=Search for a theory for beta-decay | bibcode=1934ZPhy...88..161F | journal=[[Zeitschrift für Physik A]] | volume=88 | issue=3–4 | pages=161–177 | doi=10.1007/BF01351864 | s2cid=125763380 }}</ref><ref name="Fermi's theory translation">{{cite journal | last=Wilson | first=Fred L. | date=December 1968 | title=Fermi's theory of beta decay | journal=American Journal of Physics | volume=36 | issue=12 | pages=1150–1160 | doi=10.1119/1.1974382|bibcode = 1968AmJPh..36.1150W }}</ref>

In the mid-1950s, [[Chen-Ning Yang]] and [[Tsung-Dao Lee]] first suggested that the handedness of the spins of particles in weak interaction might violate the [[conservation law]] or symmetry. In 1957, [[Chien Shiung Wu]] and collaborators confirmed the [[Parity_(physics)#Parity_violation|symmetry violation]].<ref>
{{cite web
 |title=The Nobel Prize in Physics
 |year=1957
 |website=NobelPrize.org
 |publisher=Nobel Media
 |url=http://nobelprize.org/nobel_prizes/physics/laureates/1957/
 |access-date=26 February 2011
}}
</ref>

In the 1960s, [[Sheldon Glashow]], [[Abdus Salam]] and [[Steven Weinberg]] unified the electromagnetic force and the weak interaction by showing them to be two aspects of a single force, now termed the electroweak force.<ref>{{cite web |title=Steven Weinberg, weak interactions, and electromagnetic interactions |url=http://www.osti.gov/accomplishments/weinberg.html |url-status=dead |archive-url=https://web.archive.org/web/20160809083339/https://www.osti.gov/accomplishments/weinberg.html |archive-date=2016-08-09}}</ref><ref name="Salam">{{cite press release |website=Nobel Prize |title=Nobel Prize in Physics |year=1979 |url=http://nobelprize.org/nobel_prizes/physics/laureates/1979/ |url-status=live |archive-url=https://web.archive.org/web/20140706082221/http://www.nobelprize.org/nobel_prizes/physics/laureates/1979/ |archive-date=6 July 2014 |df=dmy }}</ref>

The [[W and Z bosons#Discovery|existence of the {{math|W}} and {{math|Z}}&nbsp;bosons]] was not directly confirmed until 1983.<ref name=Cottingham-Greenwood-1986-2001>
{{cite book
 | first1=W. N.  | last1=Cottingham
 | first2=D. A.  | last2=Greenwood
 | title=An introduction to nuclear physics
 | publisher=Cambridge University Press
 | orig-year=1986
 | year=2001
 | edition=2nd  | page=30
 | isbn=978-0-521-65733-4
 | url=https://books.google.com/books?id=0VIpJPn-qWoC&pg=PA30
}}</ref>{{rp|style=ama|p=8}}

== Properties ==
[[File:Weak Decay (flipped).svg|thumb|right|280px|A diagram depicting the decay routes for the six [[quark]]s due to the charged weak interaction and some indication of their likelihood. The intensity of the lines is given by the [[CKM parameters]].]]
The electrically charged weak interaction is unique in a number of respects:
* It is the only interaction that can change the [[flavour (particle physics)|flavour]] of quarks and leptons (i.e., of changing one type of quark into another).{{efn|Because of its unique ability to change particle [[flavour (particle physics)|flavour]], analysis of the weak interaction is occasionally called ''quantum flavour dynamics'', in analogy to the name ''[[quantum chromodynamics]]'' sometimes used for the [[strong interaction|strong force]].}}
* It is the only interaction that violates [[parity (physics)|'''P''', or parity symmetry]]. It is also the only one that violates [[CP-symmetry|charge–parity ('''CP''') symmetry]].
* Both the electrically charged and the electrically neutral interactions are mediated (propagated) by [[gauge boson|force carrier particles]] that have significant masses, an unusual feature which is explained in the [[Standard Model]] by the [[Higgs mechanism]].

Due to their large mass (approximately 90&nbsp;GeV/''c''<sup>2</sup><ref name="PDG">{{cite journal |author1=Yao, W.-M. |display-authors=etal |collaboration=[[Particle Data Group]] |year=2006 |title=Review of Particle Physics: Quarks |journal=[[Journal of Physics G]] |volume=33 |issue=1 |pages=1–1232 |doi=10.1088/0954-3899/33/1/001 |arxiv=astro-ph/0601168 |bibcode = 2006JPhG...33....1Y |url=http://pdg.lbl.gov/2006/tables/qxxx.pdf}}</ref>) these carrier particles, called the {{math|W}} and {{math|Z}}&nbsp;bosons, are short-lived with a lifetime of under {{10^|−24}}&nbsp;seconds.<ref>{{cite book |title=Story of the {{math|W}} and {{math|Z}} |author=Watkins, Peter |publisher=Cambridge University Press |location=Cambridge |url=https://archive.org/details/storyofwz0000watk |url-access=registration |page=[https://archive.org/details/storyofwz0000watk/page/70 70] |year=1986 |isbn=978-0-521-31875-4}}</ref> The weak interaction has a [[coupling constant]] (an indicator of how frequently interactions occur) between {{10^|−7}} and {{10^|−6}}, compared to the [[Fine Structure Constant|electromagnetic coupling constant]] of about {{10^|−2}} and the [[strong interaction]] coupling constant of about&nbsp;1;<ref name="coupling">{{cite web |title=Coupling Constants for the Fundamental Forces |work=HyperPhysics |publisher=[[Georgia State University]] |url=http://hyperphysics.phy-astr.gsu.edu/hbase/forces/couple.html |access-date=2 March 2011}}</ref> consequently the weak interaction is "weak" in terms of intensity.<ref name="physnet"/> The weak interaction has a very short effective range (around {{10^|−17}} to {{10^|−16}}&nbsp;m (0.01 to 0.1&nbsp;fm)).{{efn|Compare to a [[femtometer|proton charge radius]] of 8.3×{{10^|−16}}&nbsp;m ~ 0.83&nbsp;fm.}}<ref name="physnet">{{cite web |author=Christman, J. |year=2001 |title=The Weak Interaction |website=Physnet |publisher=[[Michigan State University]] |url=http://physnet2.pa.msu.edu/home/modules/pdf_modules/m281.pdf |url-status=dead |archive-url=https://web.archive.org/web/20110720004912/http://physnet2.pa.msu.edu/home/modules/pdf_modules/m281.pdf |archive-date=20 July 2011 |df=dmy-all}}</ref><ref name="coupling"/> At distances around {{10^|−18}}&nbsp;meters (0.001&nbsp;fm), the weak interaction has an intensity of a similar magnitude to the electromagnetic force, but this starts to decrease [[Yukawa potential|exponentially]] with increasing distance. Scaled up by just one and a half orders of magnitude, at distances of around 3{{e|−17}}&nbsp;m, the weak interaction becomes 10,000&nbsp;times weaker.<ref>{{cite web |title=Electroweak |website=The Particle Adventure |publisher=[[Particle Data Group]] |url=http://www.particleadventure.org/electroweak.html |access-date=3 March 2011}}</ref>

The weak interaction affects all the [[fermion]]s of the [[Standard Model]], as well as the [[Higgs boson]]; [[neutrino]]s interact only through gravity and the weak interaction. The weak interaction does not produce [[bound state]]s, nor does it involve [[binding energy]]{{snd}} something that gravity does on an [[astronomical scale]], the electromagnetic force does at the molecular and atomic levels, and the strong nuclear force does only at the subatomic level, inside of [[atomic nucleus|nuclei]].<ref name="greiner">{{cite book |author1=Greiner, Walter |author2=Müller, Berndt |year=2009 |title=Gauge Theory of Weak Interactions |publisher=Springer |isbn=978-3-540-87842-1 |url=https://books.google.com/books?id=yWTcPwqg_00C |page=2}}</ref>

Its most noticeable effect is due to its first unique feature: The charged weak interaction causes [[Flavour changing processes|flavour change]]. For example, a [[neutron]] is heavier than a [[proton]] (its partner [[nucleon]]) and can decay into a proton by changing the [[flavour (particle physics)|flavour]] (type) of one of its two ''down'' quarks to an ''up'' quark. Neither the [[strong interaction]] nor [[electromagnetism]] permit flavour changing, so this can only proceed by '''weak decay'''; without weak decay, quark properties such as strangeness and charm (associated with the strange quark and charm quark, respectively) would also be conserved across all interactions.

All [[meson]]s are unstable because of weak decay.<ref name=Cottingham-Greenwood-1986-2001/>{{rp|style=ama|p=29}}{{efn|
The neutral pion ({{math|{{SubatomicParticle|pion0}}}}), however, decays electromagnetically, and several other [[meson]]s (when their quantum numbers permit) mostly decay via a [[strong interaction]].
}}
In the process known as [[beta decay]], a ''down'' quark in the [[neutron]] can change into an ''up'' quark by emitting a [[Virtual particle|virtual]] {{math|{{SubatomicParticle|W boson-}}}}&nbsp;boson, which then decays into an [[electron]] and an electron [[antineutrino]].<ref name=Cottingham-Greenwood-1986-2001/>{{rp|style=ama|p=28}} Another example is [[electron capture]]{{snd}} a common variant of [[radioactive decay]]{{snd}} wherein a proton and an electron within an atom interact and are changed to a neutron (an up quark is changed to a down quark), and an electron neutrino is emitted.

Due to the large masses of the W&nbsp;bosons, particle transformations or decays (e.g., flavour change) that depend on the weak interaction typically occur much more slowly than transformations or decays that depend only on the strong or electromagnetic forces.{{efn|
The prominent and possibly unique exception to this rule is the decay of the [[top quark]], whose mass exceeds the combined masses of the [[bottom quark]] and {{math|{{SubatomicParticle|W boson+}}}}&nbsp;boson that it decays into, hence it has a no energy constraint slowing its transition. Its unique speed of decay by the weak force is much higher than the speed with which the [[strong interaction]] (or "[[color force]]") can bind it to other quarks.
}}
For example, a neutral [[pion]] decays electromagnetically, and so has a life of only about {{10^|−16}}&nbsp;seconds. In contrast, a charged pion can only decay through the weak interaction, and so lives about {{10^|−8}}&nbsp;seconds, or a hundred million times longer than a neutral pion.<ref name=Cottingham-Greenwood-1986-2001/>{{rp|style=ama|p=30}} A particularly extreme example is the weak-force decay of a free neutron, which takes about 15&nbsp;minutes.<ref name=Cottingham-Greenwood-1986-2001/>{{rp|style=ama|p=28}}

=== Weak isospin and weak hypercharge ===
{{main article|Weak isospin}}
{| style="right; margin:0 0 .5em 1em;" class="wikitable"
|+ [[Chirality#Physics|Left-handed]] fermions in the Standard Model<ref name="baez">
{{cite journal
 |first1=John C. |last1=Baez   |author1-link=John C. Baez
 |first2=John    |last2=Huerta
 |year=2010
 |title=The algebra of grand unified theories
 |journal=Bulletin of the American Mathematical Society
 |volume=0904  |issue=3  |pages=483–552
 |bibcode=2009arXiv0904.1556B  |arxiv=0904.1556  |s2cid=2941843
 |doi=10.1090/s0273-0979-10-01294-2
 |url=http://math.ucr.edu/~huerta/guts/node9.html
 |access-date=15 October 2013
}}
</ref>
|-
!colspan="3" style="background:#339900; color:#ffffff"|Generation 1
!colspan="3" style="background:#339900; color:#ffffff"|Generation 2
!colspan="3" style="background:#339900; color:#ffffff"|Generation 3
|- style="background:#fdd;"
!Fermion
!Symbol
![[Weak isospin|Weak<br/>isospin]]
!Fermion
!Symbol
![[Weak isospin|Weak<br/>isospin]]
!Fermion
!Symbol
![[Weak isospin|Weak<br/>isospin]]
|-
|style="background:#efefef"| [[electron neutrino]]
|align="center"| {{math| {{SubatomicParticle|Electron neutrino}} }}
|align="center"| {{sfrac|+|1|2}}
|style="background:#efefef"| [[muon neutrino]]
|align="center"| {{math| {{SubatomicParticle|Muon neutrino}} }}
|align="center"| {{sfrac|+|1|2}}
|style="background:#efefef"| [[tau neutrino]]
|align="center"| {{math| {{SubatomicParticle|Tau neutrino}} }}
|align="center"| {{sfrac|+|1|2}}
|-
|style="background:#efefef"| [[electron]]
|align="center"| {{math| {{SubatomicParticle|Electron-}} }}
|align="center"| {{sfrac|&minus;|1|2}}
|style="background:#efefef"| [[muon]]
|align="center"| {{math| {{SubatomicParticle|Muon-}} }}
|align="center"| {{sfrac|&minus;|1|2}}
|style="background:#efefef"|[[Tau (particle)|tau]]
|align="center"| {{math| {{SubatomicParticle|Tau-}} }}
|align="center"| {{sfrac|&minus;|1|2}}
|-
|style="background:#efefef"| [[up quark]]
|align="center"| {{math| {{SubatomicParticle|Up quark}} }}
|align="center"| {{sfrac|+|1|2}}
|style="background:#efefef"|[[charm quark]]
|align="center"| {{math| {{SubatomicParticle|Charm quark}} }}
|align="center"| {{sfrac|+|1|2}}
|style="background:#efefef"|[[top quark]]
|align="center"| {{math| {{SubatomicParticle|Top quark}} }}
|align="center"| {{sfrac|+|1|2}}
|-
|style="background:#efefef"|[[down quark]]
|align="center"| {{math| {{SubatomicParticle|Down quark}} }}
|align="center"| {{sfrac|&minus;|1|2}}
|style="background:#efefef"|[[strange quark]]
|align="center"| {{math| {{SubatomicParticle|Strange quark}} }}
|align="center"| {{sfrac|&minus;|1|2}}
|style="background:#efefef"|[[bottom quark]]
|align="center"| {{math| {{SubatomicParticle|Bottom quark}} }}
|align="center"| {{sfrac|&minus;|1|2}}
|-
|colspan="9" style="text-align:center;"| All of the above left-handed (''regular'') particles have corresponding<br/>right-handed ''anti''-particles with equal and opposite weak isospin.
|-
|colspan="9" style="text-align:center;"| All right-handed (regular) particles and left-handed antiparticles have weak isospin of 0.
|}

All particles have a property called ''[[weak isospin]]'' (symbol {{mvar|T}}{{sub|3}}), which serves as an [[additive quantum number]] that restricts how the particle can interact with the {{math|{{SubatomicParticle|W boson+-}}}} of the weak force. Weak isospin plays the same role in the weak interaction with {{math|{{SubatomicParticle|W boson+-}}}} as [[electric charge]] does in [[electromagnetism]], and [[color charge]] in the [[strong interaction]]; a different number with a similar name, ''[[weak charge]]'', [[#weak_charge_anchor|discussed below]], is used for interactions with the {{math|{{SubatomicParticle|Z boson0}}}}. All left-handed [[fermion]]s have a weak isospin value of either {{sfrac|+|1|2}} or {{sfrac|&minus;|1|2}}; all right-handed fermions have 0 isospin. For example, the up quark has {{nowrap|{{mvar|T}}{{sub|3}} {{=}} {{sfrac|+|1|2}}}} and the down quark has {{nowrap|{{mvar|T}}{{sub|3}} {{=}} {{sfrac|&minus;|1|2}}}}. A quark never decays through the weak interaction into a quark of the same {{mvar|T}}{{sub|3}}: Quarks with a {{mvar|T}}{{sub|3}} of {{sfrac|+|1|2}} only decay into quarks with a {{mvar|T}}{{sub|3}} of {{sfrac|&minus;|1|2}} and conversely.

[[File:PiPlus muon decay.svg|thumb|right|{{SubatomicParticle|Pion+}} decay through the weak interaction]]
In any given strong, electromagnetic, or weak interaction, weak isospin is [[Conservation law (physics)|conserved]]:{{efn|
Only interactions with the [[Higgs boson]] violate conservation of weak isospin, and appear to always do so maximally: <math>\bigl| \Delta T_3 \bigr| = \tfrac{1}{2}\ .</math>
}} The sum of the weak isospin numbers of the particles entering the interaction equals the sum of the weak isospin numbers of the particles exiting that interaction. For example, a (left-handed) {{math|{{SubatomicParticle|Pion+}},}} with a weak isospin of +1 normally decays into a {{math|{{SubatomicParticle|Muon neutrino}}}} (with {{nowrap|{{mvar|T}}{{sub|3}} {{=}} {{sfrac|+|1|2}}}}) and a {{math|{{SubatomicParticle|Muon+}}}} (as a right-handed antiparticle, {{sfrac|+|1|2}}).<ref name=Cottingham-Greenwood-1986-2001/>{{rp|style=ama|p=30}}

For the development of the electroweak theory, another property, [[weak hypercharge]], was invented, defined as
: <math>Y_\text{W} = 2\,(Q - T_3),</math>
where {{mvar|Y}}{{sub|W}} is the weak hypercharge of a particle with electrical charge {{mvar|Q}} (in [[elementary charge]] units) and weak isospin {{mvar|T}}{{sub|3}}. [[Weak hypercharge]] is the generator of the U(1) component of the electroweak [[gauge group]]; whereas some particles have a [[weak isospin]] of zero, all known [[fermions|spin-{{sfrac|1|2}} particles]] have a non-zero weak hypercharge.{{efn|Some hypothesised fermions, such as the ''[[sterile neutrino]]s'', would have zero weak hypercharge{{snd}} in fact, no [[Charge (physics)|gauge charges]] of any known kind. Whether any such particles actually exist is an active area of research.}}

== Interaction types ==
There are two types of weak interaction (called ''[[Feynman diagram|vertices]]''). The first type is called the "[[charged-current interaction]]" because the ''weakly interacting'' fermions form a [[Mathematical formulation of the Standard Model#The charged and neutral current couplings and Fermi theory|current]] with total ''electric'' charge that is nonzero.  The second type is called the "[[neutral-current interaction]]" because the ''weakly interacting'' fermions form a [[Mathematical formulation of the Standard Model#The charged and neutral current couplings and Fermi theory|current]] with total ''electric'' charge of zero. It is responsible for the (rare) deflection of [[neutrino]]s. The two types of interaction follow different [[selection rule]]s. This naming convention is often misunderstood to label the electric charge of the [[W and Z boson|{{math|W}} and {{math|Z}} boson]]s, however the naming convention predates the concept of the mediator bosons, and clearly (at least in name) labels the charge of the current (formed from the fermions), not necessarily the bosons.{{efn|The exchange of a virtual {{math|W}} boson can be equally well thought of as (say) the emission of a {{math|W}}{{sup|+}} or the absorption of a {{math|W}}{{sup|−}}; that is, for time on the vertical co‑ordinate axis, as a {{math|W}}{{sup|+}} from left to right, or equivalently as a {{math|W}}{{sup|−}} from right to left.}}

=== Charged-current interaction ===
{{main|Charged current}}
[[File:Beta Negative Decay.svg|thumb|right|200px|The [[Feynman diagram]] for beta-minus decay of a [[neutron]] ({{math|n {{=}} udd}}) into a [[proton]] ({{math|p {{=}} udu}}), [[electron]] ({{math|e{{sup|−}}}}), and [[neutrino|electron anti-neutrino]] {{math|{{overline|ν}}{{sub|e}}}}, via a charged vector boson ({{math|{{SubatomicParticle|W boson-}}}}). ]]
In one type of charged current interaction, a charged [[lepton]] (such as an [[electron]] or a [[muon]], having a charge of −1) can absorb a [[W boson|{{math|{{SubatomicParticle|W boson+}}}}&nbsp;boson]] (a particle with a charge of +1) and be thereby converted into a corresponding [[neutrino]] (with a charge of 0), where the type ("flavour") of neutrino (electron {{math|ν{{sub|e}}}}, muon {{math|ν{{sub|μ}}}}, or tau {{math|ν{{sub|τ}}}}) is the same as the type of lepton in the interaction, for example:
: <math> \mu^- + \mathrm{W}^+ \to \nu_\mu </math>
Similarly, a down-type [[quark]] ({{math|d}}, {{math|s}}, or {{math|b}}, with a charge of {{sfrac|−| 1 |3}}) can be converted into an up-type quark ({{math|u}}, {{math|c}}, or {{math|t}}, with a charge of {{sfrac|+| 2 |3}}), by emitting a {{SubatomicParticle|W boson-}}&nbsp;boson or by absorbing a {{math|{{SubatomicParticle|W boson+}}}}&nbsp;boson. More precisely, the down-type quark becomes a [[quantum superposition]] of up-type quarks: that is to say, it has a possibility of becoming any one of the three up-type quarks, with the probabilities given in the [[CKM matrix]] tables. Conversely, an up-type quark can emit a {{math|{{SubatomicParticle|W boson+}}}}&nbsp;boson, or absorb a {{math|{{SubatomicParticle|W boson-}}}}&nbsp;boson, and thereby be converted into a down-type quark, for example:
: <math>\begin{align}
                 \mathrm{d} &\to \mathrm{u} + \mathrm{W}^- \\
  \mathrm{d} + \mathrm{W}^+ &\to \mathrm{u} \\
                 \mathrm{c} &\to \mathrm{s} + \mathrm{W}^+ \\
  \mathrm{c} + \mathrm{W}^- &\to \mathrm{s}
\end{align}</math>

The W&nbsp;boson is unstable so will rapidly decay, with a very short lifetime. For example:
: <math>\begin{align}
  \mathrm{W}^- &\to \mathrm{e}^- + \bar\nu_\mathrm{e} ~ \\
  \mathrm{W}^+ &\to \mathrm{e}^+ + \nu_\mathrm{e} ~
\end{align}</math>

Decay of a W&nbsp;boson to other products can happen, with varying probabilities.<ref name="PDG2">{{cite journal |author1=Nakamura, K. |display-authors=etal |collaboration=[[Particle Data Group]] |year=2010 |title=Gauge and Higgs Bosons |journal=[[Journal of Physics G]] |volume=37 |issue=7A |page=075021 |doi = 10.1088/0954-3899/37/7a/075021 |bibcode = 2010JPhG...37g5021N |url=http://pdg.lbl.gov/2010/tables/rpp2010-sum-gauge-higgs-bosons.pdf}}</ref>

In the so-called [[beta decay]] of a neutron (see picture, above), a down quark within the neutron emits a [[Virtual particle|virtual]] {{math|{{SubatomicParticle|W boson-}}}} boson and is thereby converted into an up quark, converting the neutron into a proton. Because of the limited energy involved in the process (i.e., the mass difference between the down quark and the up quark), the virtual {{math|{{SubatomicParticle|W boson-}}}} boson can only carry sufficient energy to produce an electron and an electron-antineutrino – the two lowest-possible masses among its prospective decay products.<ref name=PDG3>
{{cite journal
 |author1=Nakamura, K.
 |display-authors=etal
 |collaboration=[[Particle Data Group]]
 |year=2010
 |title={{math| {{SubatomicParticle|neutron}} }}
 |journal=[[Journal of Physics G]]
 |volume=37  |issue=7A  |page=7
 |doi=10.1088/0954-3899/37/7a/075021
 |bibcode=2010JPhG...37g5021N
 |url=http://pdg.lbl.gov/2010/listings/rpp2010-list-n.pdf
}}
</ref>
At the quark level, the process can be represented as:
: <math> \mathrm{d} \to \mathrm{u} + \mathrm{e}^- + \bar\nu_\mathrm{e} ~</math>

=== Neutral-current interaction ===
{{main|Neutral current}}
In [[neutral current]] interactions, a [[quark]] or a [[lepton]] (e.g., an [[electron]] or a [[muon]]) emits or absorbs a neutral [[Z boson|{{math|Z}} boson]]. For example:
: <math> \mathrm{e}^- \to \mathrm{e}^- + \mathrm{Z}^0</math>

Like the {{math|{{SubatomicParticle|W boson+-}}}}&nbsp;bosons, the {{math|{{SubatomicParticle|Z boson0}}}}&nbsp;boson also decays rapidly,<ref name="PDG2"/> for example:
: <math> \mathrm{Z}^0 \to \mathrm{b} + \bar \mathrm{b} </math>

Unlike the charged-current interaction, whose selection rules are strictly limited by chirality, electric charge, {{nowrap|and / or}} weak isospin, the neutral-current {{math| {{SubatomicParticle|Z boson0}} }} interaction can cause any two fermions in the standard model to deflect: Either particles or anti-particles, with any electric charge, and both left- and right-chirality, although the strength of the interaction differs.{{efn|
The only fermions which the {{math|{{SubatomicParticle|Z boson0}}}} does ''not'' interact with at all are the hypothetical [[sterile neutrino|"sterile" neutrinos]]: Left-chiral anti-neutrinos and right-chiral neutrinos. They are called "sterile" because they would not interact with any Standard Model particle, except perhaps the [[Higgs boson]]. So far they remain entirely a conjecture: As of October&nbsp;2021, no such neutrinos are known to actually exist.
: 
: "[[MicroBooNE]] has made a very comprehensive exploration through multiple types of interactions, and multiple analysis and reconstruction techniques", says co-spokesperson [[Bonnie Fleming]] of Yale. "They all tell us the same thing, and that gives us very high confidence in our results that we are not seeing a hint of a sterile neutrino."<ref name=CERN-2021-10-28/>
: 
: ...&nbsp;"eV-scale sterile neutrinos no longer appear to be experimentally motivated, and never solved any outstanding problems in the Standard Model", says theorist Mikhail Shaposhnikov of EPFL. "But GeV-to-keV-scale sterile neutrinos – so-called Majorana fermions – are well motivated theoretically and do not contradict any existing experiment."<ref name=CERN-2021-10-28>
{{cite news
 |first=Mark |last=Rayner 
 |title=MicroBooNE sees no hint of a sterile neutrino
 |date=28 October 2021
 |periodical=CERN Courier
 |url=https://cerncourier.com/a/microboone-sees-no-hint-of-a-sterile-neutrino/
 |access-date=2021-11-09
}}
</ref>
}}

{{anchor|weak_charge_anchor}}The quantum number ''[[weak charge]]'' ({{mvar|Q}}{{sub|{{sc|w}}}}) serves the same role in the neutral current interaction with the {{math| {{subatomic Particle|Z boson0}} }} that electric charge ({{mvar|Q}}, with no subscript) does in the [[electromagnetic interaction]]: It quantifies the vector part of the interaction. Its value is given by:<ref name=dzuba>
{{cite journal
 |first1=V. A. |last1=Dzuba         |first2=J. C. |last2=Berengut
 |first3=V. V. |last3=Flambaum      |first4=B.   |last4=Roberts
 |year=2012
 |title=Revisiting parity non-conservation in cesium
 |journal=Physical Review Letters
 |volume=109  |issue=20  |page=203003
 |doi=10.1103/PhysRevLett.109.203003  |arxiv=1207.5864
 |pmid=23215482  |bibcode=2012PhRvL.109t3003D  |s2cid=27741778
}}
</ref>
: <math> Q_\mathsf{w} = 2 \, T_3 - 4 \, Q \, \sin^2\theta_\mathsf{w} = 2 \, T_3 - Q + (1 - 4 \, \sin^2\theta_\mathsf{w}) \, Q ~.</math>
Since the [[weak mixing angle]] {{tmath|1= \theta_\mathsf{w} \approx 29^\circ }}, the parenthetic expression {{tmath|1= (1 - 4 \, \sin^2\theta_\mathsf{w}) \approx 0.060 }}, with its value [[Renormalization group|varying slightly with the momentum difference (called "''running''")]] between the particles involved. Hence 
: <math>\ Q_\mathsf{w} \approx 2 \ T_3 - Q = \sgn(Q)\ \big(1 - |Q|\big)\ ,</math>
since by convention {{tmath|1= \sgn T_3 \equiv \sgn Q }}, and for all fermions involved in the weak interaction {{tmath|1= T_3 = \pm\tfrac{1}{2} }}. The weak charge of charged leptons is then close to zero, so these mostly interact with the {{math|Z}}&nbsp;boson through the axial coupling.

== Electroweak theory ==
{{main article|Electroweak interaction}}
The [[Standard Model]] of particle physics describes the [[electromagnetic interaction]] and the weak interaction as two different aspects of a single electroweak interaction. This theory was developed around 1968 by [[Sheldon Glashow]], [[Abdus Salam]], and [[Steven Weinberg]], and they were awarded the [[Nobel Prize in Physics#1970s|1979 Nobel Prize in Physics]] for their work.<ref>{{cite web |publisher=Nobel Media |work=NobelPrize.org |title=The Nobel Prize in Physics 1979 |url=http://nobelprize.org/nobel_prizes/physics/laureates/1979/ |access-date=26 February 2011}}</ref> The [[Higgs mechanism]] provides an explanation for the presence of [[W and Z bosons|three massive gauge bosons]] ({{math|{{SubatomicParticle|W boson+}}}}, {{math|{{SubatomicParticle|W boson-}}}}, {{math|{{SubatomicParticle|Z boson0}}}}, the three carriers of the weak interaction), and the [[photon]] ({{math|γ}}, the massless gauge boson that carries the electromagnetic interaction).<ref name="PDGHiggs">
{{cite journal
 |author=C. Amsler ''et al.'' ([[Particle Data Group]])
 |year=2008
 |title=Review of Particle Physics – Higgs Bosons: Theory and Searches
 |url=http://pdg.lbl.gov/2008/reviews/higgs_s055.pdf
 |journal=[[Physics Letters B]]
 |volume=667 |issue= 1|pages=1–6
 |doi= 10.1016/j.physletb.2008.07.018
 |bibcode = 2008PhLB..667....1A
 |hdl=1854/LU-685594
 |s2cid=227119789
 |hdl-access=free
}}
</ref>

According to the electroweak theory, at very high energies, the universe has four components of the [[Higgs field]] whose interactions are carried by four massless scalar [[boson]]s forming a complex scalar Higgs field doublet. Likewise, there are four massless electroweak vector bosons, each similar to the [[photon]]. However, at low energies, this gauge symmetry is [[spontaneous symmetry breaking|spontaneously broken]] down to the {{math|U(1)}} symmetry of electromagnetism, since one of the Higgs fields acquires a [[vacuum expectation value]]. Naïvely, the symmetry-breaking would be expected to produce three massless [[Goldstone boson|bosons]], but instead those "extra" three Higgs bosons become incorporated into the three weak bosons, which then acquire mass through the [[Higgs mechanism]]. These three composite bosons are the {{math|{{SubatomicParticle|W boson+}}}}, {{math|{{SubatomicParticle|W boson-}}}}, and {{math|{{SubatomicParticle|Z boson0}}}}&nbsp;bosons actually observed in the weak interaction. The fourth electroweak gauge boson is the photon ({{mvar|γ}}) of electromagnetism, which does not couple to any of the Higgs fields and so remains massless.<ref name="PDGHiggs"/>

This theory has made a number of predictions, including a prediction of the masses of the {{math|{{SubatomicParticle|Z boson}}}} and {{math|{{SubatomicParticle|W boson}}}}&nbsp;bosons before their discovery and detection in 1983.

On 4&nbsp;July 2012, the CMS and the ATLAS experimental teams at the [[Large Hadron Collider]] independently announced that they had confirmed the formal discovery of a previously unknown boson of mass between 125 and {{val|127|u=GeV/c2}}, whose behaviour so far was "consistent with" a Higgs boson, while adding a cautious note that further data and analysis were needed before positively identifying the new boson as being a Higgs boson of some type. By 14&nbsp;March 2013, a Higgs boson was tentatively confirmed to exist.<ref>{{cite web |url=http://home.web.cern.ch/about/updates/2013/03/new-results-indicate-new-particle-higgs-boson |title=New results indicate that new particle is a Higgs boson |date=March 2013 |publisher=[[CERN]] |website=home.web.cern.ch |access-date=20 September 2013}}</ref>

In a speculative case where the [[Higgs mechanism|electroweak symmetry breaking]] [[Electroweak scale|scale]] were lowered, the unbroken {{math|SU(2)}} interaction would eventually become [[Color confinement|confining]]. Alternative models where {{math|SU(2)}} becomes confining above that scale appear quantitatively similar to the [[Standard Model]] at lower energies, but dramatically different above symmetry breaking.<ref>{{cite journal |last1=Claudson |first1=M. |last2=Farhi |first2=E. |last3=Jaffe |first3=R. L. |title=Strongly coupled standard model |journal=Physical Review D |date=1 August 1986 |volume=34 |issue=3 |pages=873–887 |doi=10.1103/PhysRevD.34.873 |pmid=9957220 |bibcode=1986PhRvD..34..873C }}</ref>

== Violation of symmetry ==
[[File:Right left helicity.svg|thumb|right|280px|[[Chirality (physics)|Left- and right-handed particles]]: {{mvar|p}} is the particle's momentum and {{mvar|S}} is its [[Spin (physics)|spin]]. Note the lack of reflective symmetry between the states.]]
The [[physical law|laws of nature]] were long thought to remain the same under mirror [[reflection (physics)|reflection]]. The results of an experiment viewed via a mirror were expected to be identical to the results of a separately constructed, mirror-reflected copy of the experimental apparatus watched through the mirror. This so-called law of [[parity (physics)|parity]] [[Conservation law (physics)|conservation]] was known to be respected by classical [[gravitation]], [[electromagnetism]] and the [[strong interaction]]; it was assumed to be a universal law.<ref>
{{cite book
 |first=Charles W. |last=Carey
 |year=2006
 |chapter=Lee, Tsung-Dao
 |title=American scientists
 |page=225
 |publisher=Facts on File Inc.
 |isbn=9781438108070
 |chapter-url=https://books.google.com/books?id=00r9waSNv1cC&pg=PA225
 |via=Google Books
}}
</ref> However, in the mid-1950s [[Chen-Ning Yang]] and [[Tsung-Dao Lee]] suggested that the weak interaction might violate this law. [[Chien Shiung Wu]] and collaborators in 1957 discovered that the weak interaction violates parity, earning Yang and Lee the [[Nobel Prize in Physics#1950s|1957 Nobel Prize in Physics]].<ref>
{{cite web
 |title=The Nobel Prize in Physics
 |year=1957
 |website=NobelPrize.org
 |publisher=Nobel Media
 |url=http://nobelprize.org/nobel_prizes/physics/laureates/1957/
 |access-date=26 February 2011
}}
</ref>

Although the weak interaction was once described by [[Fermi's theory]], the discovery of parity violation and [[renormalization]] theory suggested that a new approach was needed. In 1957, [[Robert Marshak]] and [[George Sudarshan]] and, somewhat later, [[Richard Feynman]] and [[Murray Gell-Mann]] proposed a '''V&nbsp;−&nbsp;A''' ([[vector (geometry)|vector]] minus [[axial vector]] or left-handed) [[Lagrangian (field theory)|Lagrangian]] for weak interactions. In this theory, the weak interaction acts only on left-handed particles (and right-handed antiparticles). Since the mirror reflection of a left-handed particle is right-handed, this explains the maximal violation of parity. The ''V&nbsp;−&nbsp;A'' theory was developed before the discovery of the Z&nbsp;boson, so it did not include the right-handed fields that enter in the neutral current interaction.

However, this theory allowed a compound symmetry '''[[CP violation|CP]]''' to be conserved. '''CP''' combines parity '''P''' (switching left to right) with charge conjugation '''C''' (switching particles with antiparticles). Physicists were again surprised when in 1964, [[James Cronin]] and [[Val Fitch]] provided clear evidence in [[kaon]] decays that ''CP'' symmetry could be broken too, winning them the 1980 [[Nobel Prize in Physics]].<ref>
{{cite web
 |title=The Nobel Prize in Physics
 |year=1980
 |website=NobelPrize.org
 |publisher=Nobel Media
 |url=http://nobelprize.org/nobel_prizes/physics/laureates/1980/
 |access-date=26 February 2011
}}
</ref> In 1973, [[Makoto Kobayashi (physicist)|Makoto Kobayashi]] and [[Toshihide Maskawa]] showed that ''CP'' violation in the weak interaction required more than two generations of particles,<ref name="KM">
{{cite journal
 |first1=M. |last1=Kobayashi
 |first2=T. |last2=Maskawa
 |year=1973
 |title=CP-Violation in the Renormalizable Theory of Weak Interaction
 |journal=[[Progress of Theoretical Physics]]
 |volume=49  |issue=2  |pages=652–657
 |doi=10.1143/PTP.49.652  |doi-access=free
 |bibcode = 1973PThPh..49..652K  |hdl=2433/66179
 |url=http://repository.kulib.kyoto-u.ac.jp/dspace/bitstream/2433/66179/1/49_652.pdf
}}
</ref> effectively predicting the existence of a then unknown third generation. This discovery earned them half of the 2008 Nobel Prize in Physics.<ref>
{{cite web
 |title=The Nobel Prize in Physics
 |year=2008
 |website=NobelPrize.org
 |publisher=Nobel Media
 |url=http://nobelprize.org/nobel_prizes/physics/laureates/2008/
 |access-date=17 March 2011
}}
</ref>

Unlike parity violation, ''CP''&nbsp;violation occurs only in rare circumstances. Despite its limited occurrence under present conditions, it is widely believed to be the reason that there is much more matter than [[antimatter]] in the universe, and thus forms one of [[Andrei Sakharov]]'s three conditions for [[baryogenesis]].<ref>
{{cite book
 |author-last=Langacker |author-first=Paul 
 |orig-year=1989
 |chapter=CP violation and cosmology
 |editor-last=Jarlskog  |editor-first=Cecilia 
 |year=2001
 |title=CP Violation
 |page=552
 |location=London, [[River Edge]]
 |publisher=World Scientific Publishing Co.
 |isbn=9789971505615
 |url=https://books.google.com/books?id=U5TC5DSWOmIC
 |via=Google Books
}}
</ref>

== See also ==
{{portal|Physics}}
* [[Weakless universe]] – the postulate that weak interactions are not [[Anthropic principle|anthropically necessary]]
* [[Gravity]]
* [[Strong interaction]]
* [[Electromagnetism]]

== Footnotes ==
{{notelist|1}}

== References ==
{{reflist|25em}}

== Sources ==
===Technical===
{{refbegin|25em|small=yes}}
* {{cite book
 | first1=W.  | last1=Greiner |  author1-link=Walter Greiner
 | first2=B.  | last2=Müller
 | year=2000
 | title=Gauge Theory of Weak Interactions
 | publisher=Springer
 | isbn=3-540-67672-4
 }}
* {{cite book
 |first1=G. D.  |last1=Coughlan
 |first2=J. E.  |last2=Dodd
 |first3=B. M.  |last3=Gripaios
 |year=2006
 |title=The Ideas of Particle Physics: An introduction for scientists
 |edition=3rd
 |publisher=Cambridge University Press
 |isbn=978-0-521-67775-2
}}
* {{cite book
 | first1=W. N.  | last1=Cottingham
 | first2=D. A.  | last2=Greenwood
 | title=An introduction to nuclear physics
 | publisher=Cambridge University Press
 | orig-year=1986
 | year=2001
 | edition=2nd  | page=30
 | isbn=978-0-521-65733-4
 | url=https://books.google.com/books?id=0VIpJPn-qWoC&pg=PA30
}}
* {{cite book
 | first=D. J.  | last=Griffiths
 | year=1987
 | title=Introduction to Elementary Particles
 | publisher=John Wiley & Sons
 | isbn=0-471-60386-4
}}
* {{cite book
 | first=G. L.  | last=Kane
 | year=1987
 | title=Modern Elementary Particle Physics
 | publisher=[[Perseus Books]]
 | isbn=0-201-11749-5
}}
* {{cite book
 | first=D. H.  | last=Perkins
 | year=2000
 | title=Introduction to High Energy Physics
 | publisher=Cambridge University Press
 | isbn=0-521-62196-8
}}
{{refend}}

=== For general readers ===
{{refbegin|25em|small=yes}}
* {{cite book
 |last=Oerter |first=R. 
 |year=2006
 |title=The Theory of Almost Everything: The Standard Model, the unsung triumph of modern physics
 |publisher=[[Plume (publisher)|Plume]]
 |isbn=978-0-13-236678-6
 |url-access=registration
 |url=https://archive.org/details/theoryofalmostev0000oert
}}
* {{cite book
 |last=Schumm |first=B. A. 
 |year=2004
 |title=Deep Down Things: The breathtaking beauty of particle physics
 |publisher=[[Johns Hopkins University Press]]
 |isbn=0-8018-7971-X
 |url-access=registration
 |url=https://archive.org/details/deepdownthingsbr00schu
}}

{{refend}}

== External links ==
{{wikiquote}}
* Harry Cheung, [http://home.fnal.gov/~cheung/rtes/RTESWeb/LQCD_site/pages/weakforce.htm The Weak Force] @[http://fnal.gov/ Fermilab]
* [http://hyperphysics.phy-astr.gsu.edu/hbase/Forces/funfor.html Fundamental Forces] @[http://hyperphysics.phy-astr.gsu.edu/hbase/index.html Hyperphysics], Georgia State University.
* [https://briankoberlein.com/ Brian Koberlein], [https://briankoberlein.com/2015/03/01/light-of-other-days/ What is the weak force?]

{{Fundamental interactions}}
{{Standard model of physics}}

{{Authority control}}

[[Category:Weak interaction]]
[[Category:Fundamental interactions]]