Editing Weak interaction (section)

== 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.}}