Editing Weak interaction (section)

=== 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.}}