Weak isospin
In particle physics, weak isospin is a quantum number relating to the weak interaction, and parallels the idea of isospin under the strong interaction. Weak isospin is usually given the symbol T or I with the third component written as , , or .[1] It can be understood as the eigenvalue of a charge operator.
Flavour in particle physics 

Flavour quantum numbers 

Related quantum numbers 

Combinations 

Flavour mixing 
The weak isospin conservation law relates to the conservation of ; all weak interactions must conserve . It is also conserved by the electromagnetic and strong interactions. However, one of the interactions is with the Higgs field. Since the Higgs field vacuum expectation value is nonzero, particles interact with this field all the time even in vacuum. This changes their weak isospin (and weak hypercharge). Only a specific combination of them, (electric charge), is conserved. is more important than T and often the term "weak isospin" refers to the "3rd component of weak isospin".
Relation with chirality
Fermions with negative chirality (also called "lefthanded" fermions) have and can be grouped into doublets with that behave the same way under the weak interaction. For example, uptype quarks (u, c, t) have and always transform into downtype quarks (d, s, b), which have , and vice versa. On the other hand, a quark never decays weakly into a quark of the same . Something similar happens with lefthanded leptons, which exist as doublets containing a charged lepton (^{}
_{}e^{−}
_{}, ^{}
_{}μ^{−}
_{}, ^{}
_{}τ^{−}
_{}) with and a neutrino (^{}
_{}ν^{}
_{e}, ^{}
_{}ν^{}
_{μ}, ^{}
_{}ν^{}
_{τ}) with . In all cases, the corresponding antifermion has reversed chirality ("righthanded" antifermion) and sign reversed .
Fermions with positive chirality ("righthanded" fermions) and antifermions with negative chirality ("lefthanded" antifermions) have and form singlets that do not undergo weak interactions.
The electric charge, , is related to weak isospin, , and weak hypercharge, , by
 .
Generation 1  Generation 2  Generation 3  

Fermion  Symbol  Weak isospin 
Fermion  Symbol  Weak isospin 
Fermion  Symbol  Weak isospin 
Electron neutrino  Muon neutrino  Tau neutrino  
Electron  Muon  Tau  
Up quark  Charm quark  Top quark  
Down quark  Strange quark  Bottom quark  
All of the above lefthanded (regular) particles have corresponding righthanded antiparticles with equal and opposite weak isospin.  
All righthanded (regular) particles and lefthanded antiparticles have weak isospin of 0. 
Weak isospin and the W bosons
The symmetry associated with weak isospin is SU(2) and requires gauge bosons with (^{}
_{}W^{+}
_{}, ^{}
_{}W^{−}
_{} and ^{}
_{}W^{0}
_{}) to mediate transformations between fermions with halfinteger weak isospin charges. implies that ^{}
_{}W^{}
_{} bosons have three different values of :
 ^{}
_{}W^{+}
_{} boson is emitted in transitions → .  ^{}
_{}W^{0}
_{} boson would be emitted in weak interactions where does not change, such as neutrino scattering.  ^{}
_{}W^{−}
_{} boson is emitted in transitions → .
Under electroweak unification, the ^{}
_{}W^{0}
_{} boson mixes with the weak hypercharge gauge boson ^{}
_{}B^{}
_{}, resulting in the observed ^{}
_{}Z^{0}
_{} boson and the photon of quantum electrodynamics; the resulting ^{}
_{}Z^{0}
_{} and the photon both have weak isospin = 0.
The sum of −isospin and +charge is zero for each of the bosons, consequently, all the electroweak bosons have weak hypercharge , so unlike gluons of the color force, the electroweak bosons are unaffected by the force they mediate.
References
 Ambiguities: I is also used as sign for the ‘normal’ isospin, same for the third component aka . T is also used as the sign for Topness. This article uses T and .
 Baez, John C.; Huerta, John (2009). "The Algebra of Grand Unified Theories". Bull. Am. Math. Soc. 0904: 483–552. arXiv:0904.1556. Bibcode:2009arXiv0904.1556B. doi:10.1090/s0273097910012942. Retrieved 15 October 2013.