Flavour in particle physics 

Flavour quantum numbers 

Related quantum numbers 

Combinations 

Flavour mixing 
In particle physics, weak isospin is a quantum number relating to the electrically charged part of the weak interaction: Particles with halfinteger weak isospin can interact with the ^{}
_{}W^{±}
_{} bosons; particles with zero weak isospin do not.
Weak isospin is a construct parallel to the idea of isospin under the strong interaction. Weak isospin is usually given the symbol T or I, with the third component written as T_{3} or I_{3}.T_{3} is more important than T; typically "weak isospin" is used as short form of the proper term "3rd component of weak isospin". It can be understood as the eigenvalue of a charge operator.
This article uses T and T_{3} for weak isospin and its projection. Regarding ambiguous notation, I is also used to represent the 'normal' (strong force) isospin, same for its third component I_{3} a.k.a. T_{3} or T_{z} . Aggravating the confusion, T is also used as the symbol for the Topness quantum number.
The weak isospin conservation law relates to the conservation of weak interactions conserve T_{3}. It is also conserved by the electromagnetic and strong interactions. However, interaction with the Higgs field does not conserve T_{3}, as directly seen by propagation of fermions, mixing chiralities by dint of their mass terms resulting from their Higgs couplings. Since the Higgs field vacuum expectation value is nonzero, particles interact with this field all the time even in vacuum. Interaction with the Higgs field changes particles' weak isospin (and weak hypercharge). Only a specific combination of electric charge is conserved. The electric charge, is related to weak isospin, and weak hypercharge, by
In 1961 Sheldon Glashow proposed this relation by analogy to the GellMann–Nishijima formula for charge to isospin.^{[1]}^{[2]}^{: 152 }
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. By convention, electrically charged fermions are assigned with the same sign as their electric charge.
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 reversed sign
Fermions with positive chirality ("righthanded" fermions) and antifermions with negative chirality ("lefthanded" antifermions) have and form singlets that do not undergo charged weak interactions.
Particles with do not interact with ^{}
_{}W^{±}
_{} bosons; however, they do all interact with the ^{}
_{}Z^{0}
_{} boson.
Lacking any distinguishing electric charge, neutrinos and antineutrinos are assigned the opposite their corresponding charged lepton; hence, all lefthanded neutrinos are paired with negatively charged lefthanded leptons with so those neutrinos have Since righthanded antineutrinos are paired with positively charged righthanded antileptons with those antineutrinos are assigned The same result follows from particleantiparticle charge & parity reversal, between lefthanded neutrinos () and righthanded antineutrinos ().
Generation 1  Generation 2  Generation 3  

Fermion  Electric charge 
Symbol  Weak isospin 
Fermion  Electric charge 
Symbol  Weak isospin 
Fermion  Electric charge 
Symbol  Weak isospin 
Electron  Muon  Tauon  
Up quark  Charm quark  Top quark  
Down quark  Strange quark  Bottom quark  
Electron neutrino  Muon neutrino  Tau neutrino  
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. 
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. ^{[4]} implies that ^{}
_{}W^{}
_{} bosons have three different values of
Under electroweak unification, the ^{}
_{}W^{0}
_{} boson mixes with the weak hypercharge gauge boson ^{}
_{}B^{0}
_{} ; both have weak isospin = 0 . This results in the observed ^{}
_{}Z^{0}
_{} boson and the photon of quantum electrodynamics; the resulting ^{}
_{}Z^{0}
_{} and ^{}
_{}γ^{0}
_{} likewise have zero weak isospin.