Weak isospin
Flavour in particle physics 
Flavour quantum numbers:
Related quantum numbers:
Combinations:

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 T_{z}, T_{3}, I_{z} or I_{3}.^{[1]} Weak isospin is a complement of the weak hypercharge, which unifies weak interactions with electromagnetic interactions.
The weak isospin conservation law relates the conservation of T_{3}; all weak interactions must preserve T_{3}. It is also conserved by the other interactions and is therefore a conserved quantity in general. For this reason T_{3} 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 T = Template:Frac and can be grouped into doublets with T_{3} = ±Template:Frac that behave the same way under the weak interaction. For example, uptype quarks (u, c, t) have T_{3} = +Template:Frac and always transform into downtype quarks (d, s, b), which have T_{3} = −Template:Frac, and viceversa. On the other hand, a quark never decays weakly into a quark of the same T_{3}. Something similar happens with lefthanded leptons, which exist as doublets containing a charged lepton (Template:SubatomicParticle, Template:SubatomicParticle, Template:SubatomicParticle) with T_{3} = −Template:Frac and a neutrino (Template:SubatomicParticle, Template:SubatomicParticle, Template:SubatomicParticle) with T_{3} = Template:Frac.
Fermions with positive chirality (also called righthanded fermions) have T = 0 and form singlets that do not undergo weak interactions.
Electric charge, Q, is related to weak isospin, T_{3}, and weak hypercharge, Y_{W}, by
Weak isospin and the W bosons
The symmetry associated with spin is SU(2). This requires gauge bosons to transform between weak isospin charges: bosons Template:SubatomicParticle, Template:SubatomicParticle and Template:SubatomicParticle. This implies that Template:SubatomicParticle bosons have a T = 1, with three different values of T_{3}.
 Template:SubatomicParticle boson (T_{3} = +1) is emitted in transitions {(T_{3} = +Template:Frac) → (T_{3} = −Template:Frac)},
 Template:SubatomicParticle boson (T_{3} = −1) is emitted in transitions {(T_{3} = −Template:Frac) → (T_{3} = +Template:Frac)}.
 Template:SubatomicParticle boson (T_{3} = 0) would be emitted in reactions where T_{3} does not change. However, under electroweak unification, the Template:SubatomicParticle boson mixes with the weak hypercharge gauge boson Template:SubatomicParticle, resulting in the observed Template:SubatomicParticle boson and the photon of Quantum Electrodynamics.