# 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 ${\displaystyle T_{\mathrm {z} }}$, ${\displaystyle T_{3}}$, ${\displaystyle I_{\mathrm {z} }}$ or ${\displaystyle I_{3}}$.[1] It can be understood as the eigenvalue of a charge operator.

The weak isospin conservation law relates the conservation of ${\displaystyle T_{3}}$; all weak interactions must preserve ${\displaystyle T_{3}}$. 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, ${\displaystyle Q=T_{3}+{\tfrac {1}{2}}Y_{\mathrm {W} }}$ (electric charge), is conserved. ${\displaystyle T_{3}}$ 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 "left-handed" fermions) have ${\displaystyle T={\tfrac {1}{2}}}$ and can be grouped into doublets with ${\displaystyle T_{3}=\pm {\tfrac {1}{2}}}$ that behave the same way under the weak interaction. For example, up-type quarks (u, c, t) have ${\displaystyle T_{3}=+{\tfrac {1}{2}}}$ and always transform into down-type quarks (d, s, b), which have ${\displaystyle T_{3}=-{\tfrac {1}{2}}}$, and vice versa. On the other hand, a quark never decays weakly into a quark of the same ${\displaystyle T_{3}}$. Something similar happens with left-handed leptons, which exist as doublets containing a charged lepton (
e
,
μ
,
τ
) with ${\displaystyle T_{3}=-{\tfrac {1}{2}}}$ and a neutrino (
ν
e
,
ν
μ
,
ν
τ
) with ${\displaystyle T_{3}=+{\tfrac {1}{2}}}$. In all cases, the corresponding anti-fermion has reversed chirality ("right-handed" antifermion) and sign reversed ${\displaystyle T_{3}}$.

Fermions with positive chirality ("right-handed" fermions) and anti-fermions with negative chirality ("left-handed" anti-fermions) have ${\displaystyle T=T_{3}=0}$ and form singlets that do not undergo weak interactions.

Electric charge, ${\displaystyle Q}$, is related to weak isospin, ${\displaystyle T_{3}}$, and weak hypercharge, ${\displaystyle Y_{\mathrm {W} }}$, by

${\displaystyle Q=T_{3}+{\tfrac {1}{2}}Y_{\mathrm {W} }}$.

## Weak isospin and the W bosons

The symmetry associated with weak isospin is SU(2) and requires gauge bosons with integral ${\displaystyle T_{3}}$ (
W+
,
W
and
W0
) to mediate transformations between fermions with half-integer weak isospin charges. This implies that
W
bosons must have ${\displaystyle T=1}$, with three different values of ${\displaystyle T_{3}}$:

• W+
boson ${\displaystyle (T_{3}=+1)}$ is emitted in transitions ${\displaystyle (T_{3}=+{\tfrac {1}{2}})}$${\displaystyle (T_{3}=-{\tfrac {1}{2}})}$.

• W0
boson ${\displaystyle (T_{3}=0)}$ would be emitted in weak interactions where ${\displaystyle T_{3}}$ does not change, such as neutrino scattering.

• W
boson ${\displaystyle (T_{3}=-1)}$ is emitted in transitions ${\displaystyle (T_{3}=-{\tfrac {1}{2}})}$${\displaystyle (T_{3}=+{\tfrac {1}{2}})}$.

Under electroweak unification, the
W0
boson mixes with the weak hypercharge gauge boson
B
, resulting in the observed
Z0
boson and the photon of quantum electrodynamics. However, the resulting
Z0
and the photon both have weak isospin 0. As a consequence of their weak isospin values and charges, all the electroweak bosons have weak hypercharge ${\displaystyle Y_{\text{w}}=0}$, so unlike gluons and the color force, the electroweak bosons are unaffected by the force they mediate.

1. ^ Ambiguities: I is also used as sign for the ‘normal’ isospin, same for the third component ${\displaystyle T_{3}}$ aka ${\displaystyle I_{\mathrm {z} }}$. T is also used as the sign for Topness. This article uses T and ${\displaystyle T_{3}}$.