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 Tz, T3, Iz or I3. It can be understood as the eigenvalue of a charge operator.
The weak isospin conservation law relates the conservation of T3; all weak interactions must preserve T3. It is also conserved by the other 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, Q = T3 + Y / 2 (electric charge), is conserved. T3 is more important than T and often the term "weak isospin" refers to the "3rd component of weak isospin".
Fermions with negative chirality (also called left-handed fermions) have T = 1⁄2 and can be grouped into doublets with T3 = ± 1⁄2 that behave the same way under the weak interaction. For example, up-type quarks (u, c, t) have T3 = + 1⁄2 and always transform into down-type quarks (d, s, b), which have T3 = − 1⁄2, and vice versa. On the other hand, a quark never decays weakly into a quark of the same T3. Something similar happens with left-handed leptons, which exist as doublets containing a charged lepton (
e−
,
μ−
,
τ−
) with T3 = − 1⁄2 and a neutrino (
ν
e,
ν
μ,
ν
τ) with T3 = 1⁄2.
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