The weak isospin in theoretical physics parallels the idea of the isospin under the strong interaction, but applied under the weak interaction. Weak isospin is usually given the symbol T or I^W. CPT-symmetry is a fundamental symmetry of physical laws under transformations that involve the inversions of charge, parity and time simultaneously. ... In the standard model of particle physics the Cabibbo Kobayashi Maskawa matrix (CKM matrix, sometimes earlier called KM matrix) is a unitary matrix which contains information on the strength of flavour changing weak decays. ... CP is the product of two symmetries: C for charge conjugation, which transforms a particle into its antiparticle, and P for parity, which creates the mirror image of a physical system. ... A phenomenon is said to be chiral if it is not identical to its mirror image (see Chirality (mathematics)). The spin of a particle may be used to define a handedness for that particle. ... Theoretical physics employs mathematical models and abstractions of physics, as opposed to experimental processes, in an attempt to understand Nature. ... Isospin (isotopic spin, isobaric spin) is a physical quantity which is mathematically analogous to spin. ... The strong interaction or strong force is today understood to represent the interactions between quarks and gluons as detailed by the theory of quantum chromodynamics (QCD). ... The weak nuclear force or weak interaction is one of the four fundamental forces of nature. ...

Leptons do not undergo strong interaction, and so isospin is not defined for them. But, in the same way that isospin creates hadron multiplets of particles that are indistinguishable under the strong interaction, all elemental fermions can be grouped in multiplets that behave the same under the weak interaction. For example, in the decay of a quark, type "u" quarks (u, c, t) always originate type "d" quarks (d, s, b) and vice versa. On the other hand, a quark never decays into a quark of the same type. Something similar happens with leptons, which are split into two groups: charged leptons versus neutrinos. In particle physics, a hadron is a subatomic particle which experiences the strong nuclear force. ... In particle physics, fermions are particles with half-integer spin. ... The weak nuclear force or weak interaction is one of the four fundamental forces of nature. ... These are the 6 quarks and their most likely decay modes. ... In physics, a particle is a lepton if it has a spin of 1/2 and does not experience the strong nuclear force. ...

Thus, fundamental fermions are grouped in pairs of particles that behave the same under the weak interaction, and differ from other pairs on their masses (i.e., they belong to different generations of matter). This means that all fundamental fermions (and in fact, all fermions) have weak isospin T = 1/2.

As in the isospin case, members of the same pair are distinguished by the third component of weak isospin (T_z). Type "u" fermions (quarks u, c, t and neutrinos) have T_z = +1/2, while type "d" fermions (quarks d, s, b and charged leptons) have T_z = −1/2.

There is also a weak isospin conservation law: all weak interactions must preserve the weak isospin.

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 W⁺, W⁻ and W⁰. This implies that W bosons have a T = 1, with three different values of T_z. In particle physics, bosons, named after Satyendra Nath Bose, are particles having integer spin. ...

• Boson W⁺ (T_z = +1) regulates transitions {(T_z = +½) → (T_z = −½)},
• Boson W⁻ (T_z = −1) is emitted in transitions {(T_z = −½) → (T_z = +½)}.
• Gauge boson W⁰ (T_z = 0) would regulate reactions where T_z (neither charge) does not change. (However boson W⁰ mixes with the electromagnetic gauge boson B, so instead of W⁰ we see the boson Z, and instead of B we observe γ).

The word light is defined here as electromagnetic radiation of any wavelength; thus, X-rays, gamma rays, ultraviolet light, microwaves, radio waves, and visible light are all forms of light. ...

See also

• Field theoretical formulation of standard model
• Weak hypercharge