WEAK ISOSPIN

The 'weak isospin' in particle 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_z'' or ''I^W''.
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.
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 "left handed" fermions have a total weak isospin ''T_z'' = 1/2 (as you can see below the projections of this weak isospin, analogous to all spins, can change).
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.

Contents

Weak isospin and the W bosons

See also

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 ''W''⁰. This implies that ''W'' bosons have a ''T_z'' = 1, with three different values of ''T_z''.

★ 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).

See also

★ Field theoretical formulation of standard model

★ Weak hypercharge