Weak isospin
From Wikipedia, the free encyclopedia
| Flavour in particle physics |
Flavour quantum numbers
Related topics: |
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 IW.
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 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 (Tz). Type "u" fermions (quarks u, c, t and neutrinos) have Tz = +1/2, while type "d" fermions (quarks d, s, b and charged leptons) have Tz = −1/2.
There is also a weak isospin conservation law: all weak interactions must preserve the weak isospin.
[edit] 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 Tz.
- Boson W+ (Tz = +1) regulates transitions {(Tz = +½) → (Tz = −½)},
- Boson W− (Tz = −1) is emitted in transitions {(Tz = −½) → (Tz = +½)}.
- Gauge boson W0 (Tz = 0) would regulate reactions where Tz (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).
