Everything about The Weak Isospin totally explained
The
weak isospin in
particle physics 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
T3,
Tz or
I3. Weak isospin is a component of the
weak hypercharge, which unifies
weak interactions with
electromagnetic interactions.
Fermions with negative
helicity (also called left-handed fermions) have
T = ½ and can be grouped into doublets with
T3 = ± ½ that behave the same under the
weak interaction. For example, "u" type
quarks (
u,
c,
t) have
T3 = +½ and always transform in "d" type quarks (
d,
s,
b) that have
T3 = -½ and vice versa. On the other hand, a quark never decays into a quark of the same type. Something similar happens with left-handed
leptons, which exist as doublets containing a charged lepton with
T3 = -½ and a
neutrino with T
3 = ½.
Fermions with positive
chirality (also called right-handed fermions) have T = 0 and form singlets that don't undergo weak interactions.
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
Tz.
- W+ boson (T3 = +1) is emitted in transitions .
- W0 boson (T3 = 0) would be emitted in reactions where T3 doesn't change. However, 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.
Further Information
Get more info on 'Weak Isospin'.
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