287
20.2. Parity violation in weak interactions, and V-A theory
Other combinations are also possible, as we shall discuss shortly. Note that
the interaction must always be Lorentz invariant.
Thus began a long period of difficult experimentation to establish the
correct form of the β-decay interaction. With the discovery of the muon (in
1937) and the pion (ten years later) more weak decays became experimentally
accessible, for example μ decay
−
μ
−
→ e + ν + ν
(20.8)
and π decay
−
π
−
→ e + ν.
(20.9)
Note that we have deliberately called all the neutrinos just ‘ν’, without any
particle/antiparticle indication, or lepton flavour label; we shall have more to
say on these matters in section 20.3. There were hopes that the couplings of
the pairs (p,n), (ν, e
− ) and (ν, μ
− ) might have the same form (‘universality’)
but the data was incomplete, and in part apparently contradictory.
The breakthrough came in 1956, when Lee and Yang (1956) suggested that
parity was not conserved in all weak decays. Hitherto, it had always been assumed that any physical interaction had to be such that parity was conserved,
and this assumption had been built into the structure of the proposed β-decay
interactions, such as (20.3), (20.5) or (20.6). Once it was looked for properly,
following the analysis of Lee and Yang, parity violation was indeed found to
be a strikingly evident feature of weak interactions.
20.2 Parity violation in weak interactions,
and V-A theory
20.2.1 Parity violation
In 1957, the experiment of Wu et al. (1957) established for the first time that
parity was violated in a weak interaction, specifically nuclear β-decay. The
experiment involved a sample of
60 Co (J = 5) cooled to 0.01 K in a solenoid.
At this temperature most of the nuclear spins are aligned by the magnetic field,
and so there is a net polarization ‹J ›, which is in the direction opposite to
the applied magnetic field.
60 Co decays to
60 Ni (J = 4), a ΔJ = 1 transition.
The degree of
60 Co alignment was measured from observations of the angular
distribution of γ-rays from
60 Ni. The relative intensities of electrons emitted
along and against the magnetic field direction were measured, and the results
were consistent with a distribution of the form
I(θ) = 1 − ‹J › · p/E
(20.10)
= 1 − P v cos θ
(20.11)
20.2. Parity violation in weak interactions, and V-A theory
Other combinations are also possible, as we shall discuss shortly. Note that
the interaction must always be Lorentz invariant.
Thus began a long period of difficult experimentation to establish the
correct form of the β-decay interaction. With the discovery of the muon (in
1937) and the pion (ten years later) more weak decays became experimentally
accessible, for example μ decay
−
μ
−
→ e + ν + ν
(20.8)
and π decay
−
π
−
→ e + ν.
(20.9)
Note that we have deliberately called all the neutrinos just ‘ν’, without any
particle/antiparticle indication, or lepton flavour label; we shall have more to
say on these matters in section 20.3. There were hopes that the couplings of
the pairs (p,n), (ν, e
− ) and (ν, μ
− ) might have the same form (‘universality’)
but the data was incomplete, and in part apparently contradictory.
The breakthrough came in 1956, when Lee and Yang (1956) suggested that
parity was not conserved in all weak decays. Hitherto, it had always been assumed that any physical interaction had to be such that parity was conserved,
and this assumption had been built into the structure of the proposed β-decay
interactions, such as (20.3), (20.5) or (20.6). Once it was looked for properly,
following the analysis of Lee and Yang, parity violation was indeed found to
be a strikingly evident feature of weak interactions.
20.2 Parity violation in weak interactions,
and V-A theory
20.2.1 Parity violation
In 1957, the experiment of Wu et al. (1957) established for the first time that
parity was violated in a weak interaction, specifically nuclear β-decay. The
experiment involved a sample of
60 Co (J = 5) cooled to 0.01 K in a solenoid.
At this temperature most of the nuclear spins are aligned by the magnetic field,
and so there is a net polarization ‹J ›, which is in the direction opposite to
the applied magnetic field.
60 Co decays to
60 Ni (J = 4), a ΔJ = 1 transition.
The degree of
60 Co alignment was measured from observations of the angular
distribution of γ-rays from
60 Ni. The relative intensities of electrons emitted
along and against the magnetic field direction were measured, and the results
were consistent with a distribution of the form
I(θ) = 1 − ‹J › · p/E
(20.10)
= 1 − P v cos θ
(20.11)