524
K. Kleinknecht and U. Uwer
a direction opposite to the external magnetic field, and that this effect disappeared
when the crystal was warmed and the nuclear polarization disappeared. Thus, at low
temperature, a nonzero pseudoscalar is observed, demonstrating parity violation.
By comparing nuclear beta decays having an electron and an antineutrino in
the final state with their counterpart with emission of a positron-neutrino pair, it
was shown that the helicity h = =
s · ·
p/|| s · ·
p| of neutrinos is opposite to the one of
antineutrinos [18].
Other experiments lead to similar results. The helicity h = =
s · ·
p/|| s · ·
p| of
the neutrino emitted in the weak electron capture by 152 Eu was measured in
an experiment by Goldhaber and Grodzins [19] to be negative; the neutrino is
“left-handed”, i.e. the spin is aligned antiparallel to the momentum. Similarly,
measurements of the polarization of electrons from β − decay showed a negative
value, with a modulus increasing with the velocity of the electron, v/c. Positrons
from β + decay were found to have a positive polarization that increases with v/c.
9.2.2.3 Violation of C Symmetry, and CP Invariance
In the realm of weak decays of particles, supporting evidence for the violation of
mirror symmetry came from the observation that parity is violated in the decay
π + → μ + ν μ , and that the muon neutrino from this decay is left-handed [15, 16].
The P-conjugate process, i.e. π + → ν μ μ + , with a right-handed neutrino, does not
occur. The same is true for the C-conjugate process, π − → μ − ν μ , with a lefthanded antineutrino. However, if we combine the C and P operations, we arrive at a
process π − → μ − ν μ with a right-handed antineutrino, which proceeds at the same
rate as the original π + decay, with a left-handed muon neutrino. Evidently, in weak
interactions, P and C are violated, while it seemed at the time of those experiments
that the process was invariant under the combined operation C · P. This argument
can be visualized as in Fig. 9.2. Here the P mirror and the C mirror act on a lefthanded neutrino, both leading to unphysical states, a right-handed neutrino and a
left-handed antineutrino. Only the combined CP mirror leads to a physical particle,
the right-handed antineutrino. This argument was made by Landau [17], suggesting
that the real symmetry was CP invariance.
9.2.2.4 CP Invariance and Neutral K Mesons
One consequence of this postulated CP invariance was predicted by Gell-Mann
and Pais [12] for the neutral K mesons: there should be a long-lived partner to
the known V 0
K 0
1
particle of short lifetime (10 −10 s). According to this proposal,
these two particles are mixtures of the two strangeness eigenstates K 0 (S = +1) and
K 0 (S = −1) produced in strong interactions. Weak interactions do not conserve
strangeness, and the physical particles should be eigenstates of CP if the weak
K. Kleinknecht and U. Uwer
a direction opposite to the external magnetic field, and that this effect disappeared
when the crystal was warmed and the nuclear polarization disappeared. Thus, at low
temperature, a nonzero pseudoscalar is observed, demonstrating parity violation.
By comparing nuclear beta decays having an electron and an antineutrino in
the final state with their counterpart with emission of a positron-neutrino pair, it
was shown that the helicity h = =
s · ·
p/|| s · ·
p| of neutrinos is opposite to the one of
antineutrinos [18].
Other experiments lead to similar results. The helicity h = =
s · ·
p/|| s · ·
p| of
the neutrino emitted in the weak electron capture by 152 Eu was measured in
an experiment by Goldhaber and Grodzins [19] to be negative; the neutrino is
“left-handed”, i.e. the spin is aligned antiparallel to the momentum. Similarly,
measurements of the polarization of electrons from β − decay showed a negative
value, with a modulus increasing with the velocity of the electron, v/c. Positrons
from β + decay were found to have a positive polarization that increases with v/c.
9.2.2.3 Violation of C Symmetry, and CP Invariance
In the realm of weak decays of particles, supporting evidence for the violation of
mirror symmetry came from the observation that parity is violated in the decay
π + → μ + ν μ , and that the muon neutrino from this decay is left-handed [15, 16].
The P-conjugate process, i.e. π + → ν μ μ + , with a right-handed neutrino, does not
occur. The same is true for the C-conjugate process, π − → μ − ν μ , with a lefthanded antineutrino. However, if we combine the C and P operations, we arrive at a
process π − → μ − ν μ with a right-handed antineutrino, which proceeds at the same
rate as the original π + decay, with a left-handed muon neutrino. Evidently, in weak
interactions, P and C are violated, while it seemed at the time of those experiments
that the process was invariant under the combined operation C · P. This argument
can be visualized as in Fig. 9.2. Here the P mirror and the C mirror act on a lefthanded neutrino, both leading to unphysical states, a right-handed neutrino and a
left-handed antineutrino. Only the combined CP mirror leads to a physical particle,
the right-handed antineutrino. This argument was made by Landau [17], suggesting
that the real symmetry was CP invariance.
9.2.2.4 CP Invariance and Neutral K Mesons
One consequence of this postulated CP invariance was predicted by Gell-Mann
and Pais [12] for the neutral K mesons: there should be a long-lived partner to
the known V 0
K 0
1
particle of short lifetime (10 −10 s). According to this proposal,
these two particles are mixtures of the two strangeness eigenstates K 0 (S = +1) and
K 0 (S = −1) produced in strong interactions. Weak interactions do not conserve
strangeness, and the physical particles should be eigenstates of CP if the weak