Elementary Particles
379
The V-A theory brings in the possibility that the weak interaction is invariant
under the combined operation of spatial reflection and charge conjugation which
changes a particle into an antiparticle. As observed before, mirror reflection
takes a left-handed neutrino into a right-handed neutrino which under charge
conjugation goes into a right-handed antineutrino which is included in the theory.
However, even this invariance under CP, i.e. the combined operations of parity
and charge conjugation, is violated to a small extent. This was observed by
Christenson, Cronin, Fitch and Turaly (1964) in the decay of the long-lived
Component, Cronin, Fitch and Turlay (1964) in the decays into three π-meson
states with CP = –1, also decays to a small extent into two π-meson states with
CP = 1, indicating a small violation of CP invariance.
10.5 UNIFIED APPROACH
There have been efforts to unify the various interactions and deduce them as
different manifestations of the same underlying theory. Such a unification was
achieved for electric and magnetic forces by Maxwell (1864). Physicists have
now succeeded in unifying electromagnetic and weak forces and further efforts
are being made to bring in strong and gravitational forces as well. Here the
ideas that have led to the unification of electromagnetic and weak forces are
briefly described.
As was noted in the lest section, the electron-antineutrino pair in the
β-decay is produced in the same angular momentum state as the photon. This
similarity between weak and electromagnetic processes can be further extended
by postulating that the neutron changes into a proton by emitting a massive
W
–
(which carries spin ) and the massive W
–
then decays into an
electron-antineutrino pair. The basic weak process here is a vector boson
W
–
interacting with a fermion pair similar to a photon interaction with a fermion
pair. The important differences are (i) the W
–
is charged and is accompanied by
its antiparticle the W
+
whereas the photon is neutral and is its own antiparticle,
(ii) the W
–
is massive since weak interaction is a short-range force. From the
analysis of the weak processes, it is estimated that
m W > ~ 7.5 × 10
4
MeV
(10.38)
(iii) The interaction of the W
±
violates parity invariance whereas the interaction
of the photon conserves parity.
In spite of the difference mentioned above, the interaction of the W bosons
and the photon with matter can be combined. This is done by allowing a triplet
and a singlet of vector bosons to interact with the lepton and quark doublets in
Eqs. (10.1) and (10.3) (the left-handed parts of charged leptons are a part of the
doublet and their right-handed parts form singlets). Demanding that the photon
does not interect directly with the neutrinos and that its interaction conserves
parity identifies the photon and relates the strengths of the weak and the
379
The V-A theory brings in the possibility that the weak interaction is invariant
under the combined operation of spatial reflection and charge conjugation which
changes a particle into an antiparticle. As observed before, mirror reflection
takes a left-handed neutrino into a right-handed neutrino which under charge
conjugation goes into a right-handed antineutrino which is included in the theory.
However, even this invariance under CP, i.e. the combined operations of parity
and charge conjugation, is violated to a small extent. This was observed by
Christenson, Cronin, Fitch and Turaly (1964) in the decay of the long-lived
Component, Cronin, Fitch and Turlay (1964) in the decays into three π-meson
states with CP = –1, also decays to a small extent into two π-meson states with
CP = 1, indicating a small violation of CP invariance.
10.5 UNIFIED APPROACH
There have been efforts to unify the various interactions and deduce them as
different manifestations of the same underlying theory. Such a unification was
achieved for electric and magnetic forces by Maxwell (1864). Physicists have
now succeeded in unifying electromagnetic and weak forces and further efforts
are being made to bring in strong and gravitational forces as well. Here the
ideas that have led to the unification of electromagnetic and weak forces are
briefly described.
As was noted in the lest section, the electron-antineutrino pair in the
β-decay is produced in the same angular momentum state as the photon. This
similarity between weak and electromagnetic processes can be further extended
by postulating that the neutron changes into a proton by emitting a massive
W
–
(which carries spin ) and the massive W
–
then decays into an
electron-antineutrino pair. The basic weak process here is a vector boson
W
–
interacting with a fermion pair similar to a photon interaction with a fermion
pair. The important differences are (i) the W
–
is charged and is accompanied by
its antiparticle the W
+
whereas the photon is neutral and is its own antiparticle,
(ii) the W
–
is massive since weak interaction is a short-range force. From the
analysis of the weak processes, it is estimated that
m W > ~ 7.5 × 10
4
MeV
(10.38)
(iii) The interaction of the W
±
violates parity invariance whereas the interaction
of the photon conserves parity.
In spite of the difference mentioned above, the interaction of the W bosons
and the photon with matter can be combined. This is done by allowing a triplet
and a singlet of vector bosons to interact with the lepton and quark doublets in
Eqs. (10.1) and (10.3) (the left-handed parts of charged leptons are a part of the
doublet and their right-handed parts form singlets). Demanding that the photon
does not interect directly with the neutrinos and that its interaction conserves
parity identifies the photon and relates the strengths of the weak and the