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R. N. Mohapatra
the theory and no right-handed neutrino was included to fit weak interaction
observations. As a result, parity symmetry was intrinsically broken by choice.
The model reproduced the known V −A charged weak interaction observed in
beta decay. As already emphasized, the absence of the right-handed neutrino of
course implied that neutrino is massless which we now know not to be correct.
We did not know that in 1967, when the standard model was proposed.
The left-right symmetric models were proposed to resolve this unsatisfactory
feature of the standard model. Since the standard model is so successful,
whatever is done to extend it must give back the same observed consequences
as the standard model. This is quite a challenge. How was this achieved in the
left–right model?
The left–right model postulated that there are new kinds of weak forces
generated by new W-like bosons. In other words, the familiar W bosons of
the standard model are accompanied by new W -like bosons called W R bosons
which couple to the right-handed helicity quarks and leptons, exactly like the
standard model W couples to left helicity fermions. Clearly this required that
there be right-handed neutrinos. The theory is therefore mirror symmetric
since, corresponding to any kind of weak process that has been observed to
emit the left-handed neutrinos, there is a mirror counterpart weak force which
emits right-handed neutrinos, making the theory respect mirror symmetry.
The other main point is that in the language of gauge theories which is the
framework for the standard model, this is done in a simple manner without
breaking any of the rules of the game.
The question then arises: why do not we see the mirror counterpart reactions
in the laboratory? These are the weak interactions that are of V + A type
as opposed to what is observed, i.e. the V − A type. The answer to this
is in the left–right models, the mass of the right-handed weak boson W R is
much heavier than that of the standard model W boson, so the weak forces
that involve the right-handed helicity neutrinos are much weaker and harder
to detect. But eventually they should be observed as the energy of colliders
increases. Figure 21.1 pictorial view of left–right symmetry compared to the
standard model.
21.1 Neutrino Mass-Mirror Symmetry Breaking
Connection
An immediate question that arises in the left–right symmetric models is the
following: the model puts neutrinos in the same footing as the quarks and
R. N. Mohapatra
the theory and no right-handed neutrino was included to fit weak interaction
observations. As a result, parity symmetry was intrinsically broken by choice.
The model reproduced the known V −A charged weak interaction observed in
beta decay. As already emphasized, the absence of the right-handed neutrino of
course implied that neutrino is massless which we now know not to be correct.
We did not know that in 1967, when the standard model was proposed.
The left-right symmetric models were proposed to resolve this unsatisfactory
feature of the standard model. Since the standard model is so successful,
whatever is done to extend it must give back the same observed consequences
as the standard model. This is quite a challenge. How was this achieved in the
left–right model?
The left–right model postulated that there are new kinds of weak forces
generated by new W-like bosons. In other words, the familiar W bosons of
the standard model are accompanied by new W -like bosons called W R bosons
which couple to the right-handed helicity quarks and leptons, exactly like the
standard model W couples to left helicity fermions. Clearly this required that
there be right-handed neutrinos. The theory is therefore mirror symmetric
since, corresponding to any kind of weak process that has been observed to
emit the left-handed neutrinos, there is a mirror counterpart weak force which
emits right-handed neutrinos, making the theory respect mirror symmetry.
The other main point is that in the language of gauge theories which is the
framework for the standard model, this is done in a simple manner without
breaking any of the rules of the game.
The question then arises: why do not we see the mirror counterpart reactions
in the laboratory? These are the weak interactions that are of V + A type
as opposed to what is observed, i.e. the V − A type. The answer to this
is in the left–right models, the mass of the right-handed weak boson W R is
much heavier than that of the standard model W boson, so the weak forces
that involve the right-handed helicity neutrinos are much weaker and harder
to detect. But eventually they should be observed as the energy of colliders
increases. Figure 21.1 pictorial view of left–right symmetry compared to the
standard model.
21.1 Neutrino Mass-Mirror Symmetry Breaking
Connection
An immediate question that arises in the left–right symmetric models is the
following: the model puts neutrinos in the same footing as the quarks and