22 Hints of Other New Physics from Neutrino Mass
169
22.3 What if Neutrinos Are not Majorana
Fermions?
What if the neutrino mass battle between Dirac and Majorana is lost by
Majorana and the neutrino is not its own anti-particle, i.e., it is a Dirac
fermion? That would be a major change in theoretical research, and the change
has already started in the last few years as the evidence for the Majorana nature
of the neutrino keeps getting farther and farther away from being established.
The Dirac neutrino will have similar characteristics as all the other fermions of
the standard model—the electron, the muon, the quarks, etc. In some sense,
we might think that it is a natural possibility, except that the neutrino mass
is so much smaller than the masses of other fermions, a fact which is hard
to understand if it is a Dirac fermion. Of course, it has zero electric charge,
unlike the other fermions of the standard model, making it unique among
all the standard model fermions. As we saw earlier, this property was one of
the main motivations for the seesaw picture of the neutrino that explained
naturally why the neutrinos are ultralight.
One way that the small Dirac neutrino masses can arise naturally is when
there are large extra dimensions. In this case, the smallness of the Dirac
neutrino mass is due to the existence of the large extra dimension [38]. Because
of the uncertainty principle which specifies that large lengths are associated
with small masses, the small neutrino mass is a consequence of the length
of extra dimensions being large. These models however lead to cosmological
difficulties. One unpleasant implication is that the ultimate temperature of the
universe in these models is only a few MeV, which is a drastically new kind
of universe. In this case, it is hard to understand many aspects of the early
universe, such as, inflation, the origin of matter, etc. At the laboratory level,
such models also lead to sizable corrections to the inverse square law of gravity,
which has been searched for, and no evidence has emerged yet [98].
How do we experimentally establish that neutrinos are Dirac rather than
Majorana fermions? If the neutrinoless double beta decay keeps getting farther
and farther from the reach of experimentalists, the interest in neutrinos
possibly being Dirac will attract more attention. However, as already stated,
for the case of the normal hierarchy of neutrino mass ordering, double beta
decay is suppressed anyway. As a result, it cannot be taken as an evidence for
Dirac neutrino. Most other experiments do not care if the neutrino is a Dirac
or Majorana particle.
One possible way to tell that the neutrino is a Dirac particle is as follows:
If the neutrino mass ordering is established from long baseline oscillation
169
22.3 What if Neutrinos Are not Majorana
Fermions?
What if the neutrino mass battle between Dirac and Majorana is lost by
Majorana and the neutrino is not its own anti-particle, i.e., it is a Dirac
fermion? That would be a major change in theoretical research, and the change
has already started in the last few years as the evidence for the Majorana nature
of the neutrino keeps getting farther and farther away from being established.
The Dirac neutrino will have similar characteristics as all the other fermions of
the standard model—the electron, the muon, the quarks, etc. In some sense,
we might think that it is a natural possibility, except that the neutrino mass
is so much smaller than the masses of other fermions, a fact which is hard
to understand if it is a Dirac fermion. Of course, it has zero electric charge,
unlike the other fermions of the standard model, making it unique among
all the standard model fermions. As we saw earlier, this property was one of
the main motivations for the seesaw picture of the neutrino that explained
naturally why the neutrinos are ultralight.
One way that the small Dirac neutrino masses can arise naturally is when
there are large extra dimensions. In this case, the smallness of the Dirac
neutrino mass is due to the existence of the large extra dimension [38]. Because
of the uncertainty principle which specifies that large lengths are associated
with small masses, the small neutrino mass is a consequence of the length
of extra dimensions being large. These models however lead to cosmological
difficulties. One unpleasant implication is that the ultimate temperature of the
universe in these models is only a few MeV, which is a drastically new kind
of universe. In this case, it is hard to understand many aspects of the early
universe, such as, inflation, the origin of matter, etc. At the laboratory level,
such models also lead to sizable corrections to the inverse square law of gravity,
which has been searched for, and no evidence has emerged yet [98].
How do we experimentally establish that neutrinos are Dirac rather than
Majorana fermions? If the neutrinoless double beta decay keeps getting farther
and farther from the reach of experimentalists, the interest in neutrinos
possibly being Dirac will attract more attention. However, as already stated,
for the case of the normal hierarchy of neutrino mass ordering, double beta
decay is suppressed anyway. As a result, it cannot be taken as an evidence for
Dirac neutrino. Most other experiments do not care if the neutrino is a Dirac
or Majorana particle.
One possible way to tell that the neutrino is a Dirac particle is as follows:
If the neutrino mass ordering is established from long baseline oscillation