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R. N. Mohapatra
experiments to be inverted, then it follows that there must be an upper limit
on the lifetime of the neutrinoless double beta decay. The actual number
depends on nuclei. So, if the searches for the neutrinoless double beta decay
continue and one violates this upper bound, then understand both long
baseline evidence for an inverted hierarchy and the neutrinoless double beta
decay results together would require that the neutrino is a Dirac fermion. So
far, neither of these conclusions are at hand and therefore we still do not know
conclusively whether the neutrino is a Dirac or Majorana type fermion. In
fact, the experimental indications now are in favor of the mass ordering being
normal which, as remarked, would make it extremely difficult to establish
that neutrinos are Majorana fermions, until the neutrinoless double beta decay
process is discovered.
In the next chapter, we discuss how the seesaw picture can provide a way to
understand the origin of matter–anti-matter asymmetry in nature.