once researcher detected the phenomina of neutrino oscillations,
it begin clear that neutrinos have a different but small mass. This
however leads to the problem of which type of mass a neutrino
has. Most ordinary matter particles particles such as electrons obey the Dirac equations, which describes a Spin-1/2 particle
and its anti-particle, each which can be in either a left or
right handed state.
A Majorana type particle, instead has only 2 types of state at
a particular mass, a left-handed particle and a right-handed
anti-particles, (or potentially vice versa). Thus if the neutrino
is a Dirac type particle, it has twice as many states as if it
where a Majorana type particle.
If the neutrino was a Dirac particle, when there would be
a right-handed version of the neutrino with the same mass as the
left-handed version. It is the detailed arcuracy of the Big Bang
theory, that seem to rule out Dirac type neutrinos. Big Bang
nucleosynthesis creates all the elements in the early universe, and
calcuations of the abunances of these elements, depend upon the
number of particles around at the time of nucleosynthesis.
Precent calcuation thus show that the number of neutrino states
is less than or equal to 4 Majorana states, or 2 Dirac states. Since
we have already found 3 neutrino types, (one of each generation).
Neutrinos can't very well be of the Dirac type.
