The Majorana neutrino hypothesis
Once Majorana had returned from abroad, where he
met several other important personalities including
Bloch, Bohr and Weisskopf, he did not publish any
papers for several years. But his research activity during
this period, which focused mainly on field theory and
quantum electrodynamics, is well testified by a number
of unpublished scientific notes that are currently
being studied.
In 1937, however, probably after being
invited by Fermi to compete for a full professorship,
Majorana published what was to become his most
famous paper [9]: “Symmetric theory of electrons and
positrons”, in which he introduced the so-called Majorana
neutrino hypothesis.
This hypothesis was revolutionary because it argued
that the antimatter partner of a given matter particle
could be the particle itself. This was in direct contradiction
to what Dirac had successfully assumed in order
to solve the problem of negative energy states in Quantum
Field Theory (i.e. the existence of the positron).
With unprecedented farsightedness Majorana suggested
that the neutrino, which had just been postulated
by Pauli and Fermi to explain puzzling features
of radioactive beta decay, could be such a particle. This
would make the neutrino unique among the elementary
particles and, moreover, enable it to have mass.
Today many experiments are still devoted to detect
these peculiar properties, which include the phenomenon
of neutrino oscillations.