Figure 5: Composition of Universe: Universe is
found to be composed of dark energy, dark matter
and visible matter (atoms and molecules). Why
“matteranti-matter” now while “matter=anti-
matter” at its beginning (Big-Bang)?
Figure 6: Majorana vs Dirac. Like the elec-
tron and positron, a particle is diﬀerent en-
tity from its anti-particle in the Dirac theory.
However, a particle is its own anti-particle in
the Majorana theory. Whether neutrino is
Dirac or Majorana must be determined ex-
directly by telescopes of any wavelengths (optical, microwave, X-rays etc.), thus the name “dark
matter”. The rest of 4% is familiar atoms. However, there are virtually no anti-atoms (or
anti-matters). From view point of the particle physics, the fact is surprising because exactly the
same amount of matter and anti-matter should have existed at the beginning of the universe
(Big Bang). We somehow lost our partner in the course of the 14-billion-year history. Thus the
fact demands physics explanation.
The most promising theory is called “lepto-genesis”. According to the theory, when the
universe was much hotter than today, a gigantic neutrino (a hypothetical partner of the standard
neutrino, yet to be conﬁrmed) made tiny imbalance between matter and anti-matter: all anti-
matters annihilated away with matters but small portion of the matter survived. There are two
key features for this theory to be viable: one is Majorana nature of neutrinos, and the other
is violation of CP-symmetry. Let’s explain these key words one by one below. All the charged
leptons and quarks are known to be the Dirac particles. In this case, a particle and its anti-
particle is a totally diﬀerent entity. The electron, for example, has a negative charge while its
anti-particle, the positron, has positive charge, and they are diﬀerent each other. In the case of
Majorana particle, however, there is no distinction between particle and anti-particle. In another
words, a particle’s anti-particle is a particle itself. See Fig.6. That neutrino is Majorana particle
is prerequisite for the “lepto-genesis” scenario. There is another reason to believe neutirnos are
Majorana. As shown in Fig.7, neutrino’s masses are exceptionally light compared with other
fundamental particles. Again the Majorana nature of neutrinos can explain this fact very well.
Ｅｎｅｒｇｙ Ｓ ｃａｌｅ
Figure 7: Masses of elementary particles in the Stadard Model. Neutrino masses are exception-
ally small compared with the others.