22
Hints of Other New Physics from Neutrino
Mass
As already emphasized, neutrino mass is a clue to new physics beyond the
standard model. One scenario of new physics described above uses the left–
right symmetry of nature at very short distances. There are many other
scenarios for neutrino masses. They are too numerous to discuss here. But
in this chapter, we focus on one other widely discussed scenario, which is also
motivated by attempts at understanding the role of mirror symmetry in nature.
22.1 Mirror Universe Versus Left–Right
Symmetry: Two Paths to Parity
When Lee and Yang proposed that weak forces break mirror symmetry, they
were so uncomfortable with the concept of mirror symmetry violation that
they proposed a way to rescue this symmetry in nature while having it remain
compatible with its violation in weak forces. They suggested that there might
be a mirror universe, a separate set of particles and forces from ours, to which
mirror symmetry takes all our particles and forces. The mirror symmetry
would therefore be preserved when both universes are taken together. On
the other hand, since we are making observations only with particles in our
universe, the weak force would violate mirror symmetry as observed. In such
a combined scenario of the universe, the number of fundamental constituents
of matter will be doubled. In this picture, there will be twice the number of
quarks and leptons that exist in the standard model. For example, there will
be a mirror proton which is the mirror partner of our proton, a mirror photon
© The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer
Nature Switzerland AG 2021
R. N. Mohapatra, The Neutrino Story: One Tiny Particle’s Grand Role in the Cosmos,
https://doi.org/10.1007/978-3-030-51846-2_22
165
Hints of Other New Physics from Neutrino
Mass
As already emphasized, neutrino mass is a clue to new physics beyond the
standard model. One scenario of new physics described above uses the left–
right symmetry of nature at very short distances. There are many other
scenarios for neutrino masses. They are too numerous to discuss here. But
in this chapter, we focus on one other widely discussed scenario, which is also
motivated by attempts at understanding the role of mirror symmetry in nature.
22.1 Mirror Universe Versus Left–Right
Symmetry: Two Paths to Parity
When Lee and Yang proposed that weak forces break mirror symmetry, they
were so uncomfortable with the concept of mirror symmetry violation that
they proposed a way to rescue this symmetry in nature while having it remain
compatible with its violation in weak forces. They suggested that there might
be a mirror universe, a separate set of particles and forces from ours, to which
mirror symmetry takes all our particles and forces. The mirror symmetry
would therefore be preserved when both universes are taken together. On
the other hand, since we are making observations only with particles in our
universe, the weak force would violate mirror symmetry as observed. In such
a combined scenario of the universe, the number of fundamental constituents
of matter will be doubled. In this picture, there will be twice the number of
quarks and leptons that exist in the standard model. For example, there will
be a mirror proton which is the mirror partner of our proton, a mirror photon
© The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer
Nature Switzerland AG 2021
R. N. Mohapatra, The Neutrino Story: One Tiny Particle’s Grand Role in the Cosmos,
https://doi.org/10.1007/978-3-030-51846-2_22
165