The goal of particle physics research is to understand the nature of constituents of matter and the forces operating among them. Our current understanding is encoded in the successful standard model of Glashow, Weinberg and Salam. The recent discovery of the Higgs boson at the Large Hadron collider has been the crowning success of this model. There are however several conceptual puzzles raised by the success of this model.
Two outstanding ones are: (i) Are all the forces truly different from each other or they are same at some small distance scale? (ii) Why do weak interactions violate mirror symmetry whereas all other forces respect it? Addressing the second issue has been one of the major highlights of Dr. Mohapatra’s research career.
Conceptually, the understanding of maximal mirror asymmetry of weak interactions generally given in early text books is as follows: the only particle that exclusively participates in weak interactions and none other is the electrically neutral particle, the neutrino. For a long time neutrinos were thought to be massless. A massless fermion, like the neutrino can exist in only one helicity state without conflicting with the theory of relativity. The reason is that it moves with the speed of light and there is no frame of reference where the other helicity state of the fermion can be generated. On the contrary, if a fermion has mass, it moves slower than the speed of light and therefore by moving to a new reference frame, its helicity can be reversed and in that case theory of relativity would require that both states exist in nature.
Mirror symmetry takes one helicity state (e.g. left-handed) and changes to the other (e.g. right handed). Thus having one helicity state only in a theory (e. g. for a massless neutrino) means that mirror symmetry must be violated in any interaction that this single helicity state participates. Thus if neutrinos were massless, the case for weak interactions being fully mirror asymmetric would be rather strong.
In 1975, it was proposed by Mohapatra, Pati and Senjanovic that it is much more appealing to have weak interactions be parity conserving at shorter distances and in such a theory observed mirror asymmetry must be only approximate and this can be understood as arising from the possibility there is an heavier analog of the known standard model (SM) W-boson (called WR ) which mediates only the right handed weak forces involving the right handed neutrino. Observations in weak interactions allowed the WR to have its mass in the TeV range. These theories are known in the literature as left-right symmetric models (LRSM).
In the LRSM due to the existence of the right handed neutrino, one expects the neutrino to have a mass. At the time this theory was proposed, there was no evidence for neutrino mass but there were strong upper limits on how heavy the neutrino could be. It was known that the lightest neutrino mass has be much smaller than that of quarks and electrons etc. To understand this, in 1979, Mohapatra and Senjanovic (as well as several other authors) proposed the seesaw mechanism which suggests that the right handed neutrino is its won anti-particle and is as heavy as the right handed WR . As a consequence, the lighter observed neutrino becomes very light much lighter than the quarks and elcrton, muon etc for which there is no seesaw mechanism. (see figure below)
The neutrino masses have now been discovered and the seesaw mechanism is now-a-days widely believed as the basis for understanding observed neutrino oscillation phenomena. It has been speculated that this mechanism may be able to explain the matter anti-matter asymmetry of the Universe as well as the origin of dark matter pervading the whole cosmos. Both these ideas remain to be fully confirmed but are the subjects of intense activity in the field of neutrino mass and dark matter physics.