21 Mirror Symmetric Weak Force and Neutrino Mass
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Fig. 21.1 Left-handed standard model and left–right symmetric model in pictures
charged leptons. So why is the neutrino mass observed to be so small? Just to
remind the reader, the lightest quark mass is about 10 MeV (i.e. one hundred
times lighter than the proton) and the heaviest quark is known as the top quark
whose mass is about 175 times the mass of proton. The heaviest charged lepton
is the τ lepton whose mass is about 1.8 times the proton mass, whereas the
lightest charged lepton, the electron, has a mass of half an MeV. However, the
neutrino oscillation experiments combined with cosmology have conclusively
established that the heaviest left-handed neutrino mass is less than an electron
volt, so about a million times smaller than the electron. On the other hand, the
left-right symmetric theories naively would suggest that the neutrino masses
also must be of similar order to the electron mass. So how do we understand
the extreme smallness of neutrino masses? The nature of the neutrino itself
provides some clues (Fig. 21.2).