14 What Have We Learned about Neutrinos...
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beta decay (see below) measures their absolute mass scale. If however they are
Dirac fermions, neutrinoless double beta decay is forbidden, in which case
other ways must be sought.
There is a way to measure the mass of the electron neutrino since it is
emitted in beta decay. How the decay proceeds depends on how massive the
electron neutrino (as well as other neutrinos) is. There is an experiment known
as KATRIN experiment in Karlsruhe, Germany, where experimentalists are
trying to look for how the electrons from the decay of a tritium nucleus behave
at different energies. This property depends on how big the neutrino mass
is. At the moment, they can tell if the electron neutrino mass is larger than
0.2 eV. That will provide a way, using the oscillation measurements to find the
absolute mass values of the various neutrino species.
There are also ways to tell what the absolute mass of neutrinos are from
looking at cosmological observations. Any value of neutrino mass will affect
how the various galaxies formed and how densely they are distributed. These
kind of detailed considerations can give mass limits of order of 0.1 eV or less.
Such cosmological observations are under way both in Europe and USA. They
are both ground based missions, such as the Large Synoptic Survey Telescope
(LSST,) and space missions, like the EUCLID mission by European Space
Agency and the WFIRST (Wide Field Infrared Survey Telescope) mission by
NASA in the USA [55]. There should be interesting results from these searches
in the next decade.