Implications for the Standard Model and Cosmology
Neutrinos are massless in the Standard Model of particle physics by
design. The Standard Model could easily be extended to include
massive neutrinos via the Higgs mechanism--the phenomenon that
physicists believe endows other particles with mass--but
particle theorists are loath to do this. The Higgs-neutrino
interaction strength would need to be a trillion times weaker than
the equivalent Higgs-top quark coupling strength, which would be
difficult to address naturally in a more fundamental theory.
Instead, most theorists support an alternative formulation of the
Higgs mechanism, called the "seesaw mechanism," which
includes neutrino interactions with a very massive hypothetical
particle. For the range of parameters indicated by Super-Kamiokande,
that heavy mass would be within a few orders of magnitude of the
scale where the strong and electroweak forces are believed to
unify. Thus featherweight neutrinos might paradoxically shed light
on ultraheavy dynamics.
Massive neutrinos also make some contribution to the "dark
much larger than the tiny splitting between flavors.