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#

Summary

Within the present framework of field
theory, a theory
``A'': ``the neutrino is a Majorana particle and
its weak-interaction is characterised
by Lagrangian eq.(7) (which is different from the
one expected in standard model)'' and theory ``B'':
``the neutrino is a Weyl particle
and the standard-model gauge sector
is strictly valid'' are phenomenologically completely
equivalent. Therefore - without a powerful theory like
the standard model that
quantitatively predicts the form of the neutrino
weak interaction *without any recourse to measured neutrino properties* -
to go from theory ``A'' to ``B'' is merely a change of designations.
This is how the equivalence between Weyl and Majorana neutrinos
became conventional wisdom.
However, 25 years of impressive experimental
confirmations of the standard model
convinced most particle physicists
that the gauge sector of future theories
is quantitatively described by this theory to
good approximation. Under this - now very plausible -
assumption, theory ``B'' is realized in nature,
i.e. Majorana's idea of hermitian fermion fields
describing neutral fermions is not realized in nature
for the neutrino.
This is a nontrivial constraint on all future theories.
I do not claim that a Majorana theory is inconsistent
in any sense: I only say that experimental results happen
to prefer a Weyl neutrino, without offering any theoretical
reason why this should be so.
The present paper does not contradict any
publication in a refereed journal, because none
analysed the *formal proof* of Weyl - Majorana equivalence
under the assumption of *quantitative* validity of the standard model.

**Acknowledgements**
I sincerely thank H.Haber, B.Kayser, W.Marciano and S.Pezzoni for
extensive discussions and explanations.

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** Up:** The non-equivalence of Weyl
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Rainer Plaga
2001-08-03