| Abstrakt: | We present a formalism describing neutrino oscillations in any application beyond
the Standard Model theory. Instead of using the standard pure states approach,
we apply the density matrix formalism. In general, in New Physics models,
neutrino states are no longer as pure as they are in the Standard Model. We discuss
the details of the appearance of a mixed state, following which possible New Physics
effects are taken at the levels of both the production and detection processes. We
present a number of examples of calculations with our formalism, using muons as
a source of neutrinos and different detection process. We also show the connection
between normal formulae, derived by assuming pure states, and proper results
based on the density matrix approach. The difference occurs at the second order
in parameters describing the departure from the Standard Model. Finally, as an
application of our formalism, we also demonstrate that it is possible to distinguish
Dirac and Majorana neutrinos in oscillations when New Physics scalar interactions
are present. |