  In this article we consider a mathematical model for the weak decay of muons
in a uniform magnetic field according to the Fermi theory of weak interactions
with V-A coupling. With this model we associate a Hamil-tonian with cutoffs in
an appropriate Fock space. No infrared regularization is assumed. The
Hamiltonian is self-adjoint and has a unique ground state. We specify the
essential spectrum and prove the existence of asymptotic fields from which we
determine the absolutely continuous spectrum. The coupling constant is supposed
sufficiently small.
