Authentic Majorana versus singlet Dirac neutrino contributions to μ + μ +→* l + l + ( l =e,τ) transitions

We revisited the computation of the cross section for the mu (+) mu (+)->* l (+) l (+) (l (+) = tau(+), e( +)) transitions in the presence of new heavy Majorana neutrinos. We focus on scenarios where the masses of the new states are around some few TeV, the so-called low-scale seesaw models. Our analysis is set in a simple model with only two new heavy Majorana neutrinos considering the current limits on the heavy-light mixings. Our results illustrate the difference between the genuine effects of two nondegenerate heavy Majorana states and the degenerate case where they form a Dirac singlet field.