O ct 1 99 6 A demonstration that the observed neutrinos are not Majorana particles

A demonstration that the observed neutrinos are not Majorana particles Abstract It is shown that Majorana neutrinos cannot couple vectorially to the neutral-current SU(2) L x U(1) gauge field of the standard model. The absence of physics beyond the standard model (in particular Higgs-field configurations which lead to Majorana mass terms) is not assumed for this argument. Since the existence of such a vector coupling was recently experimentally shown in neutral current reactions by the Charm II collaboration, it is unlikely that the observed neutrinos are predominantly Majorana particles. The answer to the question of whether the neutrino is a Dirac or a Majorana particle is considered to be one of the most important clues to physics beyond the standard model. If the neutrino were a " Majorana particle " (i.e. a particle identical to its antiparticle[1]), the so called " seesaw " mechanism could naturally explain the smallness of neutrino masses, which remains puzzling within the standard model (see ref.[2] for a recent review on neu-trino masses). The " seesaw " mechanism requires the existence of Higgs-field configurations beyond the one of the standard model, which makes the prospect of experimentally proving the Majorana nature of neutrinos (e.g. via neutrinoless double beta-decay experiments) very attractive. In this letter I present the solution to this long-standing problem, concluding that it is unlikely that the neutrino species observed up to now are predominantly Majorana particles. The basis of this demonstration will be to accept that experimentally well confirmed parts of the standard model are at least approximately valid for Majorana neutrinos. The existence of other new fields or interactions beyond the standard model do not invalidate my argument. In particular Higgs-field configurations leading to Majorana masses are not excluded by assumption. For the sake of conceptual clarity I will confine the demonstration to purely neutral-current reactions. The " modern " choice for the metric of the 4-vectors (defined e.g. in the textbooks of