ep - p h / 99 01 28 9 v 2 1 8 Fe b 19 99 Bound on the tau neutrino magnetic moment from the Super - Kamiokande data

It is shown that recent results from the Super-Kamiokande detector constrain the tau neutrino diagonal magnetic moment to µ ντ < 1.3 × 10 −7 µ B for the case of ν µ → ν τ interpretation of the atmospheric neutrino anomaly. It is pointed out that the large magnetic moment of the tau neutrino could affect further understanding of the origin of the anomaly. Recent results from the Super-Kamiokande (S-K) detector give evidence for neutrino oscillations [1]-[4]. One of the favourable interpretations of the atmospheric neutrino anomaly suggested by the S-K collaboration is related to the existence of ν µ → ν τ neutrino oscillations. This implies that tau neutrinos have non-zero masses and therefore may also have a non-zero diagonal magnetic moment (see e.g. ref. [5]). As a consequence, massive tau neutrinos could manifest themselves in terrestrial experiments through the effect of ν µ → ν τ neutrino oscillations , and, if the magnetic moment value is large enough, through tau neutrino electromagnetic interactions. In ref.[6] (see also [7]), it was shown that the combined existence of ν µ → ν τ (and/or ν e → ν τ) oscillations and non-zero magnetic moment of the tau neutrino would increase the total rate of events in ν µ (ν e) neutrino-electron scattering experiments. It was used to constrain the mixing angles of the tau neutrino with neutrinos of another flavour. In this Letter it is shown that this effect can also be used to constrain the magnetic moment of the tau neutrino from the S-K atmospheric neutrino data. Assuming that a muon neutrino beam has a component of tau neutrinos due to ν µ → ν τ oscillations, in the case of two-neutrino mixing, neutrino states evolve with time t as |ν(t) >= a(t)|ν µ > +b(t)|ν τ , µ ντ = 0 > (1)