Constraints on neutrino - photon interactions from rare Z decays

It is shown that the rare decays Z → ν ¯ νγ and Z → ν ¯ νγγ are useful to put model-independent bounds on neutrino-one-photon and neutrino-two-photon interactions. The results are then used to constrain the τ neutrino magnetic moment µ ντ and the double radiative decay ν j → ν i γγ. It is found that the decay Z → ν ¯ νγ gives a more stringent bound on µ ντ than that obtained from Z → ν ¯ νγγ; the latter decay in turn gives limits on the neutrino-two-photon interaction that are less stringent than those obtained for a sterile neutrino ν s from the analysis of ν µ N → ν s N conversion. The behavior recently observed of atmospheric [1] and solar [2] neutrinos provides rather strong evidence that neutrinos have mass. This fact has renewed the interest in neutrino electromagnetic properties, which have received considerable attention as they may shed light on some physics issues. In particular , neutrino-one-photon interactions are of interest since they may play a key role in elucidating the solar neutrino puzzle, which can be explained in part by a large neutrino magnetic moment [3]. In the simplest extension of the standard model (SM), with the presence of massive neutrinos, one-loop radiative corrections induce a small magnetic moment proportional to the neu-trino mass m ν , i.e. µ ν = 3 e G F m ν /(8 √ 2 π 2) = 3 × 10 −19 m ν µ B [4], where m ν is to be expressed in KeV and µ B stands for the Bohr magneton. Several models have been advanced in order to induce neutrino magnetic moments as large as 10 −11-10 −10 µ B [5], even with neutrino masses compatible with the mass square differences needed by atmospheric [1], solar [2] and the liquid