Constraints on neutrino-photon interactions from rare Z decays

Abstract

It is shown that the rare decays $Z \to ν\bar{nu} γ$ and $Z \to ν\barν γγ$ 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 and the double radiative decay $ν_j \to ν_i γγ$. It is found that the decay $Z \to ν\bar{nu} γ$ gives a more stringent bound on the $τ$ neutrino magnetic moment than that obtained from $Z \to ν\barν γγ$; 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 $ν_μN \to ν_s N$ conversion.