Constraints on neutrino-photon interactions from rare Z decays

It is shown that the rare decays $Z\to \nu \overline{nu}\gamma$ and $Z \to \nu \bar{\nu} \gamma \gamma$ are useful to put model-independent bounds on neutrino-one-photon and neutrino-two-photon interactions. The results are then used to constrain the $\tau$ neutrino magnetic moment and the double radiative decay ${\nu }_j\to {\nu }_i\gamma \gamma$. It is found that the decay $Z \to \nu \bar{nu} \gamma$givesamorestringentboundonthe$\tau$ neutrino magnetic moment than that obtained from $Z\to \nu \bar{\nu }\gamma \gamma$; the latter decay in turn gives limits on the neutrino-two-photon interaction that are less stringent than those obtained for a sterile neutrino ${\nu }_s$ from the analysis of ${\nu }_{\mu }N\to {\nu }_{s}N$ conversion.