Closing the Windows on Mev Tau Neutrinos

Abstract

In this note, we analyze various constraints on the ``visible'' decay modes of a massive $\tau$ neutrino, ${\nu }_{\tau }\to {\nu }^{\prime }\gamma$ and ${\nu }_{\tau }\to {\nu }^{\prime }{e}^{+}{e}^{-}$, where ${\nu }^{\prime }$ is a light neutrino. The BEBC beam dump experiment provides model-independent constraints on these modes. The lifetime for the ${\nu }^{\prime }{e}^{+}{e}^{-}$ mode is constrained to be ${\tau }_{{\nu }^{\prime }{e}^{+}{e}^{-}}\ge 0.18(m_{{\nu }_{\tau }}/MeV)sec.$ We point out that the same experiment implies a similar constraint on the ${\nu }^{\prime }\gamma$ mode. This results in a new upper limit on the transition magnetic moment of ${\nu }_{\tau }$, ${\mu }_{\mathrm{tran}}\le 1.1\times 10^{-9}(MeV/m_{{\nu }_{\tau }}{)}^2{\mu }_B$. Furthermore, a limit on the electric charge of ${\nu }_{\tau }$ may be obtained, $Q_{{\nu }_{\tau }}\le 4\times 10^{-4}e$. Combining these constraints with those arising from supernova observations and primordial nucleosynthesis calculations, we show that these ``visible'' decays cannot be the dominant decay modes of the $\tau$ neutrino.

Comments: 8 pgs. LaTeX (1 uuencoded fig., also available on request), Bartol-930XXX, JHU-TIPAC-930026, UM-TH-93-26