Search for the neutrino radiative decay with the prototype of the Borexino detector

Results of background measurements with the prototype of the Borexino detector have been used to obtain bounds on the lifetime of the radiative neutrino decay $\nu _{H}\to \nu_{L}+\gamma$. The new lower limit for the lifetime of $pp$ and $^{7}$Be neutrinos is $\tau _{c.m.}(\nu _{H}\to \nu _{L}+\gamma )/m_{\nu }\geq 4.2\cdot 10^{3}\,{\rm s\cdot eV}^{-1}$ ($\alpha=0$). It is more than one order of magnitude stronger than obtained in previous experiments using nuclear reactors and accelerators.