Double Beta Decay Constraint on Composite Neutrinos

Neutrinoless double beta decay occurs through the magnetic coupling of dimension five, $\lambda_W/m_{\nu*}$, among the excited electron neutrino $\nu^*$, electron and $W$ boson if $\nu^*$ is a massive Majorana neutrino. (\lambda_W m_W / m_{\nu*}^2 |\frac{(m_*/m_W) +2}{(m_*/m_W +1)^2} -0.129{m_*/m_W} | <2.13\times 10^{-2}, where $\lambda_W$ is the relative strength, $m_{\nu*}$ is the composite scale and $m_*$ is the mass of excited neutrino. If the coupling is not small, i.e., $\lambda_W>1$ and $m_{\nu*}=m_*$, we find $m_*>3.4 m_W$.