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Electromagnetic Properties

A neutrino electromagnetic dipole or transition moment would lead to an enhanced rate for the plasma decay process $\gamma\to\nu\bar\nu'$ which dominates the neutrino cooling of low-mass stars, notably of low-mass red giants before the helium flash. This would delay helium ignition, in contradiction with globular-cluster data, leading to
 \begin{displaymath}\mu_\nu\leq 3\times10^{-12}\,\mu_{\rm B}\end{displaymath} (1.7)
 

with $\mu_{\rm B}=e/2m_e$ the Bohr magneton. This limit applies if $m_\nu$$m_{\nu'}\leq 5\,\rm keV$ where threshold effects become important. Transition moments can be constrained by searching for radiative decays $\nu\to\nu'\gamma$. If $\nu'$ is essentially massless the decay rate is $\mu_\nu^2 m_\nu^3/8\pi$ with $\mu_\nu$ the transition moment. If neutrinos obey Eq. ([*]) the phase-space factor $m_\nu^3$ renders the radiative decay limits from reactor, accelerator, solar, supernova, and cosmic background neutrinos less restrictive than Eq. ([*]).


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