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Search for Heavy Neutrinos and Neutrino Mixing

Beside the searches for primary neutrino masses a big variety of experiments has been performed looking for neutrino mixing. Inevitable consequences of non-degenerate neutrino masses and mixing are neutrino oscillations, deviations in the spectra of weak decays, and neutrino decays. For neutrino masses below ca. 1 MeV beta-decay experiments searching for kinks in the energy spectra of the emitted electron are sensitive down to ca. . For neutrino masses above ca. 1 MeV up to 10 MeV the high neutrino flux from nuclear reactors were used to search for the neutrino decay . Most sensitive limits reached are . At higher masses the two-body kinematics in the decay or , or is used where the heavy neutrino would cause an abnormal peak in the charged lepton spectrum. In accelerator experiments again neutrino decay was investigated. Here neutrinos originate from light $\pi$ and K or the heavier D and Ds mesons produced in proton beam dumps. The LEP experiments investigating the shape of the Z-resonance and searching for neutrino decay exclude a fourth family Dirac neutrino up to a mass of 44 GeV for all mixing elements. A fourth family Majorana neutrino is excluded up to a mass of 38.2 GeV. In theories beyond the standard model isosinglet neutrinos are discussed which don't couple to the Z like ordinary neutrinos and hence are not yet excluded. Two searches for heavy isosinglet neutrinos have been performed, testing the hypothesis of mixing between isosinglet and isodoublet neutrinos. One experiment, CHARM-II, was looking for neutral current production of and its subsequent decay. Another search was done by the LEP experiments where is produced in Z boson decays. No evidence for heavy isosinglet neutrinos was found. Neutrinos which are too heavy to be directly produced at present accelerators still would affect physics at lower energies due to deviation from neutrino universality. The measured neutrino couplings for $\nu_e$$\nu_\mu$, and $\nu_\tau$ in neutral as well as charged current reactions constrain the sum of non-diagonal elements of neutrino mixing, i.e. and 
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