Neutrino Mass and an anomaly in the tritium beta-spectrum. Possible signs of a dense neutrino cloud in the Solar System

Studying the shape of the tritium spectrum near the end point provides information about the properties of low energy neutrinos including their rest mass. The facility constructed at the Institute for Nuclear Research of the Russian Academy of Sciences, operating in Troitsk since 1994, consists of an electrostatic integral spectrometer with adiabatic magnetic collimation, matched to a window less gaseous tritium source. This apparatus, named the "Troitsk nu-mass" has measured the tritium beta-spectrum to within a few eV of the end point. This gave the lowest upper limit on the electron antineutrino mass mn < 2.5 eV/c2, but revealed a strange narrow bump 5-15 eV below the end-point with branching ratio of about 10-10 of the total decay rate. Latest measurements reveal that the position of the bump varies periodically within 5-15 eV of the end-point with a period 0.504±0.003 year. Such periodicity gives rise to the speculation that this phenomenon might originate from some object within the Solar System, which could be a dense cloud of degenerate neutrinos with Fermi energy of about 5 eV. The capture of such a neutrino produces a monochromatic electron line in the beta spectrum of tritium. Further study requires the construction of more sensitive devices, and possible new facilities are discussed.