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Abstract

Neutrinos in the early Universe decoupled essentially in helicity eigenstates. As they propagate through the Universe, their helicities could be modified via two effects. First, neutrinos with a finite magnetic moment would rotate their spins with respect to their momenta as they encounter cosmic magnetic fields, modifying their helicities. Second, the bending of neutrino's spin by a gravitational field lags the bending of its momentum, again modifying its helicity. We study both effects and investigate the implications of the helicity modification on the detection of relic neutrinos using the Inverse Tritium Beta Decay (ITBD) reaction. We find that the ITBD rate depends sensitively on the neutrino mass hierarchy and on the Dirac or Majorana nature of the neutrinos. We also discuss the concept of Macroscopic Neutrinoless Double Beta Decay using ITBD as an example. Finally, we discuss the roles of angular momentum conservation in various processes including ITBD. This talk is based on several recent papers in collaboration with Gordon Baym.