  We propose a new experiment to search for a time-reversal (T) symmetry
breaking process in muon decay and the Majoranality of the neutrinos. In the
presence of V+A interactions, the Majoranality appears as a T-violating term in
the muon decay width as shown by Doi et al, while in the Standard Model such a
T-violating term is negligibly small. The presences of V+A interactions and the
corresponding heavy right-handed Majorana neutrinos give us an important clue
to solve two major issues in particle physics, the deficit of baryon asymmetry
in the universe and the Majoranality of neutrinos. In the experiment, the
polarization of positrons from muon decays is measured using a polarimeter
consisting of a magnetized foil and a segmented calorimeter. According to our
result of numerical calculation, a factor of ten improvement in sensitivity to
the T-violating process is expected by a year of measurement at J-PARC
Materials and Life Science Experimental Facility, compared to the most recent
precursor experiment.
