The Imaginary Mass of the Neutrino
  This paper proposes that the elusive neutrino is a space-like unitary irreducible representation of the Poincare Group. Although its imaginary mass is not a physical observable, its square is. This amount is found experimentally to be negative. Therefore, the neutrino is a tachyon with a measurable property; the square of its mass. This paper does not resolve the problem of how the neutrino propagates in the space. Further studies are necessary to address this question. 
Jose N. Pecina-Cruz