Landau, L.D.; On one Possibility for Polarization Properties of the Neutrino
Zh. Eksp. Teor. Fiz. 32 (1957) 407;
JETP 5 (1957) 337;

Reprinted in The Development of Weak Interaction Theory, editor P. K. Kabir, Gordon and Breach Science Publishers, New York - London, (1963) 67.

Motivation
If the law of conservation of parity is abandoned, then new properties of the neutrino become possible. In the case of zero mass, the Dirac equation separates into two uncoupled pairs of equations. In the usual theory it is impossible to restrict attention to one pair of equations, since the two pairs are interchanged by a space-inversion. But if we require only invariance under combined inversion (L. D. Landau, J. Exp. Theor. Phys. (USSR) 32 (1957) 405), then we can suppose that the neutrino
is described by a single pair of equations. In ordinary language, this implies that the neutrino is always polarized along (or always opposite to) the direction of its motion. The antineutrino is then polarized always in the opposite sense. In this scheme the neutrino is not a truly neutral particle, in agreement with the observed absence of double -decay and especially with the experiments on induced -decay. We call
this kind of neutrino a longitudinally polarized neutrino, or a longitudinal neutrino for short. In the usual theory the neutrino mass is zero "accidentally.'' And if one takes into account the neutrino interactions, a non-zero rest-mass appear automatically, although it is of negligible magnitude. The mass of a longitudinal neutrino is automatically zero, and this fact is not disturbed by an interactions. If we assume the neutrino to be longitudinal, the number of possible types of
weak interaction operator is greatly reduced. (Extracted from the introductory part of the paper.).

Related references See also L. D. Landau, Zh. Eksp. Teor. Fiz. 32 (1957) 405;
L. Michel, Proc. Roy. Soc. A63 (1950) 514;
T. D. Lee and C. N. Yang, Phys. Rev. 104 (1956) 254;
L. Michel, Proc. Roy. Soc. A63 (1950) 1371;
Analyse data from A. Bonetti et al., Nuovo Cim. 3 (1956) 33;
C. P. Sargent, M. C. Rinehart, L. M. Lederman, and K. C. Rogers, Phys. Rev. 99 (1955) 885;

Particles studied

qn, mass

qn, mass

Record comments
Suggestion of the two-component theory for the neutrino.