A hypothesis on neutrino helicity

TL;DR

This paper questions the common assumption that ultrarelativistic massive fermions, like neutrinos, can be accurately described by Weyl equations, and proposes a new hypothesis that neutrinos are strictly helicity states despite having mass.

Contribution

It challenges the validity of using Weyl equations for massive fermions with arbitrary spin orientation and introduces a hypothesis that neutrinos are strictly helicity states regardless of their mass.

Findings

Weyl equations do not accurately describe fermions with transverse polarization.

Neutrinos' near 100% longitudinal polarization cannot be explained by current relativistic quantum mechanics.

A new hypothesis suggests neutrinos are strictly helicity states despite their non-zero mass.

Abstract

It is firmly established by experimental results that neutrinos are almost 100\% longitudinally polarized and left-handed. It is also confirmed by neutrino oscillation experiments that neutrinos have tiny but non-zero masses. Since their masses are non-zero the neutrinos cannot be strictly described by pure helicity states which coincide with the chirality eigenstates. On the other hand, it is generally assumed that ultrarelativistic massive fermions can be described well enough by the Weyl equations. This assumption obviously explains why the neutrinos are almost 100\% longitudinally polarized. We discuss the validity of this assumption and show that the assumption is fallacious for a fermion with a general spin orientation. For instance, a fermion with a transverse polarization (relative to its momentum) cannot be described by one of the Weyl equations even in the ultrarelativistic limit. Hence, the fact that neutrinos are almost completely longitudinally polarized cannot be explained in the basis of relativistic quantum mechanics or quantum field theory. As a solution to this problem, we propose a new hypothesis according to which neutrinos are strictly described by pure helicity states although they are not massless.