The 1957 discovery of parity violation in the weak interactions1 swiftly brought attention to two concepts that have become ubiquitous in particle physics: Helicity is the projection of a particle’s spin along its direction of motion,and chirality is the projection of a four-component Dirac spinor onto the sum (positive chirality) or difference (negative chirality) of what in Paul Dirac’s original notation are the “large” and “small” components of the wavefunction for spin-1⁄2 particles (see box 1). Projection onto eigenstates of chirality is central to the standard model of the electroweak interactions. That’s because most weak interactions involve only the negative-chirality components of the interacting leptons and quarks.
For the spin-1⁄2 neutrino, by convention, the helicity and chiralityeigenvalues are both normalized to ±1. If the neutrino mass actually were zero, as was presumed before flavor-mixing experiments revealed otherwise in recent decades, helicity and chirality would be equal for neutrinos...
