Helicity

(quantum mechanics) The component of the spin of a particle along its momentum.

A fundamental quantized variable used in quantum mechanics to specify the relative orientations of spin and linear momentum of massless particles. It is a requirement of fundamental Dirac quantum mechanics that such particles have their spins aligned either parallel or antiparallel to their linear momentum. Particles having parallel alignment are arbitrarily assigned helicity +1; those having antiparallel alignment, −1. See also Momentum; Spin (quantum mechanics).

In a classic experiment on K electron capture by ¹⁵²Eu, M. Goldhaber, L. Grodzins, and A. Sunyar first showed that the neutrino emitted in the weak nuclear interaction had negative helicity—that its spin was aligned antiparallel to its momentum. An equivalent description of this situation is that these neutrinos are left-handed. Symmetry requires that antineutrinos be right-handed and have positive helicity. See also Electron capture; Elementary particle; Neutrino; Quantum mechanics; Symmetry laws (physics).