Helicita

Helicita, a curly-haired spiral-weaver, draws a twisting violet ribbon of light.

The Physics

In 1958 Maurice Goldhaber, Lee Grodzins and Andrew Sunyar measured the handedness of the neutrino (“Helicity of Neutrinos”, Phys. Rev. 109, 1015). Europium-152m captures an electron and emits a neutrino; the daughter nucleus recoils and then emits a gamma ray. Selecting gammas emitted against the recoil and measuring their circular polarisation with magnetised iron tied the neutrino's spin to its motion. Neutrinos turned out left-handed: spin opposite to motion.

h = −1

• h: helicity, the spin component along the direction of motion, in units of the maximum

The result fits the V−A theory of the weak force, which acts only on left-handed particles, part of the story of parity violation after Wu's 1957 experiment.

Honest caveat: Helicity −1 is exact only for massless neutrinos; with their tiny masses it is very nearly −1 at the energies measured.

On the sticker

Helicita holds up a glowing violet spiral ribbon beside her. Printed text: HELICITA with the line h = −1. Pixel-art kiss-cut vinyl sticker, white, in 2″, 3″, 4″ and 6″ sizes.