[Physics] The electron and neutrino spin


Can you please clarify some basic notions about spin?

Can you please explain what is intended when they say that the spin of a neutrino is left-handed and equal to -1/2?

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Does it mean its angular momentum is equal in direction and strength to the spin of the electron ( $h /4\pi$), even though its mass is about one million times smaller? If so, is there an explanation for that? Is there any experimental direct evidence of that magnitude?

Also, since the electron g-factor is 2, does that mean that its value is $h /2 \pi$ , equal to the orbital angular momentum in the hydrogen atom?

Best Answer

The spin of a fundamental fermionic particle always has the absolute value $\frac{\hbar}{2}$. This does not relate to its orbital angular momentum, it's just what the spin of a fundamental fermion is.

It now turns out that, for massless particles, there is the notion of helicity (see also What is polarisation, spin, helicity, chirality and parity?), which is the projection (i.e. the relative direction) of spin and momentum.

In your picture, someone decided to orient the axis along which one component of the spin is measured parallel to the particles momentum, so that $+1/2$ spin corresponds to the spin being parallel to the linear momentum, and $-1/2$ to them being anti-parallel. In situations where "neutrinos are massless" is a good approximation, this picture is Lorentz-invariant and indeed a good description for the otherwise rather technical notion of chirality.

The electron cannot correspond to an anti-neutrino because the latter has zero electrical charge. Spin is only one of many quantum numbers.