Wu’s Experiment & Parity Violation

Wu’s experiment is a renowned experiment in particle physics that confirmed parity violation in weak interactions.

Prior to the 1950s, most physicists regarded the ambidexterity of nature as self-evident: the mirror image of any physical process must also be a perfectly possible physical process.

In 1956, scrounging the literature for tests of this assumption, Lee and Yang found that although parity invariance was supported by ample evidence for strong and electromagnetic interactions, there was no experimental test to confirm it for weak interactions.

They proposed an experiment which was carried out later that year by C.S. Wu. 

She aligned the Cobalt 60 nuclei in the direction of the applied magnetic field, say, the z direction. Wu recorded the direction of the emitted electron and found that most of the electrons were emitted in the z direction i.e. parallel to the magnetic field.

$\begin{equation}\ce{^{60}_{27}Co}\rightarrow \ce{^{60}_{28}Ni} + e^{-} + \bar{\nu}_{e} + 2\gamma\end{equation}$

This simple finding had monumental implications. If we were to look at the mirror image of this process, the spin would reverse direction i.e. -z, but the electrons would still be emitted in the z-direction, implying that this process was not invariant under parity. This then is a process whose mirror image does not exist in nature. This finding was not specific to Cobalt 60, and is in fact a signature of weak decays. 

Parity Violation and Neutrinos

Parity violation is most dramatically evident in the behavior of the neutrino. In quantum mechanics, the axis of quantization is, by convention, the z-axis. Consider a particle moving in the z direction with velocity $\textbf{v}$.

• Helicity

The helicity of a particle is $m_{s}/s$ for this axis. The particle with spin $1/2$ can have a helicity of $+1(m_{s} = +1/2)$ or $-1(m_{s} = -1/2)$. The former is called “right-handed”, and the latter is called “left-handed”.

Suppose we have a right-handed electron traveling in the z-direction, and someone else is observing it from an inertial frame that is moving with a greater velocity in the z-direction.

To the observer, the electron would seem to be moving in -z-direction and thus the electron will be left-handed. Put simply, we can change the handedness of an electron by shifting to a faster-moving frame. 

However, in the case of a neutrino, which travels with the velocity of light, the observer cannot go faster than the neutrino, and so it will always appear to be moving in the z-direction. Evidently, all neutrinos in nature are left-handed, and all antineutrinos are right-handed.