Parity Violation and the Weak Force

Parity was assumed exact until 1956. The evidence was overwhelming — but it all came from strong and electromagnetic processes, where parity does hold. When a puzzle in the weak decays of strange mesons refused to resolve, Lee and Yang noticed that no experiment had ever tested parity in the weak interaction at all, and proposed that it might simply not hold there. Within a year Wu's cobalt-60 experiment showed that it does not, and that the violation is not a small effect but total: the weak interaction distinguishes left from right as sharply as it possibly can. This lesson follows that discovery to its structural conclusion — that the charged weak force couples only to the left-handed chirality of matter, so that the mirror image of a weak decay is a process nature does not run.

The tau–theta puzzle

In the early 1950s two charged strange mesons appeared in cosmic rays and accelerators with, as far as could be measured, identical mass and lifetime. They were distinguished only by their decays:

Parity analysis of the final states gave opposite answers. The pion has parity $-1$; a two-pion state in its lowest configuration has parity $(-1)(-1)=+1$, while a three-pion state has parity $(-1{)}^3=-1$ (the orbital factors vanish for the dominant low angular-momentum configuration). If parity is conserved in the decays, the parent of the two-pion state has $P=+1$ and the parent of the three-pion state has $P=-1$: they must be different particles. Yet every other property matched to the precision of the day.

The dilemma was stark. Either there were two distinct particles, the $\theta$ and the $\tau$, whose equal masses and lifetimes were an unexplained coincidence, or there was a single particle — now called the $K^{+}$ — whose decays to two and three pions both occur because parity is not conserved in weak decays.¹

Lee and Yang's proposal

In 1956 T. D. Lee and C. N. Yang examined the experimental basis for parity conservation and found something remarkable: while parity was firmly established in the strong and electromagnetic interactions, there was no evidence for it in the weak interaction, because no one had looked. They proposed that the weak interaction violates parity, resolving the puzzle at a stroke — the $\theta$ and $\tau$ are one particle — and, the decisive step, they proposed concrete experiments to test it.²

The signature of parity violation is an observable that is a pseudoscalar: a quantity that changes sign under $\vec{r}\to -\vec{r}$ and whose nonzero average therefore cannot survive in a parity-symmetric world. The cleanest is the correlation between a particle's spin $\vec{J}$ (an axial vector, unchanged by parity) and a momentum $\vec{p}$ (a polar vector, reversed by parity). The dot product $\vec{J}\cdot \vec{p}$ flips sign under parity; a nonzero value for it is direct proof that parity is broken.

The Wu experiment

C. S. Wu and collaborators tested exactly this correlation in the beta decay of polarized cobalt-60,

a nucleus whose spin can be aligned. The cobalt-60 sample was cooled to about $0.01$ K and its nuclear spins aligned by an external magnetic field; the angular distribution of the emitted electrons was then measured relative to the spin direction. Parity conservation would demand a distribution symmetric under reversing the momentum — equal numbers of electrons emitted along and against the nuclear spin. Instead the electrons came out preferentially opposite to the nuclear spin.³

The asymmetry amounts to the pseudoscalar $\vec{J}\cdot \vec{p}$ acquiring a nonzero average. Its mirror image — spin reversed, electron directions unchanged — is a physically distinct process that does not occur in nature. Parity is violated in the weak interaction, and the effect vanishes as the polarization is lost when the sample warms, confirming it is genuinely the spin–momentum correlation and not an apparatus artifact.

Maximal violation, helicity, and chirality

The violation is not merely nonzero; it is maximal. The natural measure of a particle's handedness is its helicity, the projection of spin onto momentum,

which is $+1$ (right-handed) when spin and momentum are parallel and $-1$ (left-handed) when antiparallel. Analysis of weak decays shows the emitted electron is preferentially left-handed and the positron right-handed, with the preference approaching totality in the relativistic limit.

Helicity, however, is frame-dependent for a massive particle: a fast observer who overtakes the particle sees its momentum reverse while its spin does not, flipping the sign of $h$. The frame-independent quantity the weak interaction actually couples to is chirality, the eigenvalue of the projector $\frac{1}{2}(1\mp {\gamma }^5)$ that splits a Dirac field into left- and right-handed components. For a massless particle chirality and helicity coincide; for a massive one they align in the relativistic limit, which is why the observed electron helicity approaches $-1$ as its energy grows. The distinction becomes essential in the weak-interaction module, where the $V-A$ current is built from the left-chiral projector; here it suffices that the charged weak force reaches only the left-handed chirality of a particle and the right-handed chirality of an antiparticle.

Left-handed doublets, right-handed singlets

The structural consequence organizes every fermion in the Standard Model. The charged weak current pairs the members of each generation — it turns an electron into its neutrino, an up quark into a down quark — but only for the left-handed chirality. The left-handed fields are therefore grouped into weak-isospin doublets on which the $W$ boson acts, while the right-handed fields, which the charged weak force never touches, sit as singlets:

This left–right asymmetry is not a small correction bolted onto a symmetric theory; it is the defining feature of the weak interaction and the reason the Standard Model must be written with left- and right-handed fields treated differently from the start.

The handedness of the neutrino

The sharpest demonstration is the neutrino itself. Because the neutrino interacts only weakly, its helicity is a direct readout of the weak coupling. In 1958 Goldhaber, Grodzins, and Sunyar measured it in a single ingenious experiment: an electron capture in europium-152 producing a samarium nucleus and a neutrino, followed by a photon whose circular polarization — measured by resonant scattering — was correlated through angular-momentum conservation with the neutrino's helicity. The result was unambiguous: the neutrino is left-handed, helicity $-1$, within experimental error.⁴

The antineutrino, correspondingly, is right-handed. In the massless approximation in which the neutrino was long treated, this means only left-handed neutrinos and right-handed antineutrinos exist at all — the other two states simply do not couple to anything and were, in the original Standard Model, omitted entirely. That the neutrino in fact has a small mass, and so cannot be purely one helicity, is the first crack in this picture and the seed of neutrino oscillations, taken up in a later module. But the handedness the weak force selects — left for particles, right for antiparticles — is exact, and it is the permanent legacy of the parity violation that Wu discovered.