parity violation in the weak interaction
Parity is the symmetry between left and right — the idea that the laws of physics should look exactly the same in a mirror. For a long time everyone assumed this was obviously true: a process and its mirror image should be equally allowed. The weak force shattered that assumption. It is the one fundamental interaction that can tell left from right, and it does so dramatically.
The cleanest way to see it involves a particle's 'handedness', or chirality — loosely, whether it spins clockwise or anticlockwise relative to the way it is moving. The astonishing fact is that the W boson couples only to left-handed particles (and right-handed antiparticles). A right-handed electron simply does not feel the charged weak force at all. This is not a small bias; it is a total preference. So if you filmed a weak decay and watched the mirror image, you would see something nature never actually does, because the mirror flips handedness.
This was discovered in 1956-57 when Chien-Shiung Wu observed that electrons from the beta decay of cobalt-60 nuclei flew out preferentially in one direction relative to the nuclear spin — a direction that reverses in a mirror. It was a shock to the entire physics community and won a swift Nobel Prize (for the theorists who proposed the test). Parity violation is now built into the heart of the Standard Model: the weak force is intrinsically left-handed, which is one of the deepest and strangest facts we know about nature.
In Wu's experiment, cobalt-60 nuclei were aligned by a magnetic field, and the beta-decay electrons came out preferentially against the nuclear spin. In a mirror the spin would appear unchanged but the electrons would fly the opposite way — a difference real nature shows, proving left and right are not equivalent here.
Wu's cobalt-60 experiment caught the weak force red-handed telling left from right.
Parity violation is maximal for the charged weak force, not a tiny effect; the W boson ignores right-handed particles entirely, which is why the weak force is described as fundamentally left-handed.