QB_PH_SEC9_009: Particle Physics & Fundamental Interactions

Which landmark experiment first established the violation of parity conservation in weak interactions?

Correct Answer

### Step-by-Step Derivation & Verification: 1. **Theoretical Proposal (1956)**: Tsung-Dao Lee and Chen-Ning Yang noticed that while parity is conserved in strong and electromagnetic interactions, no experimental evidence existed for weak interactions (the $\theta-\tau$ puzzle). 2. **Wu's Experimental Execution (1957)**: - $^{60}\text{Co}$ nuclei ($J^P = 5^+$) were oriented via cryogenic adiabatic demagnetization under a strong magnetic field $\vec{B}$. - Beta decay: $^{60}\text{Co} \to ^{60}\text{Ni} + e^- + \bar{\nu}_e$. - Under parity inversion $\vec{r} \to -\vec{r}$, the nuclear spin (axial vector $\vec{J}$) does not invert, while the electron momentum (polar vector $\vec{p}_e$) inverts. - If parity were conserved, the beta emission rate in the direction along $\vec{J}$ must equal that opposite to $\vec{J}$. - The experiment revealed an overwhelming preferential emission of electrons **opposite** to the nuclear spin direction. 3. **Conclusion**: Proved maximal parity violation in weak interactions. Option (A) is correct.