QB_PH_SEC9_027: Particle Physics & Fundamental Interactions

In the Standard Model, which gauge boson mediates the neutral-current weak interaction?

Correct Answer

### Step-by-Step Derivation & Verification: 1. **Electroweak Gauge Bosons ($SU(2)_L \times U(1)_Y$)**: Spontaneous symmetry breaking via the Higgs mechanism yields three massive gauge bosons: - $W^+, W^-$: Masses $\approx 80.4\text{ GeV}/c^2$ (mediate charged currents $\Delta Q = \pm 1$) - $Z^0$: Mass $\approx 91.2\text{ GeV}/c^2$ (mediates neutral currents $\Delta Q = 0$) 2. **Neutral Current Reactions**: Discovered in 1973 in the Gargamelle bubble chamber (e.g., $\nu_\mu + e^- \to \nu_\mu + e^-$ via $Z^0$ exchange). 3. **Conclusion**: $Z^0$ boson. Option (A) is correct.