GATE 2026 PH – Question 23

Correct answer: (A) $\pi^-+p\to\pi^0+n$

Explanation

Check each process against the conservation laws (charge, baryon number, lepton number, strangeness for strong interactions).

• **A. $\pi^-+p\to\pi^0+n$:** charge $-1+1=0\to0+0$ ✓; baryon number $1\to1$ ✓; strangeness 0 on both sides ✓. This is charge-exchange scattering, a strong process. **Allowed.** ✓
• **B. $\pi^0\to\gamma+\gamma+\gamma$:** forbidden by **charge-conjugation (C) invariance** of the electromagnetic interaction. $\pi^0$ has $C=+1$ and each photon has $C=-1$, so three photons have $C=-1$. ✗
• **C. $p+\bar p\to\Lambda^0+\Lambda^0$:** baryon number $1+(-1)=0$ on the left, but $1+1=2$ on the right. ✗
• **D. $p+\bar p\to\gamma$:** a single photon cannot conserve energy and momentum in the centre-of-mass frame (the photon would need to be massive at rest). ✗

Answer **A**.