Exam questions usually give an interaction and ask whether it is possible, or ask you to identify a missing particle. Use the same method every time.Step 1: write the charge, baryon number and lepton number under every particle.
Step 2: total each quantity on both sides.
Step 3: if any total differs, the interaction is impossible; name the law that is broken.
Step 4: for a decay, check energy too: the products must have less total rest mass than the parent.Worked example 1: p + π− → n + π0Q: (+1) + (−1) = 0 → 0 + 0 = 0 ✓ B: 1 + 0 = 1 → 1 + 0 = 1 ✓ L: 0 → 0 ✓ All conserved, so the interaction is possible.Worked example 2: p + p → p + π+Q: 2 → 2 ✓ B: 2 → 1 ✗ Baryon number is not conserved, so this cannot happen.Missing particles: find the Q, B and L the unknown needs to balance each law, then match a particle. In n → p + e− + X, X must have Q = 0, B = 0 and L = −1, so it is an electron antineutrino.Quark descriptions also help: in β− decay a down quark changes into an up quark, and in β+ decay an up quark changes into a down quark.
Step 2: total each quantity on both sides.
Step 3: if any total differs, the interaction is impossible; name the law that is broken.
Step 4: for a decay, check energy too: the products must have less total rest mass than the parent.Worked example 1: p + π− → n + π0