Charged pion emission in weak nuclear decay conserves lepton number via neutrino Steps: - Identify the decay type: Emission of a charged π meson (π⁺ or π⁻) from a nucleus occurs via the weak interaction. - Apply conservation laws: Weak decays must conserve charge, baryon number, lepton number, and angular momentum. - Analyze pion properties: A charged pion carries no lepton number but decays weakly, requiring a lepton (muon) and antilepton (neutrino or antineutrino) pair. - Conclude necessity: For the nuclear process to balance, a neutrino accompanies the pion emission directly from the nucleus to satisfy lepton conservation. Why D is correct: - In weak decays producing charged pions, a neutrino is emitted to conserve lepton number (lepton number change of zero overall). Why the others are wrong: - A: Antineutrino applies to π⁻ decay (π⁻ → μ⁻ + \bar{ν}_μ), but π⁺ requires neutrino; question implies general case favoring neutrino. - B: π particle is already emitted; no second pion needed. - C: α particle (helium nucleus) is unrelated to weak pion emission; it conserves baryon number differently in alpha …