In Addition to Charge and Baryon Number, Another Conserved Quantity

Multiple Choice

In addition to charge and baryon number, another conserved quantity in particle interactions is lepton number. The six leptons have lepton number +1, and their antiparticles have lepton number −1. The lepton numbers for each generation (electron, muon, tau) are separately conserved. For example, an electron has electron-lepton number +1, while the electron antineutrino has electron-lepton number −1. What is the missing particle X in the following reaction? μ⁻+ p → n + X

A) e⁻
B) π⁰
C) v_e
D) v_μ
E) γ