A muon decays via the weak interaction to produce an electron, an electron antineutrino, and an unknown particle X\text{X}X:
μ−→e−+νˉe+X \mu^- \rightarrow \text{e}^- + \bar{\nu}_e + \text{X} μ−→e−+νˉe+XWhat is particle X\text{X}X?
νe\nu_eνe
νˉμ\bar{\nu}_\muνˉμ
νμ\nu_\muνμ
γ\gammaγ