= Solution
\b[The decay is allowed in the <Standard Model>.] Begin with ordinary <muon decay>, $\mu^-\to\nu_\mu W^{-*}$, $W^{-*}\to e^-\bar\nu_e$. The outgoing <electron> can emit a virtual <photon> that converts to $e^-e^+$. The bottom panel of the figure in part (b) shows one representative diagram. Radiation from other charged lines and exchange of the identical final <electrons> complete the <decay amplitude>.
The final total <electric charge> is $-1$. Its electron-family <lepton number> is $1+1-1-1=0$, and its muon-family <lepton number> is $1$, matching the initial <muon>. The extra pair therefore violates no relevant <Standard Model> selection rule. It is also kinematically allowed since $m_\mu>3m_e$. The virtual pair costs two electromagnetic vertices: the <decay amplitude> is order $G_Fe^2$, and the <decay rate> is suppressed by roughly two powers of the <fine-structure constant>, with additional <phase space> and logarithmic factors.
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