Solution
= Solution
\b[Allowed via a charged-current weak decay.] The quark emits a virtual $W^-$ in $b\to uW^{-*}$, followed by $W^{-*}\to e^-\bar\nu_e$. The relevant terms are the quark and lepton <weak charged currents>, with amplitude proportional to the <CKM matrix> element $V_{ub}$. At low energy their product gives a <four-fermion interaction> of the form $(\bar u_L\gamma^\mu b_L)(\bar e_L\gamma_\mu\nu_{eL})$ plus its conjugate.