= Solution
\b[The dominant interaction is weak.] In panel (iv),
$$
\tau^-\longrightarrow\nu_\tau+W^{-*},\qquad
W^{-*}\longrightarrow d\bar u\longrightarrow\pi^-.
$$
The <quark> vertex carries the <Cabibbo-Kobayashi-Maskawa matrix> factor $V_{ud}$. The <down quark> and anti-up <quark> form a <color singlet> <pion>. The <pion> bracket represents its bound-state <matrix element>, conventionally specified by the <pion decay constant>; it is not an additional weak vertex. The <neutrino> carries the initial <tau lepton> number.
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