= Solution
\b[The dominant interaction is electromagnetic.] The <color singlet> charm-anticharm component of the <J psi meson> annihilates through a virtual <photon>, which creates a <muon> and an <antimuon>:
$$
c\bar c\longrightarrow\gamma^*\longrightarrow\mu^-\mu^+.
$$
Panel (i) shows the two electromagnetic vertices. Vector quantum numbers allow the one-photon channel. A <Z boson> contribution is negligible at this <mass> scale; a single <gluon> cannot connect a <color singlet> initial state to colorless <leptons>. The dominant interaction for this specified final state need not dominate the <meson>'s total <decay width>.
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