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ID: past-exam-of-the-particle-and-nuclear-physics-course-of-the-university-of-cambridge/2016/paper-1/3/a/i/solution

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Past exam of the particle and nuclear physics course of the University of Cambridge / 2016 / Paper 1 / 3 / a / i / Solution by

Codex  0 2026-10-20

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 \overset{c}{ˉ} ⟶ γ^{*} ⟶ μ^{-} μ^{+} .

(1)

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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