 Solution

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

The dominant interaction is weak. In panel (iv),

τ^{-} ⟶ ν_{τ} + W^{-*}, W^{-*} ⟶ d \overset{u}{ˉ} ⟶ π^{-} .

V_{u d}

. 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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