Up, anti-up quark annihilation Feynman diagram

[Sonko]

Homework Statement

At the Tevatron you observe the process u ̅u → τ⁺τ⁻. Draw the lowest order Feynman diagrams for the processes involved and state which interactions are
responsible. Label all internal lines, external lines and vertices

[u = up quark, τ = tau lepton]

The Attempt at a Solution

Once i have the basic feynman diagram i can eaily label the internal/external lines and the vertices, however I'm simply having a problem identifying the diagram. From what i can think of right now i would assume the weak force (W^-) is the process involved which would be just like a time reversed tau decay, however i can't picture how the feyman diagram would look?

 

[vela]

It can't be mediated by the W because you won't be able to conserve charge at the vertices. You can rule out gluons because they don't couple to leptons, so what else is left?