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**Homework Statement**

Why does the ratio:

[itex]\frac{σ(e^- + e^+ \rightarrow μ^- + μ^+) }{σ(e^- + e^+ \rightarrow τ^- + τ^+)}[/itex]

tend to unity at high energies and would you expect the same for:

[itex]\frac{σ(e^- + e^+ \rightarrow μ^- + μ^+) }{σ(e^- + e^+ \rightarrow e^- + e^+)}[/itex]

**The attempt at a solution**

So I have a good idea of the first part:

An electron positron annihilation is going to lead to a photon. If the photon's energy is large compared to twice the rest mass of a tau then both channels are available and, if the mass difference between a muon and tau is small on the scale of the photon energy, approximately the same as far as the photon is concerned.

Now for the second part. As far as I can tell my argument applies just as well to say that the second ratio should also tend to unity but the way the question is phrased implies it doesn't :tongue:.

I guess this means something is wrong with my approach to justifying the first ratio tending to unity?

Thanks in advance for any help