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I've been stumbeling recently about new unitarity methods to obtain one-loop amplitudes by cutting them in all possible channels thereby reducing the full amplitude to products of tree amplitudes (pioneered by Bern, Dixon, Kosower, Dunbar).

From what I understand from my QFT classes cutting a diagram will give you the imaginary part of the amplitude and by dispersion relations one will get the full amplitude. But why don't these guys need to do the dispersion integrals? From what I understand they get the full amplitude through these cuts from the start...no detour via imaginary parts and doing dispersion integrals...

How is this possible when both methods rely on simple unitarity? Where does the difference lie? I don't get it and would be happy if someone could enlighten me!

Thanks

earth2

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# Old vs new unitarity cut methods

Can you offer guidance or do you also need help?

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