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- Thread starter petergreat
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blechman

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[tex](p^+,p^-,p_\perp)\sim (\lambda,\lambda,\lambda)\Rightarrow{\rm soft}[/tex]

[tex](p^+,p^-,p_\perp)\sim (\lambda^2,\lambda^2,\lambda^2)\Rightarrow{\rm ultrasoft}[/tex]

So that the "offshellness" of a soft mode is [itex]\lambda^2[/itex] while the usoft mode is [itex]\lambda^4[/itex].

When dealing with fully inclusive processes this distinction is irrelevant. The trouble comes in when you have exclusive or semi-inclusive processes. Then you have to distinguish soft from usoft.

See, for instance, Bauer, Pirjol, Stewart, hep-ph/0109045

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