Soft scalar rate of emission

Question

How does one compute the rate of emission of soft massless scalar particles with energy above a certain threshold whose interaction Lagrangian density involves only massive particles in an arbitrary process, including radiative corrections?

Comments

whose interaction Lagrangian density involves only massive particles

Doesn't it mean the massless sector of the theory completely decouples? 

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I think ruadath meant no self-action of massless particles, something like $\mathcal{L}_{\rm{int}}\propto\phi \cdot\psi^*\psi$ with completely known massive field $\psi$. If so, then the massless field equation has a known source and is solved exactly via the Green's function (kind of radiation of a classical current in QED).

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Could ruadath give some clarifications?