# Soft scalar rate of emission

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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?
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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