Causality and loop-tree duality at higher loops

Understand

We relate a $l$-loop Feynman integral to a sum of phase space integrals, where the integrands are determined by the spanning trees of the original $l$-loop graph.

• Causality requires that the propagators of the trees have a modified $i\delta$-prescription and we present a simple formula for the correct $i\delta$-prescription.