Non-factorisable electroweak virtual corrections to single-resonant processes

TL;DR

This paper calculates non-factorisable electroweak virtual corrections for fermion scattering processes involving a resonance, revealing their structure and iterative nature at two-loop order.

Contribution

It provides an explicit two-loop computation showing non-factorisable corrections can be expressed as an iteration of one-loop results plus a new fermion loop contribution.

Findings

Non-factorisable corrections can be written as an iteration of one-loop results.

The two-loop correction includes a new contribution from light fermion loops.

The results are valid in dimensional regularisation and specific to single resonance exchange.

Abstract

We consider electroweak (EW) virtual corrections to $2\to 2$ fermion scattering processes mediated by a vector boson $V$ ($V=W^\pm,Z$) in the pole approximation. As is well known, the computation can be organised into factorisable and non-factorisable contributions. The factorisable corrections can be computed by evaluating the (polarised) EW form factor of the vector boson at the relevant perturbative order. The non-factorisable corrections are instead driven by soft-photon exchanges between the initial- and final-state fermions and/or the resonance. We perform an explicit two-loop computation to show that, once the heavy degrees of freedom are properly decoupled, such non-factorisable corrections can be expressed as an iteration of the one-loop result, plus a new contribution due to (light) fermion loops. The final two-loop result, which can be expected on general grounds from soft-photon factorisation, is shown to hold exactly in dimensional regularisation and is peculiar to the exchange of a single resonance. We discuss its extension to all perturbative orders.