Two-Loop Threshold Singularities, Unstable Particles and Complex Masses

TL;DR

This paper investigates how threshold singularities from unstable particles affect two-loop calculations in particle physics and demonstrates how the complex-mass scheme can mitigate these effects, with implications for Higgs processes at colliders.

Contribution

It introduces a method to handle threshold singularities in two-loop computations using the complex-mass scheme, improving accuracy in electroweak corrections.

Findings

Threshold singularities significantly impact two-loop observables.

The complex-mass scheme effectively cures threshold singularities.

Electroweak corrections can reach 10% near the WW threshold.

Abstract

The effect of threshold singularities induced by unstable particles on two-loop observables is investigated and it is shown how to cure them working in the complex-mass scheme. The impact on radiative corrections around thresholds is thoroughly analyzed and shown to be relevant for two selected LHC and ILC applications: Higgs production via gluon fusion and decay into two photons at two loops in the Standard Model. Concerning Higgs production, it is essential to understand possible sources of large corrections in addition to the well-known QCD effects. It is shown that NLO electroweak corrections can incongruently reach a 10 % level around the WW vector-boson threshold without a complete implementation of the complex-mass scheme in the two-loop calculation.