On phase-space integrals with Heaviside functions

TL;DR

This paper addresses the challenge of computing phase-space integrals involving Heaviside functions in QCD, proposing a method to evaluate the zero-jettiness soft function at high perturbative orders.

Contribution

It introduces a novel approach to handle Heaviside functions in phase-space integrals, enabling calculations of the zero-jettiness soft function at NNLO and N3LO.

Findings

Successfully computed contributions to the zero-jettiness soft function at NNLO and N3LO.

Demonstrated the effectiveness of the proposed method in complex phase-space integrals.

Provided insights into the peculiarities of real-emission contributions with Heaviside functions.

Abstract

We discuss peculiarities that arise in the computation of real-emission contributions to observables that contain Heaviside functions. A prominent example of such a case is the zero-jettiness soft function in SCET, whose calculation at next-to-next-to-next-to-leading order in perturbative QCD is an interesting problem. Since the zero-jettiness soft function distinguishes between emissions into different hemispheres, its definition involves $\theta$-functions of light-cone components of emitted soft partons. This prevents a direct use of multi-loop methods, based on reverse unitarity, for computing the zero-jettiness soft function in high orders of perturbation theory. We propose a way to bypass this problem and illustrate its effectiveness by computing various non-trivial contrbutions to the zero-jettiness soft function at NNLO and N3LO in perturbative QCD.