Two loop correction to interference in $gg \to ZZ$

TL;DR

This paper calculates the two-loop quantum chromodynamics corrections to the interference between Higgs-mediated and continuum Z boson pair production via gluon fusion, including top quark mass effects, to improve precision in theoretical predictions.

Contribution

It provides the first detailed analysis of two-loop interference effects in gg → ZZ, incorporating top quark mass effects and assessing the heavy quark expansion validity.

Findings

Two-loop corrections to gg → ZZ interference are computed.

Heavy top quark expansion is validated against exact results.

NLO corrections including real and virtual radiation are provided.

Abstract

We present results for the production of a pair of on-shell Z bosons via gluon fusion. This process occurs both through the production and decay of the Higgs boson, and through continuum production where the Z boson couples to a loop of massless quarks or to a massive quark. We calculate the interference of the two processes and its contribution to the cross section up to and including order O(alpha_s^3). The two-loop contributions to the amplitude are all known analytically, except for the continuum production through loops of top quarks of mass m. The latter contribution is important for the invariant mass of the two Z bosons, (as measured by the mass of their leptonic decay products, m_4l), in a regime where m_4l > 2m because of the contributions of longitudinal bosons. We examine all the contributions to the virtual amplitude involving top quarks, as expansions about the heavy top quark limit. Comparison with the analytic results, where known, allows us to assess the validity of the heavy quark expansion, and it extensions. We give results for the NLO corrections to this interference, including both real and virtual radiation.