Resolving gluon fusion loops at current and future hadron colliders

TL;DR

This paper investigates methods to distinguish long-distance top quark effects from potential new physics in gluon fusion loops at colliders, using EFT and various production channels, with implications for future collider experiments.

Contribution

It compares proposed techniques to resolve gluon fusion loop degeneracies and assesses EFT validity through explicit model comparisons, including top-Z coupling modifications.

Findings

Off-shell Higgs production is sensitive to top-Z coupling modifications.

Projected collider sensitivities are comparable between off-shell Higgs and $t\bar{t}Z$ processes.

EFT validity is explicitly tested with a vector-like quark extension.

Abstract

Inclusive Higgs measurements at the LHC have limited resolution on the gluon fusion loops, being unable to distinguish the long-distance contributions mediated by the top quark from possible short-distance new physics effects. Using an Effective Field Theory (EFT) approach we compare several proposed methods to lift this degeneracy, including $t\bar{t}h$ and boosted, off-shell and double Higgs production, and perform detailed projections to the High-Luminosity LHC and a future hadron collider. In addition, we revisit off-shell Higgs production. Firstly, we point out its sensitivity to modifications of the top-$Z$ couplings, and by means of a general analysis we show that the reach is comparable to that of tree-level processes such as $t\bar{t}Z$ production. Implications for composite Higgs models are also discussed. Secondly, we assess the regime of validity of the EFT, performing an explicit comparison for a simple extension of the Standard Model containing one vector-like quark.