Charting the Higgs self-coupling boundaries

TL;DR

This paper explores the potential for Higgs self-coupling measurements at colliders to reveal new physics, even if other Higgs measurements align with the Standard Model, by deriving bounds on possible deviations.

Contribution

It provides theoretical bounds on Higgs self-coupling deviations in UV-complete models without fine-tuning, highlighting the discovery potential of future collider measurements.

Findings

Higgs self-coupling deviations can exceed 200% without fine-tuning.

Self-coupling measurements probe uncharted parameter space.

Potential for discovery even if other measurements show minimal deviations.

Abstract

Could new physics first manifest itself in Higgs self-coupling measurements? In other words, how large could deviations in the Higgs self-coupling be, if other Higgs and electroweak measurements are compatible with Standard Model predictions? Using theoretical arguments supported by concrete models we derive a bound on the ratio of self-coupling to single-Higgs coupling deviations in ultraviolet completions of the Standard Model where parameters are not fine-tuned. Broadly speaking, a one-loop hierarchy is allowed. We thus stress that self-coupling measurements at the LHC and future colliders probe uncharted parameter space, presenting discovery potential even in the absence of emerging hints in single-Higgs coupling measurements. For instance, if other observables show less than two-sigma deviations by the end of the LHC programme, the Higgs self-coupling deviations could still exceed 200% in the models discussed, without introducing fine-tuning of ultraviolet parameters.