Higgs Couplings at High Scales

TL;DR

This paper investigates high-energy off-shell Higgs production at the LHC to detect potential new physics effects in Higgs couplings and propagator behavior, focusing on deviations from the Standard Model at high energy scales.

Contribution

It extends previous studies by analyzing threshold effects, the running of the top Yukawa coupling, and non-local form factors in the top-Higgs sector, proposing new methods to probe physics beyond the Standard Model.

Findings

Deviations in the $ZZ$ invariant mass distribution could be detectable at HL-LHC and HE-LHC.

Potential to observe new physics effects up to 3 TeV scale at HE-LHC.

Significant modifications in Higgs propagator and top Yukawa coupling at high energies.

Abstract

We study the off-shell production of the Higgs boson at the LHC to probe Higgs physics at higher energy scales utilizing the process $g g \rightarrow h^{*} \rightarrow ZZ$. We focus on the energy scale dependence of the off-shell Higgs propagation, and of the top quark Yukawa coupling, $y_t (Q^2)$. Extending our recent study in arXiv:1710.02149, we first discuss threshold effects in the Higgs propagator due to the existence of new states, such as a gauge singlet scalar portal, and a possible continuum of states in a conformal limit, both of which would be difficult to discover in other traditional searches. We then examine the modification of $y_t (Q^2)$ from its Standard Model (SM) prediction in terms of the renormalization group running of the top Yukawa, which could be significant in the presence of large flat extra-dimensions. Finally, we explore possible strongly coupled new physics in the top-Higgs sector that can lead to the appearance of a non-local $Q^2$-dependent form factor in the effective top-Higgs vertex. We find that considerable deviations compared to the SM prediction in the invariant mass distribution of the $Z$-boson pair can be conceivable, and may be probed at a $2\sigma$-level at the high-luminosity 14 TeV HL-LHC for a new physics scale up to $\mathcal{O}(1 {~\rm TeV})$, and at the upgraded 27 TeV HE-LHC for a scale up to $\mathcal{O}(3 {~\rm TeV})$. For a few favorable scenarios, $5\sigma$-level observation may be possible at the HE-LHC for a scale of about $\mathcal{O}(1 {~\rm TeV})$.