Use of $Z$ polarization in $e^+e^- \to ZH$ to measure the triple-Higgs coupling

TL;DR

This paper proposes using a specific angular asymmetry of leptons from Z decay in e+e- collisions to measure the triple-Higgs coupling, providing a loop-level sensitive method that is independent of other couplings.

Contribution

It introduces a novel angular asymmetry observable related to Z polarization that can constrain the triple-Higgs coupling at future colliders independently of other anomalous couplings.

Findings

Potential to constrain the triple-Higgs coupling to about 3.4 times the SM value at the ILC.

The asymmetry is sensitive to loop-level effects and independent of dominant tree-level couplings.

Method applicable at future e+e- colliders with polarized beams.

Abstract

It is shown that a certain angular asymmetry of charged leptons produced in the decay of $Z$ in the process $e^+e^- \to ZH$, related to a tensor polarization component of the $Z$, can be used to constrain the anomalous triple-Higgs coupling, independent of the other anomalous couplings like the $ZZH$ coupling which dominate at tree level. This is because the angular asymmetry is odd under na\" ive time reversal, and hence dependent on loop-level contributions. At a future $e^+e^-$ collider like the International Linear Collider (ILC), for example, a limit of about $3.4$ might be placed on the ratio of the actual triple Higgs coupling to that predicted in the standard model for a centre-of-mass energy of 500 GeV and an integrated luminosity of 30 fb$^{-1}$ with electron and positron beams having longitudinal polarization of $-80\%$ and $+30\%$, respectively.