Golden Probe of Electroweak Symmetry Breaking

TL;DR

This paper demonstrates that Higgs decays to four leptons can directly determine the sign and magnitude of the ratio of Higgs couplings to W and Z bosons, providing a unique probe of electroweak symmetry breaking.

Contribution

It introduces a method to measure the sign of the Higgs coupling ratio to W and Z bosons using interference effects in four-lepton decays, independent of other Higgs measurements.

Findings

High luminosity LHC can determine the sign of λ_{WZ}.

Higgs to four leptons is sensitive to the magnitude of λ_{WZ}.

Method is largely independent of other Higgs coupling assumptions.

Abstract

The ratio of the Higgs couplings to $WW$ and $ZZ$ pairs, $\lambda_{WZ}$, is a fundamental parameter in electroweak symmetry breaking as well as a measure of the (approximate) custodial symmetry possessed by the gauge boson mass matrix. We show that Higgs decays to four leptons are sensitive, via tree level/1-loop interference effects, to both the magnitude and, in particular, overall sign of $\lambda_{WZ}$. Determining this sign requires interference effects, as it is nearly impossible to measure with rate information. Furthermore, simply determining the sign effectively establishes the custodial representation of the Higgs boson. We find that $h\to4\ell$ ($4\ell \equiv 2e2\mu, 4e, 4\mu$) decays have excellent prospects of directly establishing the overall sign at a high luminosity 13 TeV LHC. We also examine the ultimate LHC sensitivity in $h\to4\ell$ to the magnitude of $\lambda_{WZ}$. Our results are independent of other measurements of the Higgs boson couplings and, in particular, largely free of assumptions about the top quark Yukawa couplings which also enter at 1-loop. This makes $h\to4\ell$ a unique and independent probe of the electroweak symmetry breaking mechanism and custodial symmetry.