Physics impact of ILC Higgs coupling measurements: The effect of theory uncertainties

Abstract

We study the effect of theoretical and parametric uncertainties on the ability of future Higgs coupling measurements at the International Linear Collider (ILC) to reveal deviations from the standard model (SM). To quantify the impact of these uncertainties we plot $\ensuremath{\Delta}{\ensuremath{\chi}}^{2}=25$ contours for the deviations between the SM Higgs couplings and the light Higgs couplings in the ${m}_{h}^{\mathrm{max}}$ benchmark scenario of the minimal supersymmetric standard model (MSSM). We consider the theoretical uncertainties in the SM Higgs decay partial widths and production cross section and the parametric uncertainties in the bottom and charm masses and the strong coupling ${\ensuremath{\alpha}}_{s}$. We find that the impact of the theoretical and parametric uncertainties is moderate in the first phase of ILC data-taking ($500\text{ }\text{ }{\mathrm{fb}}^{\ensuremath{-}1}$ at 350 GeV center-of-mass energy), reducing the reach in the $CP$-odd MSSM Higgs mass ${M}_{A}$ by about 10% to $\ensuremath{\sim}500\text{ }\text{ }\mathrm{GeV}$, while in the second phase ($1000\text{ }\text{ }{\mathrm{fb}}^{\ensuremath{-}1}$ at 1000 GeV) these uncertainties are larger than the experimental uncertainties and reduce the reach in ${M}_{A}$ by about a factor of 2, from $\ensuremath{\sim}1200$ down to $\ensuremath{\sim}600\text{ }\text{ }\mathrm{GeV}$. The bulk of the effect comes from the parametric uncertainties in ${m}_{b}$ and ${\ensuremath{\alpha}}_{s}$, followed by the theoretical uncertainty in ${\ensuremath{\Gamma}}_{b}$.