Measurement of the Higgs boson mass and $e^+e^- \to ZH$ cross section using $Z \to \mu^+\mu^-$ and $Z \to e^+ e^-$ at the ILC

TL;DR

This study simulates the measurement of the Higgs boson mass and the ZH production cross section at the ILC, demonstrating high precision and model independence across different energies and polarizations, guiding future collider planning.

Contribution

First demonstration of sub-percent level model independence in ZH cross section measurement at multiple energies and polarizations at the ILC.

Findings

Achieves 2.5-2.9% precision in cross section measurement at 250 GeV.

Determines Higgs mass with 37-41 MeV precision at 250 GeV.

Projects 14 MeV Higgs mass precision over a 20-year ILC program.

Abstract

This paper presents a full simulation study of the measurement of the production cross section ($\sigma_{\mathrm{ZH}}$) of the Higgsstrahlung process $\mathrm{e^{+}e^{-}\rightarrow ZH}$ and the Higgs boson mass ($M_{\mathrm{H}}$) at the International Linear Collider (ILC), using events in which a Higgs boson recoils against a Z boson decaying into a pair of muons or electrons. The analysis is carried out for three center-of-mass energies $\sqrt{s}$ = 250, 350, and 500 GeV, and two beam polarizations $\mathrm{e_{L}^{-}e_{R}^{+}}$ and $\mathrm{e_{R}^{-}e_{L}^{+}}$, for which the polarizations of $\mathrm{e^{-}}$ and $\mathrm{e^{+}}$ are $\left(P\mathrm{e^{-}},P\mathrm{e^{+}}\right)$ =($-$80\%, +30\%) and (+80\%, $-$30\%), respectively. Assuming an integrated luminosity of 250 $\mathrm{fb^{-1}}$ for each beam polarization at $\sqrt{s}$ = 250 GeV, where the best lepton momentum resolution is obtainable, $\sigma_{\mathrm{ZH}}$ and $M_{\mathrm{H}}$ can be determined with a precision of 2.5\% and 37 MeV for $\mathrm{e_{L}^{-}e_{R}^{+}}$ and 2.9\% and 41 MeV for $\mathrm{e_{R}^{-}e_{L}^{+}}$, respectively. Regarding a 20 year ILC physics program, the expected precisions for the $\mathrm{HZZ}$ coupling and $M_{\mathrm{H}}$ are estimated to be 0.4\% and 14 MeV, respectively. The event selection is designed to optimize the precisions of $\sigma_{\mathrm{ZH}}$ and $M_{\mathrm{H}}$ while minimizing the bias on the measured $\sigma_{\mathrm{ZH}}$ due to discrepancy in signal efficiencies among Higgs decay modes. For the first time, model independence has been demonstrated to a sub-percent level for the $\sigma_{\mathrm{ZH}}$ measurement at each of the three center-of-mass energies. The results presented show the impact of center-of-mass energy and beam polarization on the evaluated precisons and serve as a benchmark for the planning of the ILC run scenario.