Model independence of the measurement of the e+e- -> ZH cross section using Z->μ+μ- and Z->e+e- at the ILC

Abstract

The model independent measurement of the absolute ZH cross section of the Higgsstrahlung process e+e- -> ZH is an unique measurement at the ILC indispensable for measuring the Higgs couplings and their deviations from the Standard Model in order to identify new physics models. The performance in measuring the ZH cross section using events in which the Higgs boson recoils against a Z boson which decays into a pair of muons or electrons has been demonstrated based on full simulation of the ILD detector for three center of mass energies 250, 350, and 500 GeV, and two beam polarizations (Pe-,Pe+) =(-80%, +30%) and (+80%, -30%). This paper demonstrates in detail that the analysis which achieved these results are model independent to the sub-percent level. Data selection methods are designed to optimize the precisions of the ZH cross section and at the same time minimize the bias on the measured due to discrepancy in signal efficiencies among Higgs decay modes. Under conservative assumptions which take into account unknown Higgs decay modes, the relative bias on the ZH cross section is shown to be smaller than 0.2% for all center-of-mass energies, which is five times below even the smallest ZH cross section statistical uncertainties expected from the leptonic recoil measurements in a full 20 years ILC physics program.