Physics Motivation: Higgs mass uncertainty would become one of the important sources of systematics (as parametric error) for Higgs coupling determination, in particularly in HZZ and HWW couplings. Typically with 50MeV uncertainty, sys. error to HZZ coupling would be ~0.3%. Ideally we should achieve <20 MeV uncertainty in order to keep 0.1% sys. error. At Ecm=250 GeV, it might not be a problem by leptonic recoil mass measurement, though one should look into the new beam parameters at 250 GeV which induce larger beamstrahlung effect. At Ecm>=350 GeV, leptonic recoil method becomes limited, therefore we need new method such direct mass reconstruction from H→bb.
Search Channel: e+e- → ZH, Z→ll/qq, H→bb, at Ecm = 500 GeV
Detector Benchmark: in Z→ll channel, there is no ambiguous in jet clustering, Higgs mass measurement is largely affected by b-jet energy resolution; however in an alternative method, measurement can be affected only by jet direction resolution, lepton momentum resolution. In Z→qq channel, there is some contamination from jet clustering, which might be not a serious problem at 500 GeV; so the measurement is largely affected by JER and modeling of b-jet energy distribution.
final observable: Higgs mass (of course)
intermediate observables: jet direction resolution, jet energy resolution, lepton momentum resolution