A measurement of the Higgs boson mass with a precision of O(10 MeV) is in general sufficient to predict Higgs production cross sections and decay branching fractions with an accuracy sufficiently smaller than the statistical precision expected at FCC-ee. One notable exception is the electron Yukawa coupling determination from the e+e− →H resonant production at √s = 125GeV, for which a precision significantly smaller than the Higgs total width (∼ 4 MeV) is needed.
Traditionally, the Higgs boson mass is obtained from a fit to the distribution of the mass recoil- ing to a leptonically-decaying Z boson (Z → l+l−) in the e+e− → ZH process at √s = 240 GeV: m2recoil =s + m2ll − 2√s (El+ +El−).
The first step in this quest is therefore the determination of the centre-of-mass energy precision of O(1 MeV). The requirements on the detector design to achieve such a precision on √s, regarding in particular the lepton and jet angular resolution, as well as the detector acceptance, will be studied with a consolidated analysis of the e+e− →Z(γ) process at √s = 240GeV – as proposed in [1] – with Z → l+l− and qq ̄, and with realistic FCC-ee collision parameters (beam energy spread, beam crossing angle). The feasibility of a calibration of the method, to reduce systematic uncertainties of various origins, with e+e− → Z(γ) events recorded at the WW threshold – where the centre-of-mass energy can be determined with resonant depolarization with a few 100 keV accuracy as well – will be ascertained.
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