Abstract
The FCC-ee offers powerful opportunities to determine the Higgs boson parameters, exploiting over 10^6{ hbox {e}^+hbox {e}^- rightarrow hbox {ZH}} events and almost 10^5{ hbox {WW} rightarrow hbox {H}} events at centre-of-mass energies around 240 and 365 GeV. This essay spotlights the important measurements of the ZH production cross section and of the Higgs boson mass. The measurement of the total ZH cross section is an essential input to the absolute determination of the HZZ coupling—a “standard candle” that can be used by all other measurements, including those made at hadron colliders—at the per-mil level. A combination of the measured cross sections at the two different centre-of-mass energies further provides the first evidence for the trilinear Higgs self-coupling, and possibly its first observation if the cross section measurement can be made accurate enough. The determination of the Higgs boson mass with a precision significantly better than the Higgs boson width (4.1 MeV in the standard model) is a prerequisite to either constrain or measure the electron Yukawa coupling via direct { hbox {e}^+hbox {e}^- rightarrow hbox {H}} production at sqrt{s} = 125 GeV. Approaching the statistical limit of 0.1% and {mathcal {O}}(1) MeV on the ZH cross section and the Higgs boson mass, respectively, sets highly demanding requirements on accelerator operation (ZH threshold scan, centre-of-mass energy measurement), detector design (lepton momentum resolution, hadronic final state reconstruction performance), theoretical calculations, and analysis techniques (efficiency and purity optimization with modern tools, constrained kinematic fits, control of systematic uncertainties). These challenges are examined in turn in this essay
Highlights
Improved-Born Higgs production cross sections, as predicted by HZHA [3] as a function of the centre-of-mass energy for mH = 125 GeV
The large luminosities provided by FCC-ee at centre-of-mass energies above 200 GeV, and the possibility to simultaneously operate different detectors at various interaction points (IPs), offer multiple opportunities and techniques to measure the Higgs boson mass and production cross section with ambitious accuracies and precisions
In the baseline FCC-ee oIPpse,raatmioonumntotdoel5,th0e.2i,natengdra1t.e5dalbu−m1inaot s√itises=ex2p4e0c,te3d4t0o–b3e50d,elainvdere3d65anGdesVh.aErexdpbloeittwinegenthtewsoe integrated luminosities, and the muon momentum resolution of the IDEA drift chamber concept, a preliminary analysis of the μ+μ−X final state with the “recoil mass” method √leads to statistical precisions of 1% on σZH and 6 MeV on mH from the data collected at s = 240 GeV alone
Summary
Conservative values for the statistical precision on inclusive and exclusive ZH cross sections, obtained from preliminary FCC-ee conceptual studies with realistic beam and detector parameters [4], are indicated, and the resulting accuracy of Higgs couplings obtained from global fits to the FCC-ee measurements The precise measurement of the ZH cross section can give access to the Higgs boson self-coupling gHHH via loop diagrams (shown in the left panel of Fig. 2) as was realized for the first time in Ref. The dependence of the ZH cross section on the centre-of-mass energy allows in addition the gHZZ and gHHH couplings to be determined separately in a robust and model-independent manner [7,8], with a.
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