Study the effect of beam energy spread and detector resolution on the search for Higgs boson decays to invisible particles at a future e+e- circular collider

We study the expected sensitivity to measure the branching ratio of Higgs boson decays to invisible particles at a future circular e(+)e(-) collider (FCC-ee) in the process e(+)e(-) -> HZ with Z -> l (+)l(-) (l = e or mu) using an integrated luminosity of 3.5 ab(-1) at a center-of-mass energy root s = 240 GeV. The impact of the energy spread of the FCC-ee beam and of the resolution in the reconstruction of the leptons is discussed. The minimum branching ratio for a 5s observation after 3.5 ab(-1) of data taking is 1.7 +/- 0.1%(stat + syst). The branching ratio exclusion limit at 95% CL is 0.63 +/- 0.22%((stat + syst)).