Probing the electron Yukawa coupling via resonant Higgs boson production at FCC-ee via $e^+e^- \to H \to WW^*$ in lepton-plus-jets final states

Abstract: We report a detailed simulation study of the search for $s$-channel Higgs boson production in $e<sup>+e<sup>-$ collisions at a center-of-mass (c.m.) energy of $\sqrt{s}=125\,\mathrm{GeV}$ at the CERN Future Circular Collider (FCC-ee), as a means to constrain the electron Yukawa coupling, $y_e$. The process of interest is $$e^{+e^-\to} H\to WW^*\to \ell^\pmν+ jj$$ with four different $WW<sup>*$ final states considered, involving both on- and off-shell $W$ bosons decaying either into dileptons ($\ell^\pm = e<sup>\pm$ and $μ<sup>\pm$, including those from $τ<sup>\pm$ decays) or into dijets ($jj$). Signal and background events are discriminated through a multiclass gradient boosted decision tree exploiting a comprehensive set of kinematic and topological variables across the four final-state categories. Assuming a monochromatized c.m. energy spread of 4.1 MeV, yielding a $$σ_{e^{+e^-\to} H} = 280\,\mathrm{ab}$$ resonant cross section, and an integrated luminosity of $10\,\mathrm{ab}<sup>{-1}$, the analysis achieves a combined statistical significance of 2.0 standard deviations. This corresponds to an upper limit on the coupling modifier $κ_e = y_e/y_e^{\rm SM} \lesssim 1.35$ at 95\% confidence level, and provides the most stringent constraint on the electron Yukawa coupling achieved in simulation-based studies to date.