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

Abstract

We report a detailed simulation study of the search for $s$-channel Higgs boson production in $e^{+}{e}^{-}$ 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 $e^{+}{e}^{-}\to H\to W{W}^{\ast }\to {\ell }^{\pm }\nu +jj$ with four different $W{W}^{\ast }$ final states considered, involving both on- and off-shell $W$ bosons decaying either into dileptons (${\ell }^{\pm }=e^{\pm }$ and ${\mu }^{\pm }$, including those from ${\tau }^{\pm }$ 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 ${\sigma }_{e^{+}{e}^{-}\to H}=280\mathrm{ab}$ resonant cross section, and an integrated luminosity of $10{\mathrm{ab}}^{-1}$, the analysis achieves a combined statistical significance of 2.0 standard deviations. This corresponds to an upper limit on the coupling modifier ${\kappa }_e=y_e/y_e^{\mathrm{SM}}\lesssim 1.35$ at 95\% confidence level, and provides the most stringent constraint on the electron Yukawa coupling achieved in simulation-based studies to date.

Comments: 36 pages, 9 tables, 13 figures