Measuring the electron Yukawa coupling via resonant s-channel Higgs production at FCC-ee

Abstract

AbstractThe Future Circular Collider (FCC-ee) offers the unique opportunity of studying the Higgs Yukawa coupling to the electron, $$y_\mathrm {e}$$ y e , via resonant s-channel production, $$\mathrm {e^+e^-}\rightarrow \mathrm {H}$$ e + e - → H , in a dedicated run at $$\sqrt{s} = m_\mathrm {H}$$ s = m H . The signature for direct Higgs production is a small rise in the cross sections for particular final states, consistent with Higgs decays, over the expectations for their occurrence due to Standard Model (SM) background processes involving $$\mathrm {Z}^*$$ Z ∗ , $$\gamma ^*$$ γ ∗ , or t-channel exchanges alone. Performing such a measurement is remarkably challenging for four main reasons. First, the low value of the e$$^\pm $$ ± mass leads to a tiny $$y_\mathrm {e}$$ y e coupling and correspondingly small cross section: $$\sigma _\mathrm {ee\rightarrow H} \propto m_\mathrm {e}^2 = 0.57$$ σ ee → H ∝ m e 2 = 0.57  fb accounting for initial-state $$\gamma $$ γ radiation. Second, the $$\mathrm {e^+e^-}$$ e + e - beams must be monochromatized such that the spread of their centre-of-mass (c.m.) energy is commensurate with the narrow width of the SM Higgs boson, $$\varGamma _\mathrm {H} = 4.1$$ Γ H = 4.1  MeV, while keeping large beam luminosities. Third, the Higgs mass must also be known beforehand with a few-MeV accuracy in order to operate the collider at the resonance peak, $$\sqrt{s} = m_\mathrm {H}$$ s = m H . Last but not least, the cross sections of the background processes are many orders-of-magnitude larger than those of the Higgs decay signals. A preliminary generator-level study of 11 Higgs decay channels using a multivariate analysis, which exploits boosted decision trees to discriminate signal and background events, identifies two final states as the most promising ones in terms of statistical significance: $$\mathrm {H}\rightarrow gg$$ H → g g and $$\mathrm {H}\rightarrow \mathrm {W}\mathrm {W}^*\!\rightarrow \ell \nu $$ H → W W ∗ → ℓ ν  + 2 jets. For a benchmark monochromatization with 4.1-MeV c.m. energy spread (leading to $$\sigma _\mathrm {ee\rightarrow H} = 0.28$$ σ ee → H = 0.28  fb) and 10 ab$$^{-1}$$ - 1 of integrated luminosity, a $$1.3\sigma $$ 1.3 σ signal significance can be reached, corresponding to an upper limit on the e$$^\pm $$ ± Yukawa coupling at 1.6 times the SM value: $$|y_\mathrm {e}|<1.6|y^\mathrm {\textsc {sm}}_\mathrm {e}|$$ | y e | < 1.6 | y e S M | at 95% confidence level, per FCC-ee interaction point per year. Directions for future improvements of the study are outlined.