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

Abstract

The Future Circular Collider (FCC-ee) offers the unique opportunity of studying the Higgs Yukawa coupling to the electron, y e , via resonant s-channel production, e + e − → H, in a dedicated run at √ 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 Z * , γ * , or t-channel exchanges alone. Performing such a measurement is remarkably challenging for four main reasons. First, the low value of the e ± mass leads to a tiny y e coupling, and correspondingly small cross section: σ ee→H ∝ m 2 e = 0.57 fb accounting for initial-state γ radiation. Second, the e + e − beams must be monochromatized such that the spread of their center-ofmass (c.m.) energy is commensurate with the narrow width of the SM Higgs boson, 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, √ 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: H → gg and H → WW * → ν + 2 jets. For a benchmark monochromatization with 4.1-MeV c.m. energy spread (leading to σ ee→H = 0.28 fb) and 10 ab −1 of integrated luminosity, a 1.3σ signal significance can be reached, corresponding to an upper limit on the e ± Yukawa coupling at 1.6 times the SM value: |y e | < 1.6|y sm e | at 95% confidence level, per FCC-ee interaction point per year. Directions for future improvements of the study are outlined.

The Future Circular Collider: a Summary for the US 2021 Snowmass Process

“…A preliminary generator-level study of eleven Higgs decay channels identifies two final states as the most promising ones in terms of statistical significance: H → gg and H → WW * → ν + 2 jets. For a benchmark monochromatization with δ √ s = 4.1 MeV and 10 ab −1 of integrated luminosity, a 1.3 standard deviation signal significance can be reached, corresponding to an upper limit on the e ± Yukawa coupling at 1.6 times the SM value: |y e | < 1.6|y sm e | at 95% confidence level, per IP and per year [46]. The expected final significance of the σ e + e − →H measurement, and associated 95% CL limits on |y e |, derived for a benchmark δ √ s = 4.1 MeV collision-energy spread and L int = 10 ab −1 , can be translated into any other combination of (δ √ s , L int ) values achievable through beam monochromatization.…”

“…The FCC-ee offers a unique opportunity to measure y e via resonant s-channel Higgs production, e + e − → H, in a dedicated run at √ s = m H [45,46]. The signature for direct Higgs production is a small rise in the cross sections for final states consistent with Higgs decays over the expectations from SM background processes involving Z * , γ * , or t-channel exchanges alone.…”

Constraining the $${\mathcal {C}}{\mathcal {P}}$$ structure of Higgs-fermion couplings with a global LHC fit, the electron EDM and baryogenesis

“…For a discussion of the technical challenges involved in obtaining limits on g e at possible future lepton colliders, see Refs [186,187]…”