Regular Article

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

¹ CERN, EP Department, 1211, Geneva, Switzerland
² LMU, 80539, Munich, Germany
³ SLAC, 2575 Sand Hill Rd., 94025, Menlo Park, CA, USA

^a david.denterria@cern.ch

Received: 5 July 2021
Accepted: 4 November 2021
Published online: 4 February 2022

Abstract

The Future Circular Collider (FCC-ee) offers the unique opportunity of studying the Higgs Yukawa coupling to the electron, $y_{\mathrm{e}}$, via resonant s-channel production, ${\mathrm{e}}^{+}{\mathrm{e}}^{-}\to \mathrm{H}$, in a dedicated run at $\sqrt{s}=m_{\mathrm{H}}$. 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 ${\mathrm{Z}}^{\ast }$, ${\gamma }^{\ast }$, 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 $y_{\mathrm{e}}$ coupling and correspondingly small cross section: ${\sigma }_{\mathrm{ee}\to \mathrm{H}}\propto m_{\mathrm{e}}^2=0.57$ fb accounting for initial-state $\gamma$ radiation. Second, the ${\mathrm{e}}^{+}{\mathrm{e}}^{-}$ 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, ${\Gamma }_{\mathrm{H}}=4.1$ 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, $\sqrt{s}=m_{\mathrm{H}}$. 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: $\mathrm{H}\to gg$ and $\mathrm{H}\to \mathrm{W}{\mathrm{W}}^{\ast }\to \ell \nu$ + 2 jets. For a benchmark monochromatization with 4.1-MeV c.m. energy spread (leading to ${\sigma }_{\mathrm{ee}\to \mathrm{H}}=0.28$ fb) and 10 ab${}^{-1}$ of integrated luminosity, a $1.3\sigma$ signal significance can be reached, corresponding to an upper limit on the e${}^{\pm }$ Yukawa coupling at 1.6 times the SM value: $|y_{\mathrm{e}}|<1.6|y_{\mathrm{e}}^{\mathrm{S}\mathrm{M}}|$ at 95% confidence level, per FCC-ee interaction point per year. Directions for future improvements of the study are outlined.