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 coupling to the electron, $y_e$, via resonant s-channel production, $e^{+}{e}^{-}\to H$, in a dedicated run at $\sqrt{s}=m_H$. The signature for direct Higgs production is a small rise in cross sections for particular final states, consistent with Higgs decays, over the expectations for their occurrence due to SM background processes involving $Z^{\ast },{\gamma }^{\ast }$, or t-channel exchanges. Performing such a measurement is remarkably challenging for four main reasons. First, the low value of the e${}^{\pm }$ mass leads to a tiny $y_e$ coupling, and correspondingly small cross section: ${\sigma }_{ee\to H}\propto m_e^2=0.57$ fb accounting for initial-state radiation. Second, the $e^{+}{e}^{-}$ beams must be monochromatized such that their c.m. energy spread is commensurate with the narrow width of the SM Higgs boson, ${\Gamma }_H=4.1$ 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, $\sqrt{s}=m_H$. 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 generator-level study of 11 Higgs decays using a multivariate analysis, exploiting BDTs to discriminate signal and background events, identifies two final states as the most promising ones in terms of statistical significance: $H\to gg$ and $H\to W{W}^{\ast }\to \ell \nu$ + 2 jets. For a benchmark 4.1-MeV c.m. energy spread (leading to ${\sigma }_{ee\to H}=0.28$ fb) and ${\mathcal{L}}_{int}=10$ ab${}^{-1}$, a $1.3\sigma$ signal significance can be reached, corresponding to an upper limit on the e${}^{\pm }$ Yukawa at 1.6 times the SM value: $|y_e|<1.6|y_e^{SM}|$ at 95\% confidence level, per IP per year. Directions for future improvements of the study are outlined.

Comments: 11 pages, 6 figures. EPJ+ Special issue: "A future Higgs and Electroweak factory (FCC): Challenges towards discovery, Focus on FCC-ee"