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, $$y_\mathrm {e}$$ 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.

A special Higgs challenge: measuring the mass and production cross section with ultimate precision at FCC-ee

“…This requirement is almost met by the precision of 20 MeV ultimately reachable at HL-LHC [9]. One notable exception is the determination of the electron Yukawa coupling via resonant e + e − → H production at √ s = 125 GeV, for which a precision smaller than the SM Higgs total width of 4.1 MeV is required [10]. The opportunities and challenges to achieve such a precision on the Higgs boson mass at FCC-ee are examined in the following.…”

Possibly heteroclite electron Yukawa coupling and small $\triangle a_μ$ in a hidden Abelian gauge model for neutrino masses

“…9, it is clear that the magnitude of electron-Higgs Yukawa could be one order of magnitude larger than the SM prediction. The projected sensitivity, |ζ e | < 1.6 at 95% CL, at FCC-ee [79] is shown as the vertical dashed line. For the case of △a CS e , the sign of electron-Higgs Yukawa could even be negative.…”

The Higgs boson turns ten

“…Investigations are ongoing to establish the potential sensitivity of a future e e + − collider to electron and strange-quark Yukawa interactions (see, for example, ref. 49 ), although currently it seems that it will be challenging to obtain a statistically conclusive signal. For the coupling of up and down quarks to the Higgs boson, there are currently no concrete possibilities in sight unless those couplings are very strongly enhanced relative to the Standard Model expectation.…”