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

Abstract

Abstract The Future Circular Collider (FCC-ee) offers the unique opportunity of studying the Higgs Yukawa coupling to the electron, $$y_\mathrm {e}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub> <mml:mi>y</mml:mi> <mml:mi>e</mml:mi> </mml:msub> </mml:math> , via resonant s -channel production, $$\mathrm {e^+e^-}\rightarrow \mathrm {H}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mrow> <mml:msup> <mml:mi>e</mml:mi> <mml:mo>+</mml:mo> </mml:msup> <mml:msup> <mml:mi>e</mml:mi> <mml:mo>-</mml:mo> </mml:msup> </mml:mrow> <mml:mo>→</mml:mo> <mml:mi>H</mml:mi> </mml:mrow> </mml:math> , in a dedicated run at $$\sqrt{s} = m_\mathrm {H}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:msqrt> <mml:mi>s</mml:mi> </mml:msqrt> <mml:mo>=</mml:mo> <mml:msub> <mml:mi>m</mml:mi> <mml:mi>H</mml:mi> </mml:msub> </mml:mrow> </mml:math> . 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 $$\mathrm {Z}^*$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msup> <mml:mrow> <mml:mi>Z</mml:mi> </mml:mrow> <mml:mo>∗</mml:mo> </mml:msup> </mml:math> , $$\gamma ^*$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msup> <mml:mi>γ</mml:mi> <mml:mo>∗</mml:mo> </mml:msup> </mml:math> , or t -channel exchanges alone. Performing such a measurement is remarkably challenging for four main reasons. First, the low value of the e $$^\pm $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msup> <mml:mrow/> <mml:mo>±</mml:mo> </mml:msup> </mml:math> mass leads to a tiny $$y_\mathrm {e}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub> <mml:mi>y</mml:mi> <mml:mi>e</mml:mi> </mml:msub> </mml:math> coupling and correspondingly small cross section: $$\sigma _\mathrm {ee\rightarrow H} \propto m_\mathrm {e}^2 = 0.57$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:msub> <mml:mi>σ</mml:mi> <mml:mrow> <mml:mi>ee</mml:mi> <mml:mo>→</mml:mo> <mml:mi>H</mml:mi> </mml:mrow> </mml:msub> <mml:mo>∝</mml:mo> <mml:msubsup> <mml:mi>m</mml:mi> <mml:mrow> <mml:mi>e</mml:mi> </mml:mrow> <mml:mn>2</mml:mn> </mml:msubsup> <mml:mo>=</mml:mo> <mml:mn>0.57</mml:mn> </mml:mrow> </mml:math> fb accounting for initial-state $$\gamma $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>γ</mml:mi> </mml:math> radiation. Second, the $$\mathrm {e^+e^-}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:msup> <mml:mi>e</mml:mi> <mml:mo>+</mml:mo> </mml:msup> <mml:msup> <mml:mi>e</mml:mi> <mml:mo>-</mml:mo> </mml:msup> </mml:mrow> </mml:math> 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, $$\varGamma _\mathrm {H} = 4.1$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:msub> <mml:mi>Γ</mml:mi> <mml:mi>H</mml:mi> </mml:msub> <mml:mo>=</mml:mo> <mml:mn>4.1</mml:mn> </mml:mrow> </mml:math> 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, $$\sqrt{s} = m_\mathrm {H}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:msqrt> <mml:mi>s</mml:mi> </mml:msqrt> <mml:mo>=</mml:mo> <mml:msub> <mml:mi>m</mml:mi> <mml:mi>H</mml:mi> </mml:msub> </mml:mrow> </mml:math> . 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: $$\mathrm {H}\rightarrow gg$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mi>H</mml:mi> <mml:mo>→</mml:mo> <mml:mi>g</mml:mi> <mml:mi>g</mml:mi> </mml:mrow> </mml:math> and $$\mathrm {H}\rightarrow \mathrm {W}\mathrm {W}^*\!\rightarrow \ell \nu $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mi>H</mml:mi> <mml:mo>→</mml:mo> <mml:mi>W</mml:mi> <mml:msup> <mml:mrow> <mml:mi>W</mml:mi> </mml:mrow> <mml:mo>∗</mml:mo> </mml:msup> <mml:mspace/> <mml:mo>→</mml:mo> <mml:mi>ℓ</mml:mi> <mml:mi>ν</mml:mi> </mml:mrow> </mml:math> + 2 jets. For a benchmark monochromatization with 4.1-MeV c.m. energy spread (leading to $$\sigma _\mathrm {ee\rightarrow H} = 0.28$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:msub> <mml:mi>σ</mml:mi> <mml:mrow> <mml:mi>ee</mml:mi> <mml:mo>→</mml:mo> <mml:mi>H</mml:mi> </mml:mrow> </mml:msub> <mml:mo>=</mml:mo> <mml:mn>0.28</mml:mn> </mml:mrow> </mml:math> fb) and 10 ab $$^{-1}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msup> <mml:mrow/> <mml:mrow> <mml:mo>-</mml:mo> <mml:mn>1</mml:mn> </mml:mrow> </mml:msup>

Topics & Concepts