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^+e^-}rightarrow 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}^*, gamma ^*, 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 {eerightarrow H} propto m_mathrm {e}^2 = 0.57 fb accounting for initial-state gamma radiation. Second, the mathrm {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 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}rightarrow gg and mathrm {H}rightarrow mathrm {W}mathrm {W}^*!rightarrow ell nu + 2 jets. For a benchmark monochromatization with 4.1-MeV c.m. energy spread (leading to sigma _mathrm {eerightarrow H} = 0.28 fb) and 10 ab^{-1} of integrated luminosity, a 1.3sigma 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}| at 95% confidence level, per FCC-ee interaction point per year. Directions for future improvements of the study are outlined.
Highlights
Measurements in p-p collisions at the Large Hadron Collider (LHC), assuming the Standard Model (SM) Higgs production cross section, lead to an upper bound on the branching fraction of B(H → e+e−) < 3.6 · 10−4 at 95% confidence level (CL), corresponding to an upper limit on the Yukawa coupling ye ∝ B(H → e+e−)1/2 of 260 times the SM value [9,10]
This present work has focused on the points (ii) and (iii) above, by performing a generator-level study that has chosen as benchmark point a baseline monochromatization scheme leading to (δ√s, Lint) = (4.1 MeV, 10 ab−1), corresponding to a peak s-channel cross section of σe+e−→H = 280 ab
Large simulated event samples of signal and associated backgrounds have been generated with the pythia 8 Monte Carlo (MC) code for 11 Higgs boson decay channels
Summary
It is worth noting that the background cross sections computed with pythia 8 for twoparticle final states (e+e− → qq, cc, bb, τ τ, γ γ ) are found consistent with those obtained running alternative calculators, such as MadGraph 5 [32,33], but that those for 4-fermion processes with intermediate WW∗ and ZZ∗ are prone to ambiguities in the internal definition of the contributing diagrams, and the ISR treatment, and are not always numerically compatible among them We trust that such differences will not significantly alter our final results, given that the applied multivariate analysis will remove most non-signal-like topologies, but a dedicated study of 4-fermion backgrounds with an alternative MC generator (such as whizard [34] or kkmc [35]) is left for a forthcoming work.
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