Measurement of the mass of Higgs boson through HZ production at FCC-ee

TL;DR

This work assesses the feasibility of a model‑independent Higgs mass and cross‑section measurement via the recoil mass method in $e^+e^- \to HZ$ at FCC‑ee, focusing on $Z\to l^+l^-$ decays at $\sqrt{s} = 240$ GeV and $365$ GeV with an integrated luminosity of 5 ab$^{-1}$. The recoil approach isolates the Higgs signal from the leptonic $Z$ kinematics, enabling direct access to the $HZZ$ coupling $g_{HZZ}$ without relying on Higgs decay products. Using the IDEA detector simulation, the study demonstrates a Higgs mass resolution of about $\sigma_H \approx 0.256$ GeV at 240 GeV and $\sigma_H \approx 0.60$ GeV at 365 GeV, with a clear Higgs peak near $m_H \approx 125$ GeV and sub‑percent precision on the inclusive $HZ$ cross section. The two energy points are complementary: 240 GeV provides the best recoil resolution and 365 GeV expands sensitivity to high‑momentum and VBF‑like processes, collectively enhancing the precision tests of the SM Higgs sector and potential new physics. The results reinforce the recoil mass method as a powerful tool for precision, model‑independent Higgs physics at future lepton colliders.

Abstract

The associated production of a Higgs boson with a $Z$ boson in the collisions of electron and positron beams at Future Circular Collider (FCC-ee) have been studied. Here, the $Z$ boson decays into dileptons, while the Higgs boson decays to all possible channels, mostly to the $b\bar{b}$. The collisions provide a clean and powerful channel to probe the ($HZZ$) coupling at future lepton colliders. Following the event generation, the analysis is performed using the recoil mass method, which allows model-independent reconstruction of the Higgs boson depending on the kinematics of the final-state leptons. This method enables precise identification of the Higgs signal peak independent of its decay mode and significantly reduces systematic uncertainties. The recoil mass distributions from the signal process ($e^+e^- \to HZ$, $Z \to l^+l^-$) and the main backgrounds ($ZZ$, $WW$, $t\bar{t}$ and other standard model processes) have been analyzed using a dedicated analysis code. Monte Carlo simulations corresponding to an integrated luminosity of $5$ ab$^{-1}$ have been used for the analysis, assuming the high performance of the IDEA detector concept. The results are presented for center-of-mass energies of $\sqrt{s} = 240$ GeV and $\sqrt{s} = 365$ GeV to compare the sensitivities and highlight the potential of future $e^+e^-$ colliders in probing the $HZZ$ interaction with high precision.

Figures (5)

• Figure 1: Relevant diagrams including HZZ (or hzz) interaction vertex and leading to $h\mu^+\mu^-$ state, where $\mu^\pm$ is denoted as mu+/mu- in the Figure due to particle definition in event generator.
• Figure 2: Relevant diagrams including HZZ (or hzz) interaction vertex and leading to $he^+e^-$ state, where $e^\pm$ is denoted as e+/e- in the Figure due to particle definition in event generator.
• Figure 3: Cross section as a function of center-of-mass energy for various single Higgs production channels at electron-positron colliders. The highest cross section is observed for the $e^+e^- \rightarrow HZ$ process at $\sqrt{s} = 240$ GeV and 365 GeV.
• Figure 4: Recoil mass distribution at $\sqrt{s} = 240$ GeV with S+B fit. The events are presented for an integrated luminosity of 5 ab$^{-1}$.
• Figure 5: Recoil mass distribution at $\sqrt{s} = 365$ GeV with S+B fit. The events are presented for an integrated luminosity of 5 ab$^{-1}$.