Measurements of H$\toττ$ cross-section at FCC-ee

TL;DR

This work evaluates the precision of H$\to\tau\tau$ cross‑section measurements at FCC‑ee in ZH production at $\sqrt{s}=240$ GeV and $\sqrt{s}=365$ GeV, and in VBF at $\sqrt{s}=365$ GeV, using fast detector simulation of the IDEA concept. It compares two tau reconstruction approaches (ParticleNet and explicit decay‑mode reconstruction) and employs a shape‑based fit with a boosted decision tree framework to extract cross‑sections from multiple final states. The study projects sub‑percent precision on $\sigma_{ZH}\times\mathcal{B}(H\to\tau\tau)$ (≈0.6% at 240 GeV, ≈1.2% at 365 GeV; combined ≈0.5%), and finds a Higgs‑tau coupling precision of $\delta\kappa_\tau$ around 0.47% from the ZH channel, with larger uncertainties in VBF due to backgrounds. Overall, the results demonstrate a substantial improvement over LHC sensitivities and underscore FCC‑ee’s potential as a precision Higgs factory for tau‑related observables within the $\kappa$ framework.

Abstract

The Future Circular Collider (FCC) stands at the forefront of the European Strategy for Particle Physics as the future flagship project at CERN. The H$\toττ$ decay, featuring a large branching ratio, clean identification in the FCC-ee environment, and the possibility to reconstruct polarization information, is an excellent channel to measure Higgs properties. This work shows the expected precision for the H$\toττ$ cross-section measurement at the FCC-ee in the ZH production mechanism at $\sqrt{s}=$240 GeV and $\sqrt{s}=$365 GeV, as well as via the vector boson fusion process at $\sqrt{s}=$365 GeV. Furthermore, we explore and evaluate a set of methods for reconstructing tau decays. These techniques are critical for unlocking the full physics potential of the FCC-ee and for improving the understanding of tau-related observables in both Standard Model measurements and New Physics searches. The results obtained significantly enhance the FCC-ee outlook in the H$\toττ$ channel, improving it by at least an order of magnitude compared to the current sensitivity of measurements' performance at the LHC.

Figures (4)

• Figure 1: On the left: Feynman diagram for ZH events in $e^+e^-$ collisions, where the Z boson decays to leptons (electrons or muons considered in this study), quarks or neutrino pairs, and the Higgs decays to a tau pair. On the right: Feynman diagram for VBF events (mediated by W bosons) in $e^+e^-$ collisions, where the Higgs decays to a tau pair.
• Figure 2: Confusion matrix of truth level tau decay modes against the ID from the explicit reconstruction method for Z$\to\nu\nu$, H$\to\tau\tau$ events at $\sqrt{s}=240$ GeV, where two jets are clustered in the events. Only taus that are matched in angular position to the true ones are considered.
• Figure 3: Comparison of different methods of reconstructing the Higgs boson mass for events where Z$\to ee$ and $H\to\tau_h\tau_h$. The events have been normalized to their sum and use exclusive jets with $n_{jets}=2$ with the ParticleNet tau reconstruction.
• Figure 4: On the left: BDT score distribution for Z$\to qq$, $H\to\tau_h\tau_h$ at $\sqrt{s}=240$ GeV. On the center: BDT score distribution for Z$\to qq$, $H\to\tau_h\tau_h$ at $\sqrt{s}=365$ GeV. On the right: BDT score distribution for VBF $H\to\tau_h\tau_h$ at $\sqrt{s}=365$ GeV.