An Experimental Framework for QCD studies of Event Shapes and Inclusive Hadron Spectra at FCC-ee energies

TL;DR

The paper develops an experimental framework for QCD studies at FCC-ee energies using PYTHIA simulations to analyze event shapes and inclusive hadron spectra in $e^+e^-$ collisions. It emphasizes correcting distortions from initial-state radiation and electroweak backgrounds, and demonstrates NNLO fits to Thrust and the C-parameter to extract $α_{ ext{s}}$ with controlled systematics. The study further probes soft-gluon dynamics through charged multiplicities and the $\xi$-spectrum, testing LPHD and MLLA expectations up to multi-hundred-GeV scales. Together, these efforts establish benchmarks for precision QCD measurements at future high-energy $e^+e^-$ colliders and guide detector and analysis requirements. The framework sets the stage for refined fragmentation modeling, resummation, and detector-level studies to fully exploit FCC-ee's precision potential.

Abstract

We present a comprehensive analysis of hadronic final states produced in $e^+e^-$ annihilations at the planned FCC-ee center-of-mass energies of 91.2, 160, 240, and 365 GeV using Monte Carlo simulations with PYTHIA. The distortions on event shape variables at high center-of-mass-energies is investigated considering initial state photon radiation and backgrounds from hadronic decays of Z pairs, W pairs, top-quark pairs, and Higgs. An extraction of the strong coupling constant $α_{\text{s}}$ is performed through fits of the Thrust and C-parameter distributions to perturbative QCD predictions at next-to-next-to-leading-order (NNLO) precision. The study further probes soft gluon dynamics through charged particle multiplicities and momentum distributions, comparing the energy evolution of their mean values with previous experiments. The learning from this phenomenological work provide a reference for future experimental QCD studies at high-energy electron-positron colliders.

Figures (15)

• Figure 1: Illustration of a hadronic $e^{+}e^{-}\rightarrow \gamma/Z\rightarrow q\overline{q}$ event. Hadrons and decay channels shown are only representative.
• Figure 2: Dominant annihilation processes at the FCC-ee.
• Figure 3: Comparison of normalized event shape distributions generated in PYTHIA 8.313 with experimental data from L3 (red circles) and ALEPH (blue squares). Each subsequent distribution is displaced by a factor of $10^3$ for presentation.
• Figure 4: Process-wise contributions to $\sqrt{s'}$ at $\sqrt{s}=365\;$GeV.
• Figure 5: Impact of ISR on event shapes (1-T) and C. Each subsequent distribution is displaced by a factor of $10^3$.