Event shapes and Inclusive Hadron Spectra at FCC-ee energies

Abstract

We analyze hadronic final states of $e^+e^-$ annihilation through event shape observables, Thrust and C-parameter, and inclusive hadron spectra at the planned center-of-mass (c.m.) energies of the Future Circular Electron-Positron Collider (FCC-ee). Collision data is produced using Monte Carlo event generation in PYTHIA at 91.2, 160, 240, and 365 GeV. Distortions of event shapes due to initial-state photon radiation and background decays of Z pairs, W pairs, top-quark pairs, and Higgs bosons are investigated. An extraction of the strong coupling $\alpha_{\text{s}}$ is performed by fitting event shape distributions to perturbative QCD predictions at next-to-next-to-leading-order (NNLO) accuracy, and the sources of systematic uncertainties at high c.m. energies are discussed. Soft gluon dynamics is examined through charged particle multiplicities and momentum distributions, and energy evolution of mean values is compared with prior experimental results. The inferences from this phenomenological study provide a reference to QCD studies at future high-energy $e^+e^-$ colliders.