Unbinned measurement of thrust in $e^+e^-$ collisions at $\sqrt{s}$ = 91.2 GeV with ALEPH archived data

TL;DR

This paper reanalyzes archived ALEPH e+e- collision data at 91.2 GeV using machine learning for detector correction, providing a more detailed thrust distribution that informs quantum chromodynamics parameters and models.

Contribution

It introduces a novel unbinned measurement of thrust using machine learning corrections, offering higher granularity and new insights into QCD parameters and non-perturbative effects.

Findings

Observed a systematic shift towards larger $ au$ values.

Compared measured distributions with modern parton shower models.

Provided new constraints for $ ext{QCD}$ phenomenological models.

Abstract

The strong coupling constant ($\alpha_{S}$) is a fundamental parameter of quantum chromodynamics (QCD), the theory of the strong force. Some of the earliest precise constraints on $\alpha_{S}$ came from measurements of event shape observables, such as thrust ($T$), using hadronic $Z$ boson decays produced in $e^+e^-$ collisions. However, recent work has revealed discrepancies between event-shape-based extractions of $\alpha_{S}$ and values determined using other experimental methods. This work reexamines archived $e^+e^-$ data collected at a collision energy of $\sqrt{s}=91.2$ GeV by the ALEPH detector at the Large Electron-Positron Collider. Modern machine learning techniques are used to correct for detector effects in an unbinned manner, allowing the $T$ distribution to be measured with higher granularity than previous ALEPH measurements. The new measurement reveals a small but systematic shift towards larger values of $\tau=1-T$, and the potential implications of this shift for $\alpha_{S}$ extractions are illustrated by comparing to state-of-the-art theoretical calculations. In addition, the region of $-6<\log\tau<-2$, where poorly-understood non-perturbative effects are large, is compared to modern parton shower Monte Carlo simulations. This measurement provides unique new inputs for $\alpha_{S}$ extractions and also improves constraints on phenomenological models of QCD dynamics such as parton fragmentation and hadronization.