Determination of the strong coupling constant using matched NNLO+NLLA predictions for hadronic event shapes in e+e- annihilations

TL;DR

This paper determines the strong coupling constant from event-shape variables in e+e- collisions, using advanced NNLO+NLLA QCD predictions, and provides a precise measurement with detailed uncertainty analysis.

Contribution

It introduces a combined NNLO+NLLA approach for extracting alpha_s from multiple event shapes at various energies, improving precision over previous NLO+NLLA methods.

Findings

Measured alpha_s(MZ)=0.1224 with uncertainties.

Systematic uncertainties increased due to NLLA scale variations.

Results are consistent with previous measurements but more precise.

Abstract

We present a determination of the strong coupling constant from a fit of QCD predictions for six event-shape variables, calculated at next-to-next-to-leading order (NNLO) and matched to resummation in the next-to-leading-logarithmic approximation (NLLA). These event shapes have been measured in e+e- annihilations at LEP, where the data we use have been collected by the ALEPH detector at centre-of-mass energies between 91 and 206 GeV. Compared to purely fixed order NNLO fits, we observe that the central fit values are hardly affected, but the systematic uncertainty is larger because the NLLA part re-introduces relatively large uncertainties from scale variations. By combining the results for six event-shape variables and eight centre-of-mass energies, we find alpha_s(MZ)= 0.1224 +/- 0.0009 (stat) +/- 0.0009 (exp) +/- 0.0012 (had) +/- 0.0035 (theo), which improves previously published measurements at NLO+NLLA. We also carry out a detailed investigation of hadronisation corrections, using a large set of Monte Carlo generator predictions.