[Paper Review] $\alpha_s$(2019): Precision measurements of the QCD coupling

D. d’Enterria, S. Kluth+51 more|DESY (CERN, DESY, Fermilab, IHEP, and SLAC)|Jul 2, 2019

Particle physics theoretical and experimental studies3 references4 citations

TL;DR

This paper summarizes the 2019 workshop on precision measurements of the QCD coupling $α_{\rm s}$, reviewing six key extraction methods—lattice QCD, hadronic $τ$ decays, deep-inelastic scattering, event shapes, Z/W boson decays, and pp collisions—assessing theoretical and experimental uncertainties, and discussing improvements to the world-average $α_{\rm s}(m_{{}_{\rm Z}})$ determination. The key contribution is a detailed review of current methods and future directions for reducing uncertainties, with lattice QCD emerging as the most precise source, prompting discussion on its potential role as the sole world-average input.

ABSTRACT

This document collects a written summary of all contributions presented at the workshop "$\\alpha_s$(2019): Precision measurements of the strong coupling" held at ECT* (Trento) in Feb. 11--15, 2019. The workshop explored in depth the latest developments on the determination of the QCD coupling $\\alpha_s$ from the key categories where high precision measurements are available: (i) lattice QCD, (ii) hadronic $\ au$ decays, (iii) deep-inelastic scattering and parton distribution functions, (iv) event shapes, jet cross sections, and other hadronic final-states in $e^+e^-$ collisions, (v) Z boson and W boson hadronic decays, and (vi) hadronic final states in p-p collisions. The status of the current theoretical and experimental uncertainties associated to each extraction method, and future perspectives were thoroughly reviewed. Novel $\\alpha_s$ determination approaches were discussed, as well as the combination method used to obtain a world-average value of the QCD coupling at the Z mass pole.

Motivation & Objective

• To review the current state-of-the-art in $α_{\rm s}$ determinations from multiple high-precision experimental and theoretical methods.
• To assess theoretical and experimental uncertainties across six key categories: lattice QCD, $τ$ decays, deep-inelastic scattering, event shapes, Z/W decays, and pp collisions.
• To evaluate the feasibility of updating the Particle Data Group (PDG) world-average $α_{\rm s}(m_{{}_{\rm Z}})$ by incorporating newer, more precise results and revising selection criteria.
• To discuss improvements in the combination methodology, including alternative $χ^2$-averaging techniques and handling of correlations.
• To explore the potential of using lattice QCD as the sole basis for the PDG world average due to its superior precision and growing theoretical control.

Proposed method

• Systematic review of 25 presentations from the 2019 workshop, organized into six categories of $α_{\rm s}$ extraction methods.
• Evaluation of theoretical uncertainties, including missing higher-order pQCD corrections, electroweak effects, and hadronization corrections.
• Assessment of experimental and systematic uncertainties in each method, with emphasis on NNLO accuracy and peer-reviewed publication as criteria for inclusion in the world average.
• Discussion of alternative averaging techniques, such as $χ^2$-averaging with floating correlations and the blue/convino correlation models, to improve robustness.
• Analysis of the impact of excluding older $e^+e^-$ data from LEP in favor of re-analyzed or more recent results.
• Proposal to use lattice QCD results as a potential single reference for the world average, given their 0.5–1% uncertainty, which is 2–4 times smaller than other methods.

Experimental results

Research questions

• RQ1 What are the current theoretical and experimental uncertainties in each of the six main $α_{\rm s}$ determination methods?
• RQ2 How can the PDG world-average $α_{\rm s}(m_{{}_{\rm Z}})$ be improved through better selection criteria and updated combination procedures?
• RQ3 To what extent can lattice QCD results alone serve as the basis for the world-average $α_{\rm s}$, given their superior precision?
• RQ4 What are the implications of discrepancies between lattice QCD and other methods if their uncertainties shrink and central values remain inconsistent?
• RQ5 How can alternative averaging methods, such as the blue or convino correlation models, enhance the stability and robustness of the final $α_{\rm s}$ average?

Key findings

• Lattice QCD provides the most precise $α_{\rm s}(m_{{}_{\rm Z}})$ extractions, with total uncertainties of 0.5–1%, significantly lower than other methods (1.5–4%).
• The world-average $α_{\rm s}(m_{{}_{\rm Z}})$ is currently $0.1181 \pm 0.0011$, with a relative uncertainty of about 1%, which remains the dominant source of theoretical uncertainty in precision SM calculations.
• Several high-precision $α_{\rm s}$ determinations from non-lattice methods (e.g., event shapes, $τ$ decays, LHC jet cross sections) are now available with uncertainties below 1%.
• There is growing discussion on whether to exclude older $e^+e^-$ data from LEP in favor of newer re-analyses, due to improved theoretical treatments and reduced uncertainties.
• The possibility of using lattice QCD as the sole basis for the PDG world average is under active consideration, though concerns remain about cross-checking with perturbative data.
• Alternative averaging techniques, such as $χ^2$-averaging with the blue or convino correlation models, are suggested to improve robustness, especially for LHC-based extractions.