Extractions of the strong coupling from collider data without PDF refitting are biased

TL;DR

Determining the strong coupling constant from collider data without refitting PDFs introduces bias, affecting both the estimated value and uncertainty, even under ideal conditions.

Contribution

This paper explicitly demonstrates the bias introduced in $oldsymbol{ ext{α}_s(m_Z)}$ determination when PDFs are not refitted simultaneously with the coupling constant.

Findings

Single-process $oldsymbol{ ext{α}_s(m_Z)}$ estimates differ from global fits.

Bias persists even in ideal, tension-free datasets.

Refitting PDFs is essential for unbiased $oldsymbol{ ext{α}_s(m_Z)}$ extraction.

Abstract

We present an explicit demonstration that a determination of the strong coupling constant $\alpha_s(m_Z)$ from deep-inelastic scattering and hadron collider data without a simultaneous determination of the parton distribution functions (PDFs) leads to a biased result for both the central value and the uncertainty, even in the ideal scenario (closure test) where there are no internal tensions between datasets and where theoretical calculations describe perfectly the experimental measurements. Specifically, we show that a determination of $\alpha_s(m_Z)$ from a single process leads in general to a result that differs from the global best fit more than the value of $\alpha_s(m_Z)$ that is actually favoured by this process.