Experimental determination of the QCD effective charge $\alpha_{g_1}(Q)$

TL;DR

This paper presents an improved experimental extraction of the QCD effective charge lpha_{g_1}(Q) from Jefferson Lab data, covering low to moderate Q, showing its behavior at low Q, and comparing with theoretical predictions.

Contribution

It provides the most precise measurement to date of lpha_{g_1}(Q) over a broad Q range, including very low Q, and compares results with Dyson-Schwinger and AdS/CFT predictions.

Findings

lpha_{g_1}(Q) becomes Q-independent at very low Q.

Results agree well with Dyson-Schwinger equation predictions.

Results also match AdS/CFT duality-based predictions.

Abstract

The QCD effective charge $\alpha_{g_1}(Q)$ is an observable that characterizes the magnitude of the strong interaction. At high momentum $Q$, it coincides with the QCD running coupling $\alpha_{\rm s}(Q)$. At low $Q$, it offers a nonperturbative definition of the running coupling. We have extracted $\alpha_{g_1}(Q)$ from measurements carried out at Jefferson Lab that span the very low to moderately high $Q$ domain, $0.14 \leq Q \leq 2.18$ GeV. The precision of the new results is much improved over the previous extractions and the reach in $Q$ at the lower end is significantly expanded. The data show that $\alpha_{g_1}(Q)$ becomes $Q$-independent at very low $Q$. They compare well with two recent predictions of the QCD effective charge based on Dyson-Schwinger equations and on the AdS/CFT duality.