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

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.