Experimental Determination of the QCD Effective Charge α_g1(Q)

The QCD effective charge αg1(Q) is an observable that characterizes the magnitude of the strong interaction. At high momentum Q, it coincides with the QCD running coupling αs(Q). At low Q, it offers a nonperturbative definition of the running coupling. We have extracted αg1(Q) from measurements carried out at Jefferson Lab that span the very low to moderately high Q domain, 0.14≤Q≤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 αg1(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.