Experimental Determination of the QCD Effective Charge αg1(Q)

Abstract

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.

The Compton amplitude and nucleon structure functions

“…2,𝑢𝑢−𝑑𝑑 provides a complementary approach to determining the nucleon isovector axial charge 𝑔 𝐴 via the Bjorken sum rule [32,33], or alternatively, the QCD effective charge [34,35]. However, our current results on polarised structure functions are very preliminary.…”

Nucleon Resonance Electroexcitation Amplitudes and Emergent Hadron Mass

“…α s /π ≈ 1, known as the strong QCD (sQCD) regime, represent a crucial challenge in modern hadron physics [1]. The rapid growth of α s in the transition from the perturbative to sQCD domains and particularly its saturation, driven by gluon self-interactions, are predicted by CSMs [2,3] and supported by recent experimental results on the Bjorken sum rule [4]. These trends suggest that the generation of hadron structure in the sQCD regime is defined by emergent degrees of freedom that are related to the partons of QCD's Lagrangian in a non-trivial manner.…”

“…up to ln k 2 corrections). (Right) CSM prediction [3] (magenta band) for the process-independent QCD running coupling αs (Q) compared with the empirical results [4] for the process-dependent effective charge defined via the Bjorken sum rule, which is prominent in deep inelastic scattering.…”

“…Since the Poincaré-invariant kinetic energy operator for a vector boson has mass-dimension two and that for a spin-half fermion has mass-dimension unity, then for m 2p /k 2 → 0, M 0 (k) ∝ 1/k 2 and m 2 g (k) ∝ 1/k 2 , up to ln k 2 corrections). (Right) CSM prediction[3] (magenta band) for the process-independent QCD running coupling αs (Q) compared with the empirical results[4] for the process-dependent effective charge defined via the Bjorken sum rule, which is prominent in deep inelastic scattering.…”

Empirical Determination of the Pion Mass Distribution

“…Connections with experiment and other nonperturbative extensions of QCD's running coupling are given in Refs. [60][61][62].…”