| Summary: | The QCD effective charge <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msub><mi>α</mi><msub><mi>g</mi><mn>1</mn></msub></msub><mrow><mo>(</mo><mi>Q</mi><mo>)</mo></mrow></mrow></semantics></math></inline-formula> is an observable that characterizes the magnitude of the strong interaction. At high momentum <i>Q</i>, it coincides with the QCD running coupling <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msub><mi>α</mi><mi mathvariant="normal">s</mi></msub><mrow><mo>(</mo><mi>Q</mi><mo>)</mo></mrow></mrow></semantics></math></inline-formula>. At low <i>Q</i>, it offers a nonperturbative definition of the running coupling. We have extracted <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msub><mi>α</mi><msub><mi>g</mi><mn>1</mn></msub></msub><mrow><mo>(</mo><mi>Q</mi><mo>)</mo></mrow></mrow></semantics></math></inline-formula> from measurements carried out at Jefferson Lab that span the very low to moderately high <i>Q</i> domain, <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mn>0.14</mn><mo>≤</mo><mi>Q</mi><mo>≤</mo><mn>2.18</mn></mrow></semantics></math></inline-formula> GeV. The precision of the new results is much improved over the previous extractions and the reach in <i>Q</i> at the lower end is significantly expanded. The data show that <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msub><mi>α</mi><msub><mi>g</mi><mn>1</mn></msub></msub><mrow><mo>(</mo><mi>Q</mi><mo>)</mo></mrow></mrow></semantics></math></inline-formula> becomes <i>Q</i>-independent at very low <i>Q</i>. They compare well with two recent predictions of the QCD effective charge based on Dyson–Schwinger equations and on the AdS/CFT duality.
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