= From deep inelastic scaling to asymptotic freedom
The 1969 <SLAC-MIT deep inelastic electron scattering experiment> found approximate <Bjorken scaling>: an estimated <deep-inelastic structure function> depended mainly on a ratio of kinematic variables. Its restricted angular coverage did not separately determine both <deep-inelastic structure functions>, and the extraction assumed transverse <virtual photons>. This was motivation for a microscopic theory, not already a measurement of the <QCD coupling>.<Observed Behavior of Highly Inelastic Electron-Proton Scattering by M. Breidenbach and others>[Printed p. 935, abstract; the stated transverse-photon assumption and limited separation of W1 and W2]{ref}
The 1973 <David Gross>–<Frank Wilczek> calculation supplied a <quantum field theory> explanation of approximately free <partons> with calculable logarithmic corrections. <Asymptotic freedom> therefore permits approximate <Bjorken scaling> without requiring exact scale independence. The <QCD coupling> governs the corrections, while <parton distribution functions> and their evolution connect the short-distance calculation to a <proton>.<Asymptotic Freedom: From Paradox to Paradigm by Frank Wilczek>[Section 2, pp. 5–6: finite-energy radiation and calculable corrections]{ref}
In a later direct test, H1 collided <Positrons> and <protons> at HERA and selected <deep inelastic scattering> events. A liquid-argon <particle calorimeter> identified the outgoing <Positron> with an associated track; calorimeter deposits and charged tracks reconstructed <particle jets> in the <Breit frame>. Vertex, timing and longitudinal <energy>-balance cuts suppressed backgrounds; two simulations checked detector corrections. Fits of <particle jet> <cross sections> to next-to-leading-order <QCD> extracted $\alpha_s(E_T)$ in transverse-<energy> bins, consistent with decreasing <QCD coupling>. <Hadronization>, <parton distribution functions> and <renormalization scale> choices limited the inference. This measures rates and fits a coupling, rather than directly reading a force meter.<Measurement of Inclusive Jet Production in Deep-Inelastic Scattering at High Q2 and Determination of the Strong Coupling by H1 Collaboration>[2007, sections 2.1–2.4 and 5; Fig. 5]{ref}
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