Why $α_s$ Cannot be Determined from Hadronic Processes without Simultaneously Determining the Parton Distributions

We show that any determination of the strong coupling $α_s$ from a process which depends on parton distributions, such as hadronic processes or deep-inelastic scattering, generally does not lead to a correct result unless the parton distributions (PDFs) are determined simultaneously along with $α_s$. We establish the result by first showing an explicit example, and then arguing that the example is representative of a generic situation which we explain using models for the shape of equal $χ^2$ contours in the joint space of $α_s$ and the PDF parameters.

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