The Evolution of the Pion Distribution Amplitude in Next-to-Leading Order

The evolution of the pion distribution amplitude in next-to-leading order is studied for a fixed and a running coupling constant. In both cases, the evolution provides a logarithmic modification in the endpoint region. Assuming a simple parameterization of the distribution amplitude at a scale of $Q_0\sim 0.5\ \rm GeV$, it is shown numerically that these effects are large enough at $Q\sim 2\ \rm GeV$ that they have to be taken into account in the next-to-leading-order analysis for exclusive processes. Alternatively, by introducing a new distribution amplitude that evolves more smoothly, this logarithmic modification can be included in the hard-scattering part of the considered process.

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