Treatment of the QCD coupling in high energy processes

The treatment of the running QCD coupling in evolution equations is discussed. It is shown that the use of the virtuality of ladder (vertical) partons as the scale for QCD coupling in every rung of ladder graphs is an approximation that holds for DIS at large x only. On the contrary, in the small x region the coupling depends on the virtuality of s-channel (horizontal) gluons. This observation leads to different results for the Regge-like processes and DIS structure functions at small x.