A direct test of perturbative QCD at small x

We show that recent data from HERA on the proton structure function F_2 at small x and large Q^2 provide a direct confirmation of the double asymptotic scaling prediction of perturbative QCD. A linear rise of \ln F_2 with the scaling variable \sigma is observed throughout the kinematic region probed at HERA, and the measured slope is in excellent agreement with the QCD prediction. This provides a direct determination of the leading coefficient of the beta function. At large values of the scaling variable \rho the data display a small but statistically significant scaling violation.