The Energy Dependence of the Saturation Scale

At low $x\cong Q^2\mathrm{/}{W}^2<<1$, in deep inelastic scattering, the photon fluctuates into a $q\bar{q}$ vector state that interacts via two gluons with the proton. The energy dependence is determined by the saturation scale, in our approach given by ${\mathrm{\Lambda }}_{sat}^2(W^2)\sim (W^2{)}^{C_2}$. Imposing DGLAP evolution, we find $C_2^{theory}=0.27$ in agreement with the model-independent analysis of the HERA data. Different values of the exponent $C_2$ are correlated with different ratios of the longitudinal to transverse structure function. This stresses the need for experiments to separate longitudinal and transverse contributions in deep inelastic scattering.