An estimate of higher twist at small x and low Q² based upon a saturation model

We investigate the influence of higher twist corrections to deep inelastic structure functions in the low-$Q^2$ and small-$x$ HERA region. We review the general features of the lowest-order QCD diagrams which contribute to twist-4 at small-$x$, in particular the sign structure of longitudinal and transverse structure functions which offers the possibility of strong cancellations in $F_2$. For a numerical analysis we perform a twist analysis of the saturation model which has been very successful both in describing the structure function and the DIS diffractive cross section at HERA. As the main conclusion, twist 4 corrections are not small in $F_L$ or $F_T$ but in $F_2=F_L + F_T$ they almost cancel. This indicates the limitation on the use of the DGLAP formalism at small $x$ and $Q^2$. We point out that $F_L$ analysis needs a large twist-4 correction. We also indicate the region of validity of the twist expansion.