An Estimate of Higher Twist at Small x B and Low Q 2 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.