Evidence of quasi-partonic higher-twist effects in deep inelastic scattering at HERA at moderate Q2

The combined HERA data for the inclusive deep inelastic scattering (DIS) cross sections for the momentum transfer Q(2) > 1GeV(2) are fitted within the Dokshitzer-Gribov-Lipatov-Altarelli-Parisi (DGLAP) framework at next-to-leading order (NLO) and next-to-next-to-leading order (NNLO) accuracy, complemented by a QCD-inspired parameterisation of twist 4 corrections. A modified form of the input parton density functions is also included, motivated by parton saturation mechanism at small Bjorken x and at a low scale. These modifications lead to a significant improvement of the data description in the region of low Q(2). For the whole data sample, the new benchmark NNLO DGLAP fit yields chi(2)/d.o.f. similar or equal to 1.19 to be compared to 1.46 resulting from the standard NNLO DGLAP fit. We discuss the results in the context of the parton saturation picture and describe the impact of the higher-twist corrections on the derived parton density functions. The resulting description of the longitudinal proton structure function FL is consistent with the HERA data. Our estimates of higher-twist contributions to the proton structure functions are comparable to the leading-twist contributions at low Q(2) similar or equal to 2 GeV2 and x similar or equal to 10(-5). The x-dependence of the twist 4 corrections obtained from the best fit is consistent with the leading twist 4 quasi-partonic operators, corresponding to an exchange of four interacting gluons in the t-channel.