Study of HERA ep Data at Low Q^2 and Low x_Bj and the Need for Higher-Twist Corrections to Standard pQCD Fits

Abstract

A detailed comparison of HERA data at low Bjorken-$x$ and low four-momentum-transfer squared, $Q^2$, with predictions based on $\ln Q^2$ evolution (DGLAP) in perturbative Quantum Chromo Dynamics suggests inadequacies of this framework. The standard DGLAP evolution was augmented by including an additional higher-twist term in the description of the longitudinal structure function, $F_{\mathrm{L}}$. This additional term, $F_{\mathrm{L}}{A}_{\mathrm{L}}^{\mathrm{HT}}/Q^2$, improves the description of the reduced cross sections significantly. The resulting predictions for $F_{\mathrm{L}}$ suggest that further corrections are required for $Q^2$ less than about 2 GeV${}^2$.

24 pages, 14 figures, updated version contains small changes made during the review process