Study of HERA ep Data at Low Q^2 and Low x_Bj and the Need for   Higher-Twist Corrections to Standard pQCD Fits
  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, $F_{\rm L}$. This additional term, $F_{\rm L}~A_{\rm L}^{\rm HT}/Q^2$, improves the description of the reduced cross sections significantly. The resulting predictions for $F_{\rm L}$ suggest that further corrections are required for $Q^2$ less than about 2 GeV$^2$. 
I. Abt, A.M. Cooper-Sarkar, B.Foster, V. Myronenko, K. Wichmann, M.   Wing
hep-ph hep-ex