A model for $F_L$ and $R=F_L/F_T$ at low $x$ and low $Q^2$

Abstract

A model for the longitudinal structure function $F_L$ at low $x$ and low $Q^2$ is presented, which includes the kinematical constraint $F_L\sim Q^4$ as $Q^2\to 0$. It is based on the photon-gluon fusion mechanism suitably extrapolated to the region of low $Q^2$. The contribution of quarks having limited transverse momentum is treated phenomenologically assuming that it is described by the soft pomeron exchange mechanism. The ratio $R=F_L/(F_2-F_L)$, with the $F_2$ appropriately extrapolated to the region of low $Q^2$, is also discussed.

14 pages, Latex, 10 ps figures