On the $A$ dependence of $R = \sigma_L/\sigma_T$ and the $Q^2$ dependence of Shadowing

Abstract

A higher--twist nuclear enhancement of $R = \sigma_L/\sigma_T$, as might be expected to arise due to fermi motion and whose magnitude is within the error-bars of recent experiment, is shown to lead to a monotonic {\it decrease} in the ratio of nuclear vs. nucleon cross--sections at small Bjorken $x$ for {\it increasing} $Q^2$. This effect at small $x$, comparable in magnitude to those reported for shadowing, is driven mainly by kinematic factors and essentially vanishes for $x > .1$. Its unusual $Q^2$ dependence rather complicates the unravelling from present data in the shadowing region the corresponding dependence in $Q^2$ of the nuclear structure functions, $F_2^{A}(x,Q^2)$.