Measurement of kinematic and nuclear dependence of $R=\frac{{\sigma }_L}{{\sigma }_T}$ in deep inelastic electron scattering

We report results on a precision measurement of the ratio $R=\frac{{\sigma }_L}{{\sigma }_T}$ in deep inelastic electron-nucleon scattering in the kinematic range $0.2\le x\le 0.5$ and $1\le Q^2\le 10$ ${(\mathrm{G}\mathrm{e}\mathrm{V}/\mathit{c})}^2$. Our results show, for the first time, a clear falloff of $R$ with increasing $Q^2$. Our $R$ results are in agreement with QCD predictions only when corrections for target mass effects and some additional higher twist effects are included. At small $x$, the data on $R$ favor structure functions with a large gluon contribution. We also report results on the differences $R_A-R_D$ and the cross section ratio $\frac{{\sigma }^A}{{\sigma }^D}$ between Fe and Au nuclei and the deuteron. Our results for $R_A-R_D$ are consistent with zero for all $x$, $Q^2$ indicating that possible contributions to $R$ from nuclear higher twist effects and spin-0 constituents in nuclei are not different from those in nucleons. The ratios $\frac{{\sigma }^A}{{\sigma }^D}$ from all recent experiments, at all $x$, $Q^2$ values, are now in agreement.