Measurement of kinematic and nuclear dependence of<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>R</mml:mi><mml:mo>=</mml:mo><mml:mfrac><mml:mrow><mml:mrow><mml:msub><mml:mrow><mml:mi>σ</mml:mi></mml:mrow><mml:mrow><mml:mi>L</mml:mi></mml:mrow></mml:msub></mml:mrow></mml:mrow><mml:mrow><mml:mrow><mml:msub><mml:mrow><mml:mi>σ</mml:mi></mml:mrow><mml:mrow><mml:mi>T</mml:mi></mml:mrow></mml:msub></mml:mrow></mml:mrow></mml:mfrac></mml:math>in deep inelastic electron scattering

We report results on a precision measurement of the ratio $R=\frac{{\ensuremath{\sigma}}_{L}}{{\ensuremath{\sigma}}_{T}}$ in deep inelastic electron-nucleon scattering in the kinematic range $0.2\ensuremath{\le}x\ensuremath{\le}0.5$ and $1\ensuremath{\le}{Q}^{2}\ensuremath{\le}10$ ${(\mathrm{G}\mathrm{e}\mathrm{V}/\mathit{c})}^{2}$. 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 ${R}_{A}\ensuremath{-}{R}_{D}$ and the cross section ratio $\frac{{\ensuremath{\sigma}}^{A}}{{\ensuremath{\sigma}}^{D}}$ between Fe and Au nuclei and the deuteron. Our results for ${R}_{A}\ensuremath{-}{R}_{D}$ 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 $\frac{{\ensuremath{\sigma}}^{A}}{{\ensuremath{\sigma}}^{D}}$ from all recent experiments, at all $x$, ${Q}^{2}$ values, are now in agreement.