Measurement of kinematic and nuclear dependence of [ital R]=[sigma][sub [ital L]]/[sigma][sub [ital T]] in deep inelastic electron scattering

We report results on a precision measurement of the ratio [ital R]=[sigma][sub [ital L]]/[sigma][sub [ital T]] in deep inelastic electron-nucleon scattering in the kinematic range 0.2[le][ital x][le]0.5 and 1[le][ital Q][sup 2][le]10 (GeV/[ital c])[sup 2]. Our results show, for the first time, a clear falloff of [ital R] with increasing [ital Q][sup 2]. Our [ital 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 [ital x], the data on [ital R] favor structure functions with a large gluon contribution. We also report results on the differences [ital R][sub [ital A]][minus][ital R][sub [ital D]] and the cross section ratio [sigma][sup [ital A]]/[sigma][sup [ital D]] between Fe and Au nuclei and the deuteron. Our results for [ital R][sub [ital A]][minus][ital R][sub [ital D]] are consistent with zero for all [ital x],[ital Q][sup 2] indicating that possible contributions to [ital R] from nuclear higher twist effects and spin-0 constituents in nuclei are not different from those in nucleons. The ratios [sigma][sup [ital A]]/[sigma][sup [ital D]] from all recent experiments, at all [ital x],[ital Q][sup 2] values, are now in agreement.