Precision Measurement of X, Q-2 and A-Dependence of R = Sigma Longitudinal/sigma Transverse and F(2) in Deep Inelastic Scattering.

We report on results for the quantities: the ratio R = sigma_{rm L}/ sigma_{rm T} of the longitudinal (sigma_{rm L}) and transverse (sigma_ {rm T}) virtual photon absorption cross section, the structure function F_2 , the difference R^{rm A}-R^{rm D} , and the cross section ratio sigma ^{rm A}/sigma^{ rm D}, measured in deep inelastic electron scattering from targets of deuterium, iron and gold. The results for R obtained at x = 0.2, 0.35 and 0.5 show a clear falloff with increasing Q^2 , in the range of 1 <=q Q ^2<=q 10 GeV^2 . The x and Q^2 dependence of the quantity R, and the scaling behavior of F _2 are inconsistent with the naive parton model, and the perturbative Quantum Chromodynamics predictions. However, when the effects due to target mass a la Georgi and Politzer are included with the QCD predictions the results are in good agreement. The possible spin-0 diquark content of the nucleons, and any large effects from higher twist terms, beyond those from kinematic target mass effects, are therefore not required to explain our data. The results on the differences R^ {rm A}-R^{rm D} are consistent with zero, and are in agreement with the models for the EMC effect, including those based on Quantum Chromodynamics, which predict negligible difference. These results indicate that there are no significant spin-0 constituents or higher twist effects in nuclei as compared to free nucleons. The EMC effect is confirmed with very small errors and all data (electron and muon scattering) are now in agreement. Our results indicate that this effect is due to a non-trivial difference in the quark distributions between heavy nuclei and the deuteron.