Small-x structure functions from NMC

Abstract

The NMC measurements of the proton and deuteron structure functions for 0.006<x<0.6 and 0.5<Q2<55 GeV2 and of the neutron-to-proton structure function ratio down to x=0.0008 are presented. The data agree very well with the earlier results of the BCDMS, SLAC (reanalysed) and EMC NA28 experiments, but disagree with the reanalysed results of the EMC NA2. A parametrization of the low-x structure function F2, incorporating vector meson dominance dynamics and the QCD-improved parton model, valid at arbitrary Q2 and reproducing well the data, is also presented. Results of a quantitative analysis of the shadowing effect in the deuteron are given. The shadowing is found to be about a 2% effect, in qualitative agreement with the NMC results on the neutron-to-proton structure function ratio at low x. The NMC results on the Q2 dependence of the structure functions are compared with the predictions of perturbative QCD in next-to-leading order. Parton distributions and the strong coupling constant are determined from the data. The QCD evolution in the low-x region is driven largely by gluons, and shows only a moderate contribution of 'higher twists' at scales around 1 GeV2.