Nuclear parton distributions: a perturbative QCD analysis

Abstract

In this work the nuclear modifications of parton distributions are studied within the framework of perturbative QCD. It consists of four publications, of which the first two concentrate on the theoretical and numerical analysis of nuclear effects. In the last two articles further constraints for the nuclear effects are sought. This thesis follows the same structure: After a brief introduction in the chapter 1, the theoretical background for the processes in question is dealt in the chapter 2. In chapter 3, the analysis of nuclear effects is presented, and finally in the chapters 4 and 5 further constraints for the sea quark and gluon distributions are studied. The nuclear effects are parametrized at the initial scale using only data from deep inelastic scattering and Drell-Yan experiments together with momentum and number density conservation as constraints [I]. QCD scale evolution of nuclear effects is studied using Dokshitzer-Gribov-Lipatov-Altarelli-Parisi (DGLAP) evolution equations. As a result, a scale dependent parametrization of nuclear effects is obtained for each parton flavour and nuclear mass number A [II]. The sea quark distribution is further studied in the Drell-Yan process in pA and Pb-Pb collisions in the region where the momentum fraction x > 0.2 [III]. In this region no direct experimental constraints for the parametrization are available; parametrization is fixed by the momentum conservation together with assumption of stable evolution. By considering different scenarios for the sea quark modifications in the region x > 0.2, we find that in Pb-Pb collisions at the SPS the effects of the order of 20 % could be seen at large M. Finally, the lepton pairs produced via heavy mesons in pA collisions are studied at different energies in order to find out to what extent they could be used to further restrict the nuclear gluon modifications [IV]. The dominant process for heavy meson production in pA collision is gg → QQ. As the mesons decay to leptons, the lepton pair cross section will thus reflect the properties of the gluon distribution. The results indicate that the ratios of lepton pair cross sections agree well with the nuclear modifications for gluons. Thus, sufficiently accurate measurements at different collider energies could be used to improve the accuracy of nuclear gluon distributions over a larger kinematical range.