Parton Distributions in Nuclei and Polarised Nucleons

Abstract

I review the emerging information on the way quark, antiquark and gluon distributions are modified in nuclei relative to free nucleons. Recent data claiming that quarks carry a vanishingly small fraction of the proton’s spin are criticised and the need for better understanding of x → 0 behaviour is emphasised.

If we are to identify the formation of quark-gluon plasma in heavy ion collisions by changes in the production rates for ψ relative to Drell-Yan lepton pairs then it is important that we first understand the “intrinsic” changes in parton distributions in nuclei relative to free nucleons. So, in the first part of this talk I will review briefly our emerging knowledge on how quark, antiquark and gluon distributions are modified in nuclei relative to free nucleons.

In the second part of the talk I will review the status of quark distributions in polarised nucleons relative to those in unpolarised nucleons. In Bjorken’s presentation⁽¹⁾ we heard about claims made at the Uppsala conference by the EMC collaboration⁽²⁾ that, taken at face value, seem to imply that valence quarks may carry a vanishingly small percentage of the proton’s spin. If this is true then it could be the most significant clue that our understanding of the proton is all at sea.