The Quark Structure of Matter
Anthony W. Thomas
Progress of Theoretical Physics Supplement
We provide an overview of recent work at the University of Adelaide aimed at using deep-inelastic scattering to probe the quark-level structure of nucleons and nuclei. § 1. Introduction These lectures are primarily based on the research performed with my colleagues and students in Adelaide, as well as with a number of overseas colleagues who have spent one or more extended periods in Adelaide. I should like to acknowledge in particular the insight that I have gained from R. P. Bickerstaff, L.
... e approach to the problem of strong interactions in the absence of realistic solutions to QCD is to build models. For one school the more mathematically beautiful the theory the better. Indeed, the Skyrme model, where the nucleon is supposed to be a topological soliton constructed from meson fields is a veritable industry as seen in the many contributions to the PANIC Conference. On the other hand, in this quest for the modern day holy grail there is a tendency to forget that the roots of physics lie in experimental data. We believe in QCD primarily because we have "seen" quarks. Deep-inelastic scattering of leptons from nucleons and nuclei revealed the presence of quarks almost two decades ago. 2 >' 3 > It has long been my belief that this same tool can and must be used to distinguish between models which purport to approximate QCD. Until a model can be used to calculate not only the nucleon mass and charge radius but also its structure function we will not have a satisfactory, phenomenological understanding of how QCD is realized. The plan of these lectures is as follows. In § 2, we review the theory of deepinelastic lepton scattering and the basic ideas of the parton model. We also briefly by guest on September 21, 2014 http://ptps.oxfordjournals.org/ Downloaded from ). In order to see how DIS can reveal the substructure of a target, let us consider electron scattering from 56 Fe, which for the present we think of as 56 nucleons. Elastic scattering on 56 Fe will occur at X56= Q 2 /2m( 56 Fe)v=l. On the other hand, elastic scattering on a nucleon in 56 Fe (ignoring small binding and Fermi motion corrections) would occur at xN= Q 2 /2mNv=1, which means X56"' 1/56. Following the argument we gave above we would then expect that for 1/Q~ size by guest on September 21, 2014 http://ptps.oxfordjournals.org/ Downloaded from U + D+S+ (] + D+ S=1-G, (2·14) where by guest on September 21, 2014 http://ptps.oxfordjournals.org/ Downloaded from Fzn(x)=g-[u(x)+ u(x)] +g-[d(x)+ d(x)] +g-[s(x)+ s (x)]. (2·20) Clearly one cannot use an electromagnetic probe to separate the quark and antiquark by guest on September 21, 2014 http://ptps.oxfordjournals.org/ Downloaded from perturbative strange quarks is S=0.52%, while S =0.28%. With R=0.6 and 1.0 fm the results are (1.4%, 1.0%) and (0.18%, 0.08%) respectively. This may be compared with the data of CDHS group (at Q 2 ::::::5GeV 2 ), which found (0+15+2,$')=7+0.5% and 2S/ ( U + 15) =52+ 9%. If, as assumed in almost all phenomenology, one stets S = S this implies S = 1.2 + 0.2%, which clearly restricts the allowed values of R. A more thorough analysis of the data requires more effort-includiing a proper by guest on September 21, 2014 http://ptps.oxfordjournals.org/ Downloaded from