X and Y Scaling

  • Geoffrey B. West


Until the discovery almost twenty years ago that the structure functions of nucleons, as measured in deep-inelastic electron scattering exhibited a point-like scaling behavior, the idea that hadrons really were bound states of quarks was not generally taken seriously.[1] Quarks somehow were thought of as fictitious objects carrying certain quantum numbers which could explain the general features of the hadronic spectrum.[2] In spite of the great success of this “naïve” quark model, quarks as real hard objects which could be “seen” in electron scattering were not given much credence by the cognoscenti. Indeed the “real” nucleon was still thought of as a “bare” nucleon surrounded by a mesonic cloud and so was expected to respond as a soft spongy object when probed by a hard scattering. Thus the observation of a point-like scaling behavior in deep-inelastic electron scattering played a crucial rôle in establishing quarks as “real” objects from which all hadronic matter is constructed.[1,3] Indeed these experiments completely re-oriented our thinking about strong interactions and opened the way to re-establishing quantum field theory as the paradigm for describing all fundamental interactions. Since that time enormous progress has been made and it is fair to say that we now have an almost universally accepted realistic quantum field theory describing all the fundamental interactions with the possible exception of gravity.[4] Quarks enter as a fundamental fermionic field with strong interactions mediated by massless vector bosons, called gluons. This strong interaction part of the theory is called quantum chromodynamics (QCD) because it closely mimics QED. The crucial difference is that the gluons, as well as the quarks, carry the fundamental “charge” of the theory, here called color. In QED, the photons are not, of course, electrically charged (unlike the electrons) and so do not directly interact with each other. It is generally believed that it is this curious complexity, namely that gluons are colored (and therefore self-interact) that ultimately leads to the confinement of quarks (and more generally of color itself).


Momentum Distribution Quark Model Asymptotic Freedom Hadronic Spectrum Fictitious Object 
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Copyright information

© Springer Science+Business Media New York 1989

Authors and Affiliations

  • Geoffrey B. West
    • 1
  1. 1.Theoretical Division, T-8, MS B285Los Alamos National LaboratoryLos AlamosUSA

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