Most physicists are convinced that Quantum Chromodynamics (QCD) is the correct theory of strong interactions, which also accounts for the nuclear forces. But is this really true? Quantum Chromodynamics can not be 'proven' in the way one proves a theorem in mathematics. However, the more numerous checks of the theory become and the more accurate they are, the more one gains confidence, until it is finally accepted as a valid description of nature. The measurements at HERA have provided new and accurate checks of QCD, and together with measurements at CERN and FNAL, so much progress has been made in the last few years, that QCD is now considered as firmly established.

Examples:

  1. Measurement of the strength of the strong interaction, as expressed by the coupling constant alphas between quarks and gluons. Precise measurements at CERN, FNAL and at HERA, based on different processes and with different methods all lead to the same value of alphas.
  2. Does the strong interaction get weaker at high energies? Yes! This is a pivotal prediction of QCD, connected with its character as a gauge theory. The figure, based on measurements at HERA, shows that this is indeed the case.
Different methods to measure the strength of the strong interaction
Different methods to measure the strength of the strong interaction by looking for the appearance of a gluon. The storage rings at LEP at CERN, the proton-antiproton collider at FNAL and HERA use different reactions for this measurement.

Survey of different measurements of the strength of strong interactions
Survey of different measurements of the strength of strong interactions, expressed by alphas, the quark gluon coupling constant. Measurements with a variety of methods all give the same results, if normalized to the same energy. Those based on the HERA measurements labeled 'Deep inelastic Scattering' are among the most accurate.
(From 'Review of Particle Physics', upgrade 1999)
Alpha_s as a function of the energy
The strength of the strong interaction, expressed by alphas, plotted as a function of the energy, expressed by the squared momentum transfer Q2. The HERA measurements indicate, that the strong force gets weaker with increasing energy.


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