Proton spin structure and anomalous glue

This project is aimed at understanding the internal quark structure of matter and fundamental issues involving spin and dynamical symmetry breaking. The interpretation of data from new and ongoing experiments in Europe and the United States will lead to new and exciting insight into the fundamental structure of matter. Understanding the spin structure of the proton and the energy of the subatomic vacuum are among the most challenging open questions in particle physics. Key particle physics experiments at CERN (Geneva), DESY (Hamburg) and SLAC (Stanford) have revealed that just about 30% of the proton`s spin is carried by the intrinsic spin of its quark constituents - half the prediction of relativistic constituent quark models (~ 60%). This experimental discovery has challenged our understanding about the internal structure of the proton and inspired a global programme of experimental and theoretical activity to understand the role of spin in the proton`s internal structure. Theoretical studies performed in close contact with experimental groups working at CERN and RHIC (New York) are making genuine new progress understanding how the proton`s spin is distributed between its quark and gluon constituents. The "missing spin" appears to be a property of the valence quarks (the leading Fock component in the proton`s wavefunction). Exactly why the spin is suppressed depends on the details of dynamical symmetry breaking in the proton. The project emphasises synergies and mutually beneficial close interaction between theory and experiment, and also the connections between the proton spin problem and the famous axial U(1) problem of QCD: the role of gluonic degrees of freedom in dynamical chiral symmetry breaking.

The project made new advances in understanding key puzzles in quantum chromodynamics (QCD, the theory of quarks and gluons): the internal spin structure of the proton and the dynamics of the eta-prime meson, a special particle because of its strong coupling to gluons. Experiments tell us that the quarks in the proton contribute just 35% of its spin. Where is the missing spin and why is the quark spin contribution so small? In a Reviews of Modern Physics article written together with leading experimentalists (C.A. Aidala, D. Hasch and G. K. Mallot) we bring together the theoretical and experimental progress in this field from a vast global programme in the last 25 years and set the framework for future, next generation, experiments. New experiments studying eta-prime nucleus interactions have revealed that the eta-prime behaves like its mass is 4% less when the eta-prime is inside a nucleus compared to in free space. This mass shift was predicted by A. W. Thomas and myself in 2006. New theoretical work in close contact to experiments in Bonn, FZ-Jülich and GSI has led to greater understanding of this effect and the role of gluons in the interactions of the eta-prime meson with nucleons and nuclei. The first measurement of the eta-prime nucleon scattering length, an important quantity that determines the low-energy interaction of the eta-prime with protons, was published in Physical Review Letters together with the COSY-11 Collaboration (P. Moskal et al.). In addition to QCD, new ideas about the cosmological constant or dark energy and its connections to particle physics were developed linking the dark energy which describes the accelerating expansion of the Universe to LHC measurements of the Higgs boson at CERN and neutrino physics.