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Abstract. The accelerating expansion of the Universe points to a small positive vacuum energy density and negative vacuum pressure. A strong candidate is the cosmological constant in Einstein's equations of General Relativity. Possible contributions are zero-point energies and the condensates associated with spontaneous symmetry breaking. The vacuum energy density extracted from astrophysics is 10 56 times smaller than the value expected from quantum fields and Standard Model particle physics. Is the vacuum energy density time dependent ? We give an introduction to the cosmological constant puzzle and ideas how to solve it.Confidential: not for distribution.

This article reviews the present understanding of the QCD spin structure of the proton. The author first outlines the proton spin puzzle and its possible resolution in QCD. Then the review explores the present and next generation of experiments being undertaken to resolve the proton's spin-flavor structure, explaining the theoretical issues involved, the present status of experimental investigation, and the open questions and challenges for future investigation.

We argue that η bound states in nuclei are sensitive to the singlet component in the η. The bigger the singlet component, the more attraction and the greater the binding. Thus, measurements of η bound states will yield new information about axial U(1) dynamics and glue in mesons. η − η ′ mixing plays an important role in understanding the value of the η-nucleon scattering length.