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Spin is everywhere… Evidence for dark matter in the universe… Why do cats always land on their feet? Even they were puzzled… N. Bohr W. Pauli … but this is purely classic! Spin in quantum mechanics…… G. Uhlenbeck O. Klein A. Goudsmit P. Ehrenfest (1925): “This is a good idea. Your idea may be wrong, but since both of you are so young without any reputation, you would not loose anything making a stupid mistake.” …. spin as a fundamental property of elementary particles: Proton spin • The value: S = 1 2 • The original idea: • The applications: p S p S quark 12 Example: MRI Alzheimer’s disease © VUMC Alzheimercentrum The origin of proton spin • European Muon Collaboration (EMC) @ CERN (1985): Quarks carry only a small fraction of the proton spin • ZEUS and H1 experiments @ DESY (early ’90’s): In the proton abundant quark-antiquark pairs and gluons are found. gluon Quark-antiquark pair Search the carriers of proton spin • Three possible sources: quark-antiquarks (sea quarks) gluons orbital momentum • Mathematically: ½ = ½ Sq + DG + Lq ~ 30% (Valence Sea Quarks?) ? ? HERA e-p collider at DESY 6.2 km circumference ring, 27 GeV e+ & 920 GeV p’s The HERMES experiment at HERA: Outlook • Spin is everywhere • The origin of proton spin: – gluons & orbital motion • Importance QCD physics: mquark 5 MeV 938 MeV m proton The of mass and spin of the constituents of the visible universe are rooted in QCD dynamics