Quark spin and dynamical symmetry breaking

This project was about spin in the subatomic world.Matter is constructed from elementary particles: protons, neutrons and electrons. Protons and neutrons reside in the nucleus of the atom surrounded by a cloud of electrons. Protons have size about 10-15 meters. They are made out of more fundamental particles: three quarks bound together by gluons.Working together with key experiments at major world particle physics laboratories (CERN in Geneva, DESY in Hamburg and RHIC at Brookhaven, Long Island, New York) this project focused on understanding a key question in particle physics: Where does the spin of the proton come from? Protons, electrons and neutrons behave like quantum spinning tops. There is rotation in the subatomic world as well as in the macroscopic world. Quantum spin plays an important role in determining the phases of matter in low temperature physics and the structure of neutron stars. It is responsible for the stability of the known Universe. In the static quark model the spin of the proton is built up from the spin of the 3 quarks. However, experiments have shown that only 35% of the spin of the proton comes from the spin of the quarks? Where is the missing 65%? This question has inspired a worldwide programme of new and ongoing particle physics experiments. The deeper one probes inside the proton, with greater resolution ones finds in addition to the quarks, also antiquarks and gluons. Work done in co-operation with the experimental groups suggests that the gluon and antiquark spin contributions are small, too small to resolve the spin puzzle. The spin puzzle is now understood in terms of the interaction of the three quarks with the complex vacuum of Quantum Chromodynamics, producing significant orbital motion of the quarks inside the proton and partial dissolution of the quarks spin in the vacuum.A key publication from the project is a new Reviews of Modern Physics article with leading experimental colleagues bringing together the results and conclusions of the 25 years global programme to understand the spin structure of the proton.