The mysterious quantity that is spin took centre stage at Brookhaven for the SPIN2002 meeting last September. Yousef Makdisi and Thomas Roser report.
The 15th biennial International Spin Physics Symposium (SPIN2002) was held at Brookhaven National Laboratory on 9-14 September 2002. Some 250 spin enthusiasts attended, including experimenters and theorists in both nuclear and high-energy physics, as well as accelerator physicists and polarized target and polarized source experts. The six-day symposium included 23 plenary talks and 150 parallel talks. SPIN2002 was preceded by a one-day spin physics tutorial for students, postdocs, and anyone else who felt the need for a refresher course.
In the opening talk, “A Beautiful Spin”, Xiang-dong Ji of Maryland reviewed the history of spin, starting with the 1925 publication by George Uhlenbeck and Samuel Goudsmit that introduced spin as a fundamental property of most subatomic particles. Ji noted that our 3+1-dimensional space-time is symmetrical under translation and rotation; this symmetry results in two universal observables, mass and spin. Understanding these two fundamental quantities has been a central goal of particle and nuclear physics throughout much of the 20th century. He went on to describe the invaluable role that spin plays in uncovering physics beyond the Standard Model, nucleon structure and nonperturbative quantum chromodynamics (QCD).
Over the last decade, great progress has been made in measuring the quark spin structure functions of the nucleon with the SMC experiment at CERN, HERMES at Hamburg’s DESY laboratory, and at the Stanford Linear Accelerator Center (SLAC) in California. Todd Averett of the University of William and Mary and Andrew Miller of TRIUMF summarized this progress. Inclusive deep inelastic scattering (DIS) measurements have established that quarks and antiquarks combined (valence and sea) contribute only a small fraction of the nucleon spin on average. Therefore there must be a significant contribution from gluons and/or orbital angular momentum. It is possible that the valence quark contribution to the nucleon spin is large, but is offset by a negative sea quark polarization.
Semi-inclusive DIS data, where an outgoing hadron is observed, should separate the contributions from valence and sea quarks. New results were presented from HERMES showing little or no sea polarization, but with large errors, so it is difficult to draw a conclusion at this stage. The indication of a positive strange quark polarization is quite interesting. Future parity-violating W-boson production from high-energy polarized proton collisions should directly measure antiquark polarization, separated by flavour. Scaling violations in DIS data at different energies provide a first glimpse of gluon polarization, which appears to be significant. Future results from the experiments COMPASS at CERN, E-160 at SLAC, and PHENIX and STAR at Brookhaven, with complementary kinematic coverage, are eagerly awaited. Richard Milner of MIT-Bates showed recent plans for the next-generation measurements of polarized DIS using a proposed high-luminosity electron-ion collider (EIC).
On the theory front, Marco Stratmann of Regensburg reviewed the theoretical framework for describing longitudinal spin asymmetries in perturbative QCD, and the progress made in the corresponding higher-order calculations. He then ventured into the domain of high-energy polarized proton collisions and outlined the framework for global QCD analyses. Kostas Orginos of the RIKEN-Brookhaven Research Center (RBRC) reviewed the progress, results and future prospects for learning about nucleon spin structure from lattice QCD. Philip Ratcliffe of the University of Insubria expanded on the latest attempts to explain the large observed single-spin transverse asymmetries in inclusive hyperon and pion production, as well as lepton-induced production. Recent theoretical advances calculate such asymmetries perturbatively in terms of intrinsic transverse-momentum degrees of freedom in hadrons, and of higher-twist effects. He highlighted the re-emerging domain of transversity and how to access it in polarized hadron production processes such as Drell-Yan, as well as semi-inclusive processes in polarized lepton-nucleon collisions. Inevitably, he pleaded for additional experimental data. Marc Vanderhaeghen of Mainz reviewed the relatively new and exciting field of DIS exclusive processes and generalized parton distributions, which could provide information on the orbital angular momentum of partons in the nucleon.
Klaus Helbing of Erlangen reviewed the status of the experimental verification of the Gerasimov-Drell-Hearn (GDH) sum rule, which relates a weighted integral of the spin-dependent photon-nucleon absorption cross-section to the anomalous magnetic moment of the nucleon, and is widely believed to stand on very solid ground in QCD. Proton results from the Mainz Microtron and ELSA machine at Bonn have now verified this sum rule at the 5% level. Further experimental information in the resonance region and at high energies from laboratories such as Jefferson Laboratory (JLab) in Virginia will be important. This is especially true for the neutron. The rich spectrum of low-energy electron and photon-induced experiments from HIGS at Duke University, LEGS at Brookhaven, GRAAL in Grenoble and LEPS at Harima, Japan (listed in increasing energy from 0.6 to 2400 MeV) were covered by Andrew Sandorfi of Brookhaven. He detailed work on the GDH sum rule and lambda production with an effort to understand the 1405 MeV resonance. He looked to the future EIC and back-scattered photons to extend the reach to 6500 MeV.
Brookhaven’s Haixin Huang reported on the first operation of the Relativistic Heavy Ion Collider (RHIC) as a polarized proton collider. Polarized proton beams in RHIC were recently accelerated to 100 GeV without significant loss of polarization; this confirmed that the two Siberian snakes installed in each ring indeed work as predicted at Novosibirsk by Yaroslav Derbenev and Anatoly Kondratenko about 25 years ago. The world’s first polarized proton collisions in a collider were observed at RHIC at a centre-of-mass energy of 200 GeV and a luminosity of about 1.5 x 1030 cm_2s_1. However, due to a significant polarization loss in the Alternating Gradient Synchrotron (AGS), which serves as the injector for RHIC, the maximum beam polarization was only about 25%. This can be partly ascribed to a weak temporary replacement for the AGS’s failed 30 MW motor-generator. The proton beam polarization was measured by the proton-carbon Coulomb nuclear interference reaction, a topic that was also covered by Boris Kopeliovich of the Max Planck Institute in Heidelberg in his theoretical review of proton-proton elastic scattering. Brookhaven’s Les Bland reported on the hadron spin physics experiments during RHIC’s first short three-week polarized data run. These included the observation of large asymmetries in neutral pion production by the STAR experiment, and in neutron production by the PHENIX experiment at large Feynman-x (the ratio of observed longitudinal momentum to the maximum allowed) in polarized proton-proton collisions at 200 GeV in the centre of mass.
Representing the next generation of spin physicists, Michigan graduate student Vassili Morozov presented some other impressive polarized beam efforts. During the venerable Indiana University Cyclotron Facility (IUCF) cooler ring’s final year, Morozov and his colleagues spin-flipped both vector and tensor polarized deuterons for the first time. They also spin-flipped polarized protons with a measured spin-flip efficiency of 99.93 ± 0.02%. There were many reports of the impressive progress on polarized sources and polarized solid and gas targets. Their ever-increasing intensities and polarizations are essential to the field of spin physics, and were highlighted in a review by Erhard Steffens of Erlangen, and in workshop summaries by Vladimir Derunchuck of IUCF) and Manouchehr Farkhondeh of MIT-Bates.
Beyond the Standard Model
Turning to probes of physics beyond the Standard Model, Ernst Sichtermann of Yale presented the recent highly precise measurement of the muon anomalous magnetic moment (g-2) using the high-intensity polarized positive muon storage ring that is fed by the AGS. The latest result is about a two-sigma deviation from the Standard Model. Considerable theory work is in progress, and negative muon data are being analysed by the experiment. New experiments are planned to measure the neutron electric dipole moment using polarized neutrons at Los Alamos.
Krishna Kumar of Massachusetts reviewed parity violation in polarized electron scattering. Two major experiments – SAMPLE at Bates and HAPPEX at JLab – are using parity violation to study the strangeness content of the proton. The new high-precision E158 experiment’s measurement of parity violation in Moller scattering at SLAC will test the Standard Model predictions for the electron’s weak charge.
The last day of SPIN2002 focused mostly on the future. Gudrid Moortgat-Pick of DESY discussed polarized electron-positron linear colliders and supersymmetry. Gordon Cates of Virginia described some impressive advances using spin physics in the field of medicine. Jacques Soffer of Marseilles ended the symposium with confidence that we are poised to witness significant progress in our understanding of spin and QCD in the near future. He also echoed the sentiment of the opening presentation: that the universe without spin would collapse. This happy note set the stage for the 16th International Spin Physics Symposium, SPIN2004, which will take place at the International Centre for Theoretical Physics in Trieste, Italy, in early autumn 2004. The many dedicated spin physicists attending SPIN2002 can now look forward to more exciting results, and a deeper understanding of the mysterious quantity that is spin.
Proceedings to be published by the American Institute of Physics.