SPIN2010 provides a showcase for what’s new in spin physics.
Image credit: FZ Jülich.
The international conference series on spin originated with the biannual Symposia on High Energy Spin Physics, launched in 1974 at Argonne, and the Symposia on Polarization Phenomena in Nuclear Physics, which started in 1960 at Basle and were held every five years. Joint meetings began in Osaka in 2000, with the latest, SPIN2010, being held at the Forschungszentrum Jülich, chaired by Hans Ströher and Frank Rathmann. The 19th International Spin Physics Symposium was organized by the Institut für Kernphysik
(IKP), host of the 3 GeV Cooler Synchrotron, COSY – a unique facility for studying the interactions of polarized protons and deuterons with internal polarized targets. Research there is aimed at developing new techniques in spin manipulation for applications in spin physics, in particular for the new Facility for Antiproton and Ion Research (FAIR) at GSI, Darmstadt. The 250 or so talks presented at SPIN2010 covered all aspects of spin physics – from the latest results on transverse spin physics from around the world to spin-dependence at fusion reactors.
The conference started with a review of the theoretical aspects of spin physics by Ulf-G Meißner, director of the theory division at IKP, who focused on the challenges faced by the modern effective field-theory approach to few-body interactions at low and intermediate energies. Progress here has been tremendous but old puzzles such as the analysing power, Ay, in proton-deuteron scattering, refuse to be fixed. These were discussed in more detail in the plenary talks by Evgeny Epelbaum of Bochum and Johan Messchendorp of Groningen. In the second talk of the opening plenary session, Richard Milner of the Massachusetts Institute of Technology (MIT) highlighted the future of experimental spin physics.
It is fair to say that the classical issue of the helicity structure of protons has decided to take a rest, in the sense that rapid progress is unlikely. During the heyday of the contribution of the Efremov-Teryaev-Altarelli-Ross spin anomaly to the Ellis-Jaffe sum rule, it was tempting to attribute the European Muon Collaboration “spin crisis” to a relatively large number of polarized gluons in the proton. Andrea Bressan of Trieste reported on the most recent data from the COMPASS experiment at CERN, on the helicity structure function of protons and deuterons at small x, as well as the search for polarized gluons via hard deep inelastic scattering (DIS) reactions. Kieran Boyle of RIKEN and Brookhaven summarized the limitations on Δg from data from the Relativistic Heavy Ion Collider (RHIC) at Brookhaven. The non-observation of Δg within the already tight error bars indicates that gluons refuse to carry the helicity of protons. Hence, the dominant part of the proton helicity is in the orbital momentum of partons.
The extraction of the relevant generalized parton distributions from deeply virtual Compton scattering was covered by Michael Düren of Gießen for the HERMES experiment at DESY, Andrea Ferrero of Saclay for COMPASS and Piotr Konczykowski for the CLAS experiment at Jefferson Lab. Despite impressive progress, there is still a long road ahead towards data that could offer a viable evaluation of the orbital momentum contribution to Ji’s sum rule. The lattice QCD results reviewed by Philipp Hägler of Munich suggest the presence of large orbital-angular momenta, Lu ≈ –Ld ≈ 0.36 (1/2), which tend to cancel each other.
The future of polarized DIS at electron–ion colliders was reviewed by Kurt Aulenbacher of Mainz. The many new developments range from a 50-fold increase in the current of polarized electron guns to an increase of 1000 in the rate of electron cooling.
Transversity was high on the agenda at SPIN2010. It is the last, unknown leading-twist structure function of the proton – without it the spin tomography of the proton would be forever incomplete. Since the late 1970s, everyone has known that QCD predicts the death of transverse spin physics at high energy. It took quite some time for the theory community to catch up with the seminal ideas of J P Ralston and D E Soper of some 30 years ago on the non-vanishing transversity signal in double-polarized Drell-Yan (DY) processes; it also took a while to accept the Sivers function, although the Collins function fell on fertile ground. Now, the future of transverse spin physics has never been brighter. During the symposium, news came of the positive assessment by CERN’s Super Proton Synchrotron Committee with respect to the continuation of COMPASS for several more years.
Image credit: E Schneevoigt.
Both the Collins and Sivers effects have been observed beyond doubt by HERMES and COMPASS. With its renowned determination of the Collins function, the Belle experiment at KEK paved the way for the first determination of the transversity distribution in the proton, which turns out to be similar in shape and magnitude to the helicity density in the proton. Mauro Anselmino reviewed the phenomenology work at Turin, which was described in more detail by Mariaelena Boglione. Non-relativistically, the tensor/Gamow-Teller (transversity) and axial (helicity) currents are identical. The lattice QCD results reported by Hägler show that the Gamow-Teller charge of protons is indeed close to the axial charge.
The point that large transverse spin effects are a feature of valence quarks has been clearly demonstrated in single-polarized proton–proton collisions at RHIC by the PHENIX experiment, as Brookhaven’s Mickey Chiu reported. The principal implication for the PAX experiment at FAIR from the RHIC data, the Turin phenomenology and lattice QCD is that the theoretical expectations of large valence–valence transversity signals in DY processes with polarized antiprotons on polarized protons are robust.
The concern of the QCD community about a contribution of the orbital angular momentum of constituents to the total spin is nothing new to the radioactive-ion-beam community. Hideki Ueno of RIKEN reported on the progress in the production of spin-aligned and polarized radioactive-ion beams, where the orbital momentum of stripped nucleons shows itself in the spin of fragments.
The spin-physics community is entering a race to test the fundamental QCD prediction of the opposite sign of the Sivers effect in semi-inclusive DIS and DY on polarized protons. As Catarina Quintans from Lisbon explained, COMPASS is well poised to pursue this line of research. At the same time, ambitious plans to measure AN in DY experiments with transverse polarization at RHIC, which Elke-Caroline Aschenauer of Brookhaven presented, have involved scraping together a “yard-sale apparatus” for a proposal to be submitted this year. Paul Reimer of Argonne and Ming Liu of Los Alamos discussed the possibilities at the Fermilab Main Injector.
Following the Belle collaboration’s success with the Collins function, Martin Leitgab of Urbana-Champaign reported nice preliminary results on the interference fragmentation function. These cover a broad range of invariant masses in both arms of the experiment.
In his summary talk, Nikolai Nikolaev, of Jülich, raised the issue of the impact of hadronization on spin correlation. As Wolfgang Schäfer observed some time ago, the beta decay of open charm can be viewed as the final step of the hadronization of open charm. In the annihilation of e+e– to open charm, the helicities of heavy quarks are correlated and the beta decay of the open charm proceeds via the short-distance heavy quark; so there must be a product of the parity-violating components in the dilepton spectrum recorded in two arms of an experiment. However, because the spinning D* mesons decay into spinless Ds, the spin of the charmed quark is washed out and the parity-violating component of the lepton spectrum is obliterated.
The PAX experiment to polarize stored antiprotons at FAIR featured prominently during the meeting. Jülich’s Frank Rathmann reviewed the proposal and also reported on the spin-physics programme of the COSY-ANKE spectrometer. Important tests of the theories of spin filtering in polarized internal targets will be performed with protons at COSY, before the apparatus is moved to the Antiproton Decelerator at CERN – a unique place to study the spin filtering of antiprotons. Johann Haidenbauer of Jülich, Yury Uzikov of Dubna and Sergey Salnikov of the Budker Institute of Nuclear Physics reported on the Jülich- and Nijmegen-model predictions for the expected spin-filtering rate. There are large uncertainties with modelling the annihilation effects but the findings of substantial polarization of filtered antiprotons are encouraging. Bogdan Wojtsekhowski of Jefferson Lab came up with an interesting suggestion for the spin filtering of antiprotons using a high-pressure, polarized 3He target. This could drastically reduce the filtering time but the compatibility with the storing of the polarized antiprotons remains questionable.
Kent Paschke of Virginia gave a nice review on nucleon electromagnetic form factors, where there is still a controversy between the polarization transfer and the Rosenbluth separation of GE and GM. He and Richard Milner of MIT discussed future direct measurements of the likely culprit – the two-photon exchange contribution – at Jefferson Lab’s Hall B, at DESY with the OLYMPUS experiment at DORIS and at VEPP-III at Novosibirsk.
Spin experiments have always provided stringent tests of fundamental symmetries and there were several talks on the electric dipole moments (EDMs) of nucleons and light nuclei. Experiments with ultra-cold neutrons could eventually reach a sensitivity of dn ≈ 10–28 e⋅cm for the neutron EDM, while new ideas on electrostatic rings for protons could reach a still smaller dp ≈ 10–29 e⋅cm. The latter case, pushed strongly by the groups at Brookhaven and Jülich, presents enormous technological challenges. In the race for high precision versus high energy, such upper bounds on dp and dn would impose more stringent restrictions on new physics (supersymmetry etc.) than LHC experiments could provide.
Will nuclear polarization facilitate a solution to the energy problem? There is an old theoretical observation by Russell Kulsrud and colleagues that the fusion rate in tokomaks could substantially exceed the rate of depolarization of nuclear spins. While the spin dependence of the 3HeD and D3H fusion reactions is known, the spin dependence of the DD fusion reaction has never been measured. Kirill Grigoriev of PNPI Gatchina reported on the planned experiment on polarized DD fusion. Even at energies in the 100 keV range, DD reactions receive substantial contributions from higher partial waves and, besides possibly meeting the demands of fusion reactors, such data would provide stringent tests of few-body theories – in 2010 the existing theoretical models predict quintet suppression factors which differ by nearly one order in magnitude.
• The proceedings will be published by IOP Publishing in Journal of Physics: Conference Series (online and open-access). The International Spin Physics Committee (www.spin-community.org) decided that the 20th Spin Physics Symposium will be held in Dubna in 2012.
