Spin and snakes come to the land of Jefferson

The International Spin Physics Symposia series started at the Argonne National Laboratory in 1974, just after its 12 GeV Zero Gradient Synchrotron (ZGS) had accelerated the world’s first polarized proton beam. Paul Dirac gave the keynote lecture in which he reviewed the history of spin, starting with the first ideas in the 1920s. The 18th Symposium, SPIN 2008, was held in October 2008 at the University of Virginia, which was founded by Thomas Jefferson in 1821. Jefferson became third president of the US and is best known as the main author of the US Declaration of Independence. He had a keen interest in science and was president of the American Philosophical Society. As a fitting tribute, an appropriately-dressed person – who claimed to be Jefferson – gave the after-dinner talk at SPIN 2008. Speaking with a polite 1800s Virginian accent, he gave wise scientific advice that is as relevant now as it was in the early 19th century.

Symposia highlights

SPIN 2008 was attended by 282 high-energy and nuclear spin physicists from around the world. There were 195 parallel talks and 37 plenary talks, so this report mentions only a few of the exciting highlights. After the welcoming talks, the ever-enthusiastic Elliot Leader of Imperial College, London, opened the symposium with a rousing lecture on “The power of spin: a scalpel-like probe of theoretical ideas”. He was followed by Klaus Rith of Erlangen, who gave a detailed experimental overview in his talk that addressed selected highlights of spin experiments and their technological challenges.

The main highlight of the symposium was the success of Brookhaven’s RHIC in its operations as a polarized-proton collider. The machine has produced a great deal of high-quality data in 100 GeV-on-100 GeV collisions and had a brief but successful test of stored 250 GeV polarized protons. For this impressive achievement, Thomas Roser, Mei Bai and their team of polarized-beam experts used two Siberian snakes in each RHIC ring together with two partial Siberian snakes in the venerable Alternating Gradient Synchrotron, which serves as the injector for RHIC. These operations were possible thanks to some vital external contributions. James Simons, mathematics professor at Stony Brook, a Brookhaven trustee and now a “renaissance-technologies” billionaire, provided $13 million to allow a 6 month polarized run of RHIC. Moreover, the long-term support of Akito Arima – a nuclear theorist who became a member of the Japanese Diet and science minister – resulted in more than $20 million for RHIC’s four superconducting Siberian snakes and other essential hardware. The funds were transferred from Japan to Brookhaven via the RIKEN research institute.

One interesting result was the measurement by the BRAHMS experiment at RHIC of the left–right spin asymmetry, A_n, in the inclusive production of π⁺ and π^– mesons, which was presented by Christine Aidala of the University of Massachusetts at Amherst. The data show that, despite the prediction of perturbative QCD (PQCD) that spin would be unimportant at high energy, the inclusive A_n at large Feynman-x reached the same value of about 40% at 3900 GeV² (P_Lab ≈ 2 TeV/c) as at Argonne’s ZGS, Brookhaven’s AGS and Fermilab at momentum values of 12, 22 and 200 GeV/c, respectively (figure 1). This result might encourage PQCD theorists to define more clearly what is meant by “high” energy.

In an overview of the transverse spin structure of the nucleon, Mario Anselmino of Turin reported on the very large observed transverse spin effects, which are still not fully understood. Karl Slifer of the University of Virginia gave a talk on what polarized electron scattering has revealed about the spin content of the nucleon. He discussed the theoretical implications of recent polarized-electron experiments, many of which were done at the 6 GeV polarized-electron ring at the Thomas Jefferson National Accelerator Facility (Jefferson Lab). The use of polarized radioactive beams was the subject of an interesting talk by Koichiro Asahi of Tokyo Institute of Technology.

Speakers also covered the more experimental aspects of spin studies, namely the production of polarized beams and polarized targets. Richard Milner of the Massachusetts Institute of Technology gave an excellent review of the progress towards a future polarized-electron ring, which would allow collisions with either polarized protons or polarized nuclear ions stored in a much larger ring – possibly one of the rings at RHIC. Erhard Steffens of Erlangen, the new chair-elect of the International Spin Physics Committee, summarized the discussions at the second workshop on ‘How to Polarize Antiprotons’, held in August 2007 at the Cockcroft Institute at the Daresbury Laboratory in the UK. There has been significant progress on this challenging topic during the 22 years since Owen Chamberlain and Alan Krisch organized the first Polarized Antiproton Workshop in 1985 at Bodega Bay near Berkeley, but there is still no clearly defined solution to this difficult problem.

Werner Meyer of Bochum reviewed the continuing progress, since SPIN 2006 in Kyoto, on cryogenic polarized proton and deuteron targets. Brookhaven’s Anatoly Zelenski then described the recent progress on polarized-ion sources – the subject of a joint paper with Alexander Belov of the Institute for Nuclear Research, Troitsk. This progress is important because these polarized sources feed RHIC. Indeed, Brookhaven and the Spin Physics Committee sponsored a recent workshop on this topic at Brookhaven. Matt Poelker of Jefferson Lab reviewed progress on polarized-electron sources and polarimeters, which was the subject of another recent spin workshop at the laboratory. These sources and polarimeters are vital to progress in polarized-electron experiments.

Anatoly Kondratenko of Novosibirsk, who along with Yaroslav Derbenev invented Siberian snakes in the 1970s, gave an interesting talk on his more recent idea, now named Kondratenko Crossing (KC). This uses a symmetric spin-resonance crossing pattern that forces the resonance’s depolarizing effects to cancel themselves. Richard Raymond of Michigan reported on new data from the SPIN@COSY team at the Cooler Synchrotron (COSY) at the Forschungszentrum in Jülich. The results show that KC works, at least for RF-solenoid-induced resonances with deuterons. In the same parallel session accelerator pioneer Ernest Courant, of Brookhaven, still going strong at 89, discussed his new theoretical work on the behaviour of stored polarized beams.

There was also a special evening plenary session where the director (or proxy) of each of the major laboratories involved in spin-physics studies reported on their plans at GSI-Darmstadt, Brookhaven, Jefferson Lab, IHEP-Protvino, JINR-Dubna, J-PARC, and COSY-Jülich. On the last day, Thomas Roser, the new past-chair of the Spin Physics Committee, gave an excellent lecture on the future of high-energy polarized beams. The symposium ended with closing remarks from committee chair, Kenichi Imai of Kyoto. He announced that SPIN 2010 would be hosted by Forschungszentrum Jülich, while a high priority would be given to SPIN 2012 being hosted somewhere in Russia.

• For more information about SPIN 2008, see www.faculty.virginia.edu/spin2008/.