CERN: Circular solution to the enigma of nucleon spin

Over half the nucleon’s spin is unaccounted for, but circularly-polarized photons could soon reveal where it is. Physicists working on the new NA59 experiment at CERN aim to study the production of high-energy circularly polarized photon beams using crystals. If they are successful, such beams could be used to probe directly the contribution to nucleon spin carried by gluons, answering a question first posed by a European Muon Collaboration experiment in 1988 which found that quarks contribute less than half the nucleon’s spin.

Circularly-polarized photon beams are traditionally produced from polarized electron beams. But the sort of physics NA59 has in mind would require much higher electron energies than are currently available. The solution proposed by NA59 is to start with unpolarized electrons and produce circular polarization in a two-step process using a pair of crystals. The first crystal would generate a linearly polarized photon beam, the second would act as a “quarter-wave plate”, converting linear into circular polarization. Quarter-wave plates are frequently used to analyse polarized light ­ a feature which makes the work of NA59 potentially interesting to prospective builders of future high-energy electron­positron colliders. Quarter-wave plates might also be used to measure beam polarization in such machines.

The first step has already been demonstrated by the earlier NA43 experiment which demonstrated that linearly polarized light can be produced by firing a high-energy unpolarized electron beam into a crystal. The degree of polarization, however, was not determined. Measuring it will be the first goal of NA59 during a three-week run in 1999 using a 180 GeV electron beam. A second experiment to produce circularly-polarized light is planned for 2000 if the 1999 results are encouraging.

Circularly-polarized photon beams are not the only way to measure the gluon contribution to nucleon spin. The COMPASS experiment at CERN will soon begin to attack the question using a high-energy muon beam. The photon route, however, offers an attractive alternative with the advantage that for photons of 70 GeV and above, the experimental signature is particularly clear. In a polarized nucleon target, photons fuse with gluons to produce charm­anticharm quark pairs. Measuring the asymmetry between production rates for opposite polarizations of the target gives the gluon contribution to the nucleon’s spin. This means that despite the large attenuation of the beam caused by passing it through two crystals, the final measurement would still be competitive with alternative approaches.