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Determining how spin arises in the nucleon



In scattering experiments, the momentum transferred to a nucleon target from the incident electron is a primary characteristic of the interaction. Large momentum transfer reactions probe the fundamental quarks and gluons (collectively known as partons) that make up the nucleon.

At this high-energy scale, the scattering occurs over a very short time and spatial extent within the nucleon. The partons thus have little time to interact before the incident electron departs. As an analogy, consider a high-speed bullet shot through an apple: The apple retains its basic shape and characteristics for some short time after the bullet has passed. This is because the reaction has not had enough time to propagate throughout the apple. By examining the trajectory of the scattered bullet, an observer could learn something about the innards of the apple, whether it had a hard or soft core, for example, but not much about the whole apple. In a low momentum transfer reaction however, the partons behave collectively as a nucleon. To continue the earlier analogy, consider a pellet shot from a BB gun with much less energy, which simply bounces off the outer skin. In this case, a clever observer could learn something about the global properties of the apple, its mass for example, but not much about what is inside. Likewise, in low momentum transfer electron scattering, only the gross properties of the nucleon are seen. The characteristics of the partons within are not observed.

At this time, nuclear physicists have collected lots of information about the global properties of the nucleon and also about the individual characteristics of the partons. But we don't know exactly how to join these two pictures. JLab is uniquely positioned with one foot on the edge of the high momentum transfer region and the other on the edge of the low energy region. Experiment E94-010 took advantage of this fact to measure the extended Gerasimov-Drell-Hearn sum rule. "GDH" is one of the only spin-dependent observables that can be measured at arbitrary momentum transfer and can also be calculated by theorists from first principles.

Even more importantly, it has previously been measured at very high momentum transfer, and is also being investigated at very low (zero) momentum transfer by independent experiments. Observing how the GDH sum evolves in the intermediate range will be an important first step in understanding how partons form nucleons, and specifically how the nucleon's spin is formed from the partons and their interactions. High precision neutron data from this experiment and complementary proton data from Halls B and C have helped to stimulate a tremendous theoretical response, resulting in a series of international conferences dedicated to the GDH sum.

Experiment E94-010 (spokesmen G. Cates, J.P. Chen and Z.-E. Meziani) ran during the last three months of 1998 in Hall A and measured the polarized spin structure functions of the neutron. This international collaboration involved more than 100 physicists from 32 different institutions and led to 5 Ph.D.s. We evaluated the extended GDH sum as a function of momentum transfer, and found a smooth but dramatic transition from the value previously measured at high energy. Intriguingly, at the lowest measured energy of this experiment, where QCD-based calculations are available, our data is at odds with the expectation from theory. This has presented a significant challenge to the theorists, and should help to refine our understanding of the neutron's spin. From the data, we were also able to evaluate the size and strength of the quark-gluon interactions.

To gain access to the neutron, the collaboration built the JLab polarized helium-3 target. Free neutrons decay on average in about 15 minutes, but are stable within a nucleus such as helium. Helium-3 was chosen because it consists of two protons and a single neutron. When polarized, the protons align themselves in such a way that their spin-dependent properties nearly cancel, leaving the single unpaired neutron. The E94-010 target is the highest luminosity polarized target in the world, and by providing a convenient source of highly polarized neutrons, it has opened up an exciting new avenue of polarized studies at JLab.

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