^ Medium Energy Nuclear Physics Research A. M. Nathan,* D. H. Beck, P. T. Debevec, D. W. Hertzog, P. Kammel, N. C. R. Makins, S. E. Williamson, M. Bouwhuis, T. H. Chang, F. Ignatov, Z. Lu, C. J. G. Onderwater, P. Bailey, D. Chitwood, A. Danagoulian, F. Gray, R. Hasty, K. Nakahara, C. Polly, M. Roedelbronn, R. Roy, E. Schulte, G. Selvaggi, M. Sossong, L. B. Wang National Science Foundation, PHY 00-72044
The group doing experimental medium energy nuclear physics at Illinois pursues its studies at a variety of accelerator facilities throughout the world, using high-energy beams of electrons, photons, and muons.
Nuclear Physics Studies Using Beams of Photons at Jefferson Laboratory The primary experiment is high energy scattering of photons (Compton scattering) from the nucleon, the goal of which is to learn about the momentum distribution of quarks inside the proton. In particular, the Compton scattering process is one of the few reactions that is sensitive to correlations among the quarks. The experiment is a multi-institute collaboration led by Illinois. It involves the construction of a calorimeter and data acquisition system for detection of high-energy photons.
Nuclear Physics Studies Using Beams of Electrons There are two programs in which this research team is heavily involved. First is a program to measure the contribution of strange quarks to the vector current of the nucleon. These experiments utilize the parity-nonconserving interference between electromagnetic and weak neutral currents in the scattering of longitudinally polarized electrons from the proton. One of these experiments, called SAMPLE, has measured the strange quark contribution to the magnetic moment of the proton with data taken at the MIT/Bates Linear Accelerator. A far more ambitious program is the G0 experiment, which will take place at the new national electron facility, Jefferson Laboratory. The entire effort involves a large international collaboration led by the Illinois group.
Among other things, this group is responsible for the construction of the key piece of instrumentation that makes the experiment possible, a novel superconducting magnetic spectrometer. Second is a series of experiments (HERMES), which will measure the spin structure of the proton and neutron by scattering longitudinally polarized electrons from polarized protons and neutrons. This will allow researchers to determine how much each type of quark contributes to the spin of the nucleon. An exciting new initiative is an experiment to measure the contribution of the gluons to the spin of the proton. This involves upgrading the HERMES experiment to detect "charmed" mesons, an effort in which the Illinois group plays a major role. The experiments are under way at the DESY facility in Hamburg, Germany.
Atomic Parity Violation and Metastable Hydrogen/Deuterium Beam As a follow-up both to earlier work developing laser-driven polarized hydrogen/deuterium targets and the group's interest in parity-violating electron scattering and in studying tests of the Standard Model, researchers have initiated a program to develop a high-intensity thermal, metastable hydrogen beam. The primary goal is to study parity violation in atomic hydrogen in order to place stringent constraints on physics beyond the Standard Model.
Precision Measurement of the Anomalous Magnetic Moment of the Muon Measurements of the magnetic dipole moments of particles have played an important role in understanding the structure of matter. Deviations from the expected characteristics of "point-like" particles appear as so-called anomalous moments and are sensed by observation of the precession rates of such particles in magnetic fields. For protons and neutrons, anomalous magnetic moments are big, as expected for these particles, which are each built from three quarks. But for electrons and muons, the anomaly is tiny and so far is in agreement with theoretical expectations to an extraordinary degree of precision. This group is participating in a new experimental effort to measure the muon anomalous magnetic moment 20 times better than previous work; this will result in a test of the relevant quantity termed "(g-2)" to a level of 0.35 ppm. If achieved, the result will test the contributions of the weak interaction to the muon (g-2) factor, an essential component of the electroweak theory, which has not yet been detected experimentally.
Deviations from the theory may occur only by invoking new physics phenomena. The experiment is being mounted at the Brookhaven National Laboratory. The Illinois group has built the major detectors, constructed novel electronics simulation systems, and developed a unique electron traceback system from state-of-the-art particle-tracking devices. In addition, this group is spearheading the "Midwest analysis team" in the analysis of the data. Preliminary results have already been published. Data taking and analysis will continue for the next few years. Early in 2001, the collaboration reported a new result at the 1.3 ppm level, which disagrees with the Standard Model by 2.6 standard deviations. This result was widely reported as the first "kink" in the Standard Model in the 30 years of its existence. If confirmed, it will signal exciting new physics.
The Muon Lifetime Experiment The Standard Model is a concise description of elementary particles, quarks and leptons, and their interactions, the strong and electroweak. Experiments must provide three independent quantities to characterize completely the electroweak interaction, and these quantities are taken from the most precise measurements. The Illinois group has proposed to measure the muon lifetime to a higher precision and with this measurement determine the Fermi coupling constant to a precision of one part per million. The measurement will be done at the Paul Scherrer Institute (near Zurich, Switzerland).
Precision Measurement of Muon Capture in Hydrogen The aim of this new experiment is a 1% measurement of the capture rate of negative muons, which is a process that is sensitive to the so-called induced pseudoscalar coupling constant gP. The measurement will provide a rigorous test of theoretical predictions based on the Standard Model and Chiral Perturbation Theory. The experiment is being led in part by the Illinois group and is on a fast track for production running in 2002.
