J. Miller, B.L. Roberts
The laws of nature are generally invariant under the symmetry operations of parity (mirror reflection), charge conjugation (matter --> antimatter), and time reversal, usually indicated by P, C, and T. However, P is violated maximally by the weak force. CP is violated at the level of 0.1% by the weak interaction, and has only been observed in the decays of neutral kaons.
Finding the source of CP violation is one of the most important goals of particle physics. The Medium-Energy group has been participating in the CP Lear experiment at CERN which seeks to measure several manifestations of CP violation in the neutral kaon system. The technique is to study differences in partial decay rates for initial Ko and Ko beams. Most previous experiments instead compare KS and KL beams. Some of these measurements will be improvements on previous experiments, such as the measurement of (the difference between the +- phase and the 00 phase), while others will be first-time measurements, such as the first observations of CP violation in 3-pion decays and the first direct observation of T invariance violation. We have completed the first two of four years of data collection. The Boston University group designed and built the time-of-flight detectors and electronics for this experiment.
E. Booth, J. Miller
The photon scattering group has designed a world-class high-resolution sodium-iodide detector, located in Saskatoon, Canada, which has recently been used to detect gamma rays in the 150 MeV to 300 MeV region in a series of experiments. (The Saskastoon accelerator is a 300 MeV high duty factor electron machine.) These experiments measure the elastic and inelastic scattering patterns made on targets of hydrogen, deuterium, helium, and carbon. These are pioneering measurements designed to study the dynamics of the strong interaction of the delta(1232) resonance particle with nuclear matter and to extend our understanding of the mechanisms for resonance and non-resonance photon scattering from the pion production threshold to the peak of the delta(1232) resonance.
Two other measurements have just been made at Saskatoon. In one of these, our detector was used with 64 lead-glass detectors (supplied by Boston University) to measure the angular distribution of neutral pions produced from carbon near the pion threshold. This data measures the effect of the nuclear medium on pion production. In another experiment, the sodium iodide detector was used to measure the production of negative pions very near the threshold in deuterium. This data can check low-energy theorems which are very general in application but were recently put in some doubt by measurements of the neutral pion production near threshold. Our high-resolution detector was used to measure the capture gamma ray when the pion stopped in the thick target.
Our group will participate with the University of Illinois and the University of Alberta in a second experiment on deuterium. This research will attempt to detect a coincidence neutron and make a measurement of the neutron's polarizability. This will complement our previous measurement of the proton's polarizability. Boston university is also involved in an extensive program of neutral pion production experiments to be carried out over the next few years using the lead glass array, again with the motivation of testing the low-energy theorems.
E. Booth, J. Miller
The high-resolution sodium-iodide detector (see above) has been used, and will continue to be used, at the TRIUMF laboratory in Vancouver, Canada. TRIUMF makes intense beams of negative pions which can be captured in flight on nuclear targets with the subsequent emission of a high-energy gamma ray. This is the inverse process to the pion production reactions which are done at Saskatoon, and thus gives complementary information. Experiments have been performed on oxygen, and on both normal and polarized hydrogen at pion energies from near-zero to 400 MeV. The resulting new information on the photo pion cross sections for the neutron adds to our sparse knowledge of that fundamental process. This data is still in preparation for publication. The next experiment at TRIUMF will be negative pion capture on deuterium, with the detection of the outgoing neutron. The goal is a high-precision measurement of the neutron-neutron scattering length, which will lead us to test different quark model predictions. This experiment should run in 1994-95, while other experiments, such as pion capture in very heavy nuclei, are under discussion.