Fundamental Symmetry Measurements with Resonance Reactions
PNC and TRV Expts based on Nuclear Resonance Reactions
There are many rules governing how a physical system evolves: conservation of energy, conservation of momentum, conservation of angular momentum, etc. Given this collection of rules, one could examine all systems and extract "philosophical" fundamental principles which are embodied in all the rules. One arrives at three fundamental symmetries. Parity symmetry is invariance of the rules with respect to the choice of a left- or right-handed coordinate system. Time-reversal symmetry, deals with an invariance of the rules with respect to forward-backward flow of time. Charge conjugation symmetry involves an invariance with respect to the interchange of particles with antiparticles. A violation of any of the three symmetries requires a revision of the rules of physics.
Parity violation (PV) was first observed in 1957 in the b-decay measurements of C.S. Wu, and has since been observed in many nuclear and elementary particle reactions. The existence of parity violation was explained on a fundamental level by the Weak Interaction. Typically the strength of Weak Interaction is only ~10-7 the strength of "normal" interactions. However in nucleon scattering on heavy nuclei, the nuclear structure and dynamics serves to amplify the Weak Interaction to produce ~ 5 % effects upon observables. Large parity violation effects have been seen at resonances in the nuclei: 81Br, 111Cd, 117Sn, 139La, 232Th,and 238U.
Time-reversal violation (TRV) has never been directly observed, but is implied only in the decay of two particular elementary particles -- the neutral kaons. This isolated case makes it difficult to draw any direct conclusions about time-reversal symmetry. Time-reversal violation effects are thought to be extremely small (Å10-15 or less of the "normal" interactions), and some mechanism to enhance the signal is required.
Experiments are under development at Los Alamos National Laboratory, the Joint Institute of Nuclear Research, Dubna (USSR), and the KEK Laboratory (Japan). These experiments seek to gain a good understanding of parity violation before beginning the search for the much weaker time-reversal effects. Obviously there are experimental problems that must be overcome to do the measurements. There are also problems to consider in analyzing the data to extract the parity violation matrix elements. I have focused on the analysis problems over the past ~7 years.
Details of the PV kinematic enhancement were the focus of a series of papers in 1987, 1988, 1989, and 1993. S- and P-wave scattering amplitudes enter coherently to produce the observable effect in the cross sections. It was shown that the enhancements are sensitive to the amplitudes of the individual partial waves that contribute, and that in certain situations the effect reaches extremes. The interference effect is not always constructive -- in special cases the observable effects will vanish identically regardless of whether parity is violated or conserved.
Trident Scholar Paul Larsen investigated systematic difficulties associated with the neutron beam propagating through the target. Results are summarized in a paper published in Zeitschrift fur Physik. The lesson was clear: one always needs to know the resonance parameters well to extract a reliable value of the PNC matrix element. The importance of the detailed resonance spectroscopy was also indicated in the sensitivity of the PNC observables to experimental errors such as improper spin reversal. The sensitivity to incorrect spin reversal changes more than an order of magnitude depending on the resonance parameters. Depolarization effects also can be important. Since the details of the depolarization process depend not only on the characteristics of the target material, but also on the details of the specific sample, we simply note that depolarization can be included in the present approach.
Similar or related effects with sensitive dependence on the resonance parameters may be crucial in proposed time reversal invariance tests which utilize a polarized neutron beam and a polarized target. Effects such as acquired polarization and depolarization should be considered in depth for these experiments. Our results show that explicit inclusion of actual resonance parameters and detailed calculation of the dependence of experimental observables on these parameters provides additional insight into symmetry-breaking measurements. How to perform similar calculations for time-reversal violation experiments is not clear. Proper treatment of spin precession in the individual magnetic domains of the target is a major concern.
These calculations are closely tied to the status of the experiments at Los Alamos, Dubna, and KEK. Funding for future experiments at Dubna and Los Alamos looks bleak at present, but the KEK Laboratory in Japan is taking up the slack by starting construction on TRV apparatus. Experimental groups in the US and FSU are transferring their support to the KEK Laboratory. Experiments at KEK will come online in a few years. As the TRV experiments progress, it will be necessary to examine similar effects which will occur there.