Kaons have played an important role in many important discoveries in particle physics, including the hypotheses of parity violation by Lee and Yang in 1956 and of the charm quark by Glashow, Iliopoulos, and Maiani in 1970. The long-lived (KL) kaon's long lifetime and the ability to be generated copiously are characteristics which make kaons particularly useful to experiment.
Even though much exciting research is being done at the world's highest energy accelerators, the low energy kaon is still able to contribute to physics in three areas: (1) Precise tests of lepton flavor conservation; (2) Precise tests of CPT and of quantum mechanics; (3) Tests of CP violation in the only system in which this has been found. The lepton "flavors" refer to the three distinct ones: the electron, the muon, and the tau. They have identical properties, except that they differ widely in mass. No direct transitions have ever been observed between them.
The experiment in which we are involved is being performed at the Alternating Gradient Synchrotron at Brookhaven National Lab (BNL). It involves five institutions: the College of William and Mary, Stanford, the University of Richmond, the University of Texas at Austin, and UC Irvine. The head of the Irvine group is professor Bill Molzon and the group includes Mark Bachman, a post-doctoral researcher, and myself, Roy Lee, a graduate student. Further, we are seeking to replace two post-doctoral researchers who recently left the group.
The main focus of our experiment is to test lepton flavor conservation through the decay KL to muon plus electron. This decay mode has been searched for in past experiments with null results. The current upper limit on the branching ratio comes from BNL-791 (many groups on 791 are now involved with the current experiment, BNL-871) and is 3.3 × 10-11. Experiment 871 has data which should either observe an event or improve the upper limit by at least an order of magnitude.
If we hypothesize a new gauge boson to mediate such a process (this is necessary since none of the currently known interactions could mediate this process at this branching ratio), the current upper limit would imply a lower limit to the gauge boson's mass of 90 TeV (trillion electron volts). Thus, by probing very rare processes, we are in effect probing very high mass scales--much higher than anything that can be directly probed with existing accelerators. Since this decay (essentially converting a muon to an electron) is forbidden within the Standard Model, observation of this decay would be a startling indication of a very new physics.
Our spectrometer consists of two momentum analyzing magnets, twelve drift chambers, trigger scintillators, a Cerenkov counter, a lead glass calorimeter, and more scintillators and wire chambers at the end of the spectrometer to identify muons. UCI has built and operated four of the drift chambers and their associated electronics in addition to developing and maintaining a cluster of eight SGI computers for online event reconstruction.
UCI is heavily involved in the magnetic spectrometer (tracking) part of the analysis (as opposed to particle identification). Tracking involves determining charged particle tracks and their momenta. For the lepton flavor violating decay, a precise measurement of momentum is extremely important in eliminating background. Thus we measure a particle's momentum twice with the two spectrometer magnets.
In addition to taking part in the running of the experiment at Brookhaven during data taking periods (which last several months out of the year), the main focus of my contributions to the experiment has been in kinematic analysis. The aspects of the kinematic analysis to which I have contributed include the spatial alignment of the drift chambers, the development of computer code to determine track momenta, and analysis of the magnetic field map. These are crucial components of the experimental analysis, as we heavily rely on kinematic cuts to eliminate background. Finding the one flavor violating event out of 1012 total events (that's one trillion events) is not an easy task and requires precise measurements.
We took physics data in 1995 and in 1996 for a total period of 41 weeks. Based on preliminary results, we expect to see one KL-> muon + electron event for a branching ratio of 10-12. In addition, we will record about 6,000 KL-> muon + muon events at a branching ratio of 7 × 10-9 and expect a handful of KL-> electron + electron events, a standard model decay mode which has never been observed.
We are considering a continuation of the rare kaon decay program. Other proposals for the existing spectrometer include a search for the R0 hadron, a light particle having supersymmetric constituents, and a search for the H dibaryon, a particle composed of six quarks, including two strange quarks.
The World-Wide-Web page for the experiment, BNL E871.
