Valparaiso University's

contributions to the MEGA experiment

 

The Valparaiso University group joined this experiment in 1986 with the expectation that the design and construction of the detector would be much less difficult than it proved to be in practice. In addition, the analysis of the very large body of data collected turned out to be considerably more complicated than anticipated owing to detector noise which complicated pattern recognition and to resolutions which were consequently degraded. These effects contributed to a protracted time scale for the completion of the experiment and conspired to prevent us from reaching the ambitious result which appeared in our proposal. Nonetheless, in the end the experiment was a triumph in detector technology and a success in physics results.

While the exact date for the completion of the MEGA experiment was not known at the time the 1996 proposal was submitted for the present three year period, it was expected that MEGA would be concluding its analyses in this three year period, as indeed we have done. As a result, the effort our group would make toward the completion of the MEGA experiment would gradually diminish over this three year grant period, freeing us to devote more time to our other commitments as these were needed.

In the course of the preparations to bring this experiment from its conceptual stages to its completion, the Valparaiso University group, including scientific staff, technical specialist, and undergraduate students have assumed numerous responsibilities. At the conclusion of this extended experiment it may be worth noting some of the many contributions which our group has made.

  • We designed, tested, and implemented a Labview-based controlling program on a Macintosh computer for the LeCroy 1445 high voltage system used for the positron detector MWPC's through our three years of data collection. The controller managed and monitored the voltages and currents simultaneously on all positron wire chambers. Using a graphical user interface, this program set high voltages and current limits for all of the MWPC's. It recorded all chamber over-current trips and all voltage changes chronologically. The program automatically, and slowly issued commands to ramp up the voltage on each MWPC and managed programs of gradual chamber conditioning to increasing high voltages. Alarms were sounded to alert experimenters to the possible need for user intervention and/or for recording specific events in the log book. The controller program proved to be entirely successful in meeting the design goals for this function and, compared to an earlier network of individual manually controlled high voltage supplies, this essentially made the management of this critical aspect of the MWPC operation possible.

 

  • We designed, tested, and fabricated all of the muon stopping targets for the MEGA experiment. The unique planar elliptical design, which required precise positioning with minimal mass support, prompted the development of a special fabrication technique. The design was established along with several colleagues at Los Alamos National Laboratory and all of the fabrication and testing was done at Valparaiso University. The targets met all of the design criteria for the experiment and included those used for the MEGA running periods as well as special targets to measure resolutions. An article describing the unique target design and fabrication is in preparation and will be submitted to Nuclear Instruments and Methods A in 1999.

 

  • We have made major contributions to the research, development, and fabrication of the unique MWPC positron detectors. Profs. Koetke and Manweiler spent sabbatical leaves at LAMPF working on this project, and Prof. Stanislaus was a central figure in the successful completion of this work. A Valparaiso University undergraduate student was instrumental in the unique design and testing of glass anode wire restraints which were implemented to ensure electro-mechanical stability of the positron detector MWPC's. Without a doubt the greatest technical challenge in the experiment, the fabrication, testing, and handling of the wire chambers proved to be a major hurdle on our path to mount the experiment. The details of the chamber design, fabrication, testing, and performance can be found in reference [42].

 

  • We took full responsibility for the development of an online filter pattern recognition algorithm to make decisions in real time on whether to retain triggered events or to discard them. The algorithm needed to be fast, reliable, and provide an unbiased rejection of triggers which failed to have sufficient information for a candidate. The speed with which these decisions could be made was the ultimate throttle on the speed with which data could be collected. The Valpariaso University group developed the algorithm, wrote all of the requisite software, tested it extensively with the Monte Carlo codes (which we modified and improved substantially) and installed and maintained it throughout our data collection in 1993-1995. Details of this online filter will appear in our Physical Review D paper.

 

  • We have written all of the codes for the analysis of all cosmic ray data used for positron detector performance studies and have analyzed all of these cosmic ray data taken over the course of the experiment. These data have resulted in MWPC wire-by-wire efficiency measurements , high voltage plateau curves for each MWPC, delay curves for MWPC gates relative to the trigger time, threshold settings, and precision chamber alignment studies and delay line calibrations for the photon spectrometer [42].

 

  • We have carried out extensive data analyses using cosmic rays and Michel positrons to achieve precision intra-detector and inter-detector alignment, essential for obtaining good position and energy resolutions [42].

 

  • We were responsible for the systematic determination of the denominator in the branching ratio, i.e., the number of useful muons which stopped and decayed in our target for our entire data sample. This required a detailed study of all of the data runs taken over the three year data collection period to estimate the various detector inefficiencies, variations in the detector acceptances, corrections for deadtimes, effective beam related effects, etc. This was a major focus during the past 12 months as we were bringing the data analysis to a conclusion and were calculating our final measured branching ratio.

 

  • We have done most of the Monte Carlo studies required for our estimation of the detector acceptances and resolutions over the past 12 months because many of these estimates were dependent on using the very last analysis codes applied to the data and to the Monte Carlo.

 

  • We took responsibility for developing a track reconstruction code which is based on "swimming" the positron through the non-uniform magnetic field of the MEGA magnet. This code was relatively slow and was used on a highly selected data set representing the best candidates from the experiment. This resulted in substantially better resolutions for the positron momentum and position at the muon decay point in the target.

 

  • We have given several of the talks presented at scientific meetings, the most recent being at the Symposium on Flavor Changing Neutral Currents - FCNC-97. Of the papers published by the MEGA collaboration, one of us (DDK) was the authoring editor for the comprehensive article on the positron spectrometer [42].

Over the course of our work on the MEGA experiment, ten Valparaiso University undergraduate students have participated in this experiment, some for more than one summer. Nine have gone on to graduate school, and are either in Ph.D. programs or have completed their degree. We call particular attention to Mr. Keith Stantz who graduated from Valparaiso University in 1988 and who has now completed his Ph.D. at Indiana University, writing his thesis on a preliminary measurement of the branching ratio for the MEGA experiment based on approximately 14% of the data. His analysis methods and specific codes were used to analyze the much larger body of data to complete the experiment.

The nature of this experiment, i.e., a search for a single rare decay process, with essentially no additional physics measurements to come from the experiment, results in very few physics publications. Our commitment to remain with the MEGA experiment has thereby limited our list of publications in preceding years. The experiment was judged by external reviewers to be of exceptional scientific value as a test of the Standard Model and a means of setting limits on extensions to the Standard Model. The scientific merit is as valid today as it was in 1986. Our collaboration has demonstrated that an experiment of this precision can be done, but it is by no means straightforward. What we have learned in the doing may be as important to the scientific community as the result, as others contemplate the next search for .

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