This research was sponsored by the National Science Foundation Grant NSF PHY-931478

Knott, J.E.

Stantz, K.M.

Szymanski, J.J.

English

IUScholarWorks

In addition, recent studies have shown very long relaxation times in polarized solid xenon.' 
If xenon is shown to exhibit large parity violating effects, then a polarized xenon target 
could be an attractive target in which to search for time reversal violating effects. 
The Indiana group is responsible for constructing the xenon target. The target will 
consist of 1 kg of liquid xenon of natural isotopic abundance immersed in a 120 g holding 
field for maintaining the neutron polarization. Target design is nearing completion, and 
tests of the target will begin in the summer of 1994. The experiment is projected to run 
at LANCSE in 1995. 
1. C. M. Frankle, et al., Phys. Rev. Lett. 67, 564 (1991). 
2. M. Gatzke, et al., Phys. Rev. Lett. 70, 690 (1993). 
MEGA: A SEARCH FOR THE DECAY p -+ ey 
J.E. Knott, K.M. Stantz, and J.J. Szvmanski 
Indiana University Cyclotron Facility, Bloomington, Indiana 474 08 
We continue to play a major role in the MEGA rare muon-decay experiment at 
LAMPF. The experiment is now in production, with runs having taken place or scheduled 
during the summers of 1992-1994. We now have sufficient data that, after a complete 
analysis, the sensitivity of the MEGA experiment to the decay mode p -+ ey is a factor 
of 12-15 better than the existing best measurement. We have also taken data to improve 
the measurement of the muon decay parameter p by a factor of three or better. The new 
world-best measurement of p + ey is the thesis project of IU student Keith Stantz. 
In previous years, the IUCF group has built substantial portions of the hardware for 
the first- and second-level trigger systems. The hardware was installed in spring, 1992 
and has run for the last two years with few problems. Our group has the responsibility to 
install, debug and maintain the entire hardware trigger system. One of us (KMS) has also 
had a leadership role in the installation and commissioning of the electron spectrometer 
multiwire proportional chambers (MWPCs). These chambers, as discussed below, are truly 
state-of-the-art in terms of their rate capability and their low mass (3 x radiation 
lengths). (Note that in this report we use the word electron to represent e+ or e-). Our 
final hardware responsibility was to diagnose problems in the (more than 130) FASTBUS 
modules, and to calibrate the FASTBUS TDC modules. 
During 1993, we received funding from The Foundation to upgrade the computing 
power of the MEGA workstation farm. The workstation farm provides the third-level 
(software) trigger for the experiment. We purchased upgrades to six existing DECstation 
5000-200 workstations, as well as purchasing one new DECstation 5000-240 and memory 
upgrades to the DECstations and the VAX-station used for data logging. The workstation 
farm worked well during the 1993 data-taking run and provided sufficient computing power 
for the t hird-level trigger algorithms. 
Because the MEGA experiment is now concentrating on production dat a-t aking, and 
not hardware development, the Indiana Group has moved strongly into data analysis tasks. 
Our analysis responsibilities include: precision location of the target with respect to the 
electron chambers, analysis of the electron arm chambers and scintillators for the high-rate 
p -+ ey production data, analysis of the muon inner-bremstrahlung decay mode p -+ ey vF, 
and extraction of the final p -+ ey branching ratio (or limit). 
The summer, 1992 and 1993 data-taking runs were quite successful, and proved that 
the MEGA detector works. All of the positron detectors were in place and, during the 1993 
run, operated at high rates. The stop rate used for production data-taking was 250 MHz, 
which is a rate on the chamber of 104/mm2/s (or, in excess of 3 MHz per wire!). The 
chambers performed well in these high-rate conditions and it appears that only 10% of 
the events have contamination of the tracks by rate-induced bursting effects. In addition, 
all three of the photon, pair spectrometers were in place in 1993 and performed well. 
Production data were taken for 4 weeks during 1993. 
A portion of each summer's run was used to measure the Michel p parameter, which 
describes the energy spectrum of positrons emitted in muon decay. Our present data set 
should improve the measurement precision of this parameter by a factor of 3. The p pa- 
rameter is sensitive to non-V-A terms in the weak interaction; for the left-right symmetric 
model, the p parameter sets a limit on the mixing angle between left- and right-handed W 
bosons. 
The p-parameter measurement is done by precisely measuring the shape of the posi- 
tron energy spectrum from muon decay. Analysis of the p-parameter data set is proceeding. 
Of particular concern are the effects of scintillator and chamber efficiencies on the shape 
of the energy spectrum. Monte Carlo studies of these effects are an important component 
of the analysis. 
The decay p -+ ey is separated from backgrounds by determing that the e and y are 
produced at the same time, at the same point, are produced back-to-back, and each have 
an energy equal to half the muon rest mass, or 52.8 MeV. Thus, the parameters that the 
data analysis must determine are the energy, direction and decay time for each electron 
and photon considered in the analysis. In addition, the resolution functions in electron 
and photon energy, direction and decay time need to be determined (actually, it is the 
time difference between the electron and photon decay times that is relevant). 
The analysis of the p -+ ey data set is divided into the photon spectrometer analysis 
and electron spectrometer analysis. The two analyses have the goals of measuring the 
energy, direction and decay time of the photon and electron. Figs. 1 and 2 show magni- 
fied views of the photon and electron spectrometers, respectively, for the same p + ey 
production data event. The photon spectrometer works by converting a photon into an 
e+ e- pair. The e+ and e- are then tracked in chambers and scintillators. The event in 
Fig. 1 shows a fit to the chambers hits for the first loop of the trajectory of the e+ and the 
e-. Subsequent loops are smaller, and give the chamber hits shown within the first loop. 
The event in Fig. 2 shows the electron spectrometer hits. The track superimposed on the 
hits is the trajectory the electron must follow, according to the information measured in 
Figure 1. An end view of a p -+ ey production data event as seen in the photon spec- 
trometer. Hits in drift chambers (small square boxes), MWPCs (rectangles with crosses 
inside) and scintillators (rectangles within rectangles) are shown. The two tracks are fits 
to a subset of the drift chamber hits, as discussed in the text. 
the photon spectrometer and if the event is a p - ey decay. These two figures show the 
typical environment within which the tracking algorithms need to work. 
Because we are doing a substantial portion of the electron spectrometer analysis at 
Indiana, we are interested in good graphics tools to display event information. The electron 
trajectories are helices, and we decided that three-dimensional display of the helices would 
be helpful in underst anding the reconstruction algorithms and the general quality of the 
data. To this end, we have developed software that uses the Advanced Visualization System 
(AVS) from the company of the same name. AVS allows the user to define objects, which 
AVS then manipulates in an extremely flexible way (e.g., rotation, translation, coloring, 
and the transparency of objects is done by AVS). At this point, we have displays of the 
electron spectrometer and associated detector hits in 3-D form. Devlopment is continuing 
on this system, to make it a complete single-event display for the electron spectrometer. 
Figure 2. An end view of a p - ey production data event as seen in the electron spec- 
trometer. Hits in MWPCs (crosses on the circular chamber outlines) and scintillators 
(small parallelograms) are shown. The track is the trajectory the electron must follow, 
according to the information measured in the photon spectrometer, if the event is a p -+ ey 
decay. 
Reconstruction algorithms are undergoing development. Very preliminary results are 
shown in Figs. 3 and 4 for the photon and electron energy spectra. The photon energy 
spectrum is characteristic of inner bremsstrahlung decay of the muon, and drops at high 
energy. The recontruction algorithms are improving steadily. The electron energy spec- 
trum is characteristic of the normal Michel decay of the muon, p -+ evp, and would have 
a peak at 52.8 MeV, the fall-off point. Efficiency considerations reduce the spectrum near 
the fall-off. The fall-off rate at 52.8 MeV is intrinsically sharp, and is used to measure 
the electron energy resolution. This resolution is improving steadily as the recontruction 
algorithms are improved and magnetic-field non-uniformities are taken into account. 
The timing resolution function of the MEGA detector is measured using the decay 
p --+ eyvp (muon inner-bremsstrahlung decay). Figure 5 shows a spectrum of the time- 
' 11bF 
Entriee 13733, 
Mean 42.97 . 
RMS 7.042- 
- - 
- - 
- - 
- - 
- 
- 
- - 
q-  ,. ,;
20 30 40 50 60 70 80 90 IOU 
LY 1 VISIBLE ENERGY (MeV) 
Figure 3. Photon energy spectrum taken with p -+ ey production data. The photons in 
this spectrum are produced in the decay p -+ eyvc. 
ENERGY ( M ~ v )  
Figure 4. Electron energy spectrum taken at low rates. The electrons are produced in 
the decay p --+ evfi and have a peak at high energy, which is characteristic of the Michel 
spectrum. 
TIME (ns) 
Figure 5. Time difference spectrum taken between the photon and positron spectrome- 
ters. 
difference taken between the positron and photon spectrometers for inner-bremsst r ahlung 
decay. The peak at zero time difference clearly shows the presence of the muon inner- 
bremsstrahlung decay in the data set. The observation of this decay mode proves that the 
detector is capable of measuring a process that has a single muon decaying into a positron 
and a gamma, as for p += eey. The resolution is expected to improve to 1 ns or better after 
walk corrections in the positron scintillators are made and the optimal use of all scintillator 
times is included. 
The experiment will continue to take data in 1994. The only major hardware change 
is to the electron spectrometer MWPCs, where the readout electronics has been changed 
according to the specifications of Indiana Group member Keith Stantz. The modifications 
were made to improve the rate capability of the electron spectrometer. By running for a 
longer time, running more efficiently and at higher rates, and with a higher proportion of 
good events, we expect to acquire twice as much data in 1994 as we have from 1993. This 
will lead to a sensitivity, after analysis, 40-50 times better than the existing world-best 
measurement. 
The MEGA Collaboration consists of the following member institutions: UCLA, Univ. 
of Chicago, Fermilab, Hampton Univ., Univ. of Houston, Indiana Univ., Los Alamos Na- 
tional Laboratory, Queens Univ., Stanford Univ., Texas A&M Univ., Univ. of Virginia, 
Virginia Polytechnical Inst. and State Univ., Univ. of Wyoming, and Yale Univ. 


Mega: A Search for the Decay μ → e γ

https://scholarworks.iu.edu/dspace/bitstream/2022/516/1/iucf1993-1994_72-77.pdf

Mega: A Search for the Decay μ → e γ

Abstract

Similar works

Full text

Available Versions

IUScholarWorks