Tracing CP-violation in Lepton Flavor Violating Muon Decays

Although the Lepton Flavor Violating (LFV) decay $\mu^+\to e^+ \gamma$ is forbidden in the Standard Model (SM), it can take place within various theories beyond the SM. If the branching ratio of this decay saturates its present bound [{\it i.e.,} Br$(\mu^+ \to e^+\gamma)\sim 10^{-11}$], the forthcoming experiments can measure the branching ratio with high precision and consequently yield information on the sources of LFV. In this letter, we show that for polarized $\mu^+$, by studying the angular distribution of the transversely polarized positron and linearly polarized photon we can derive information on the CP-violating sources beyond those in the SM. We also study the angular distribution of the final particles in the decay $\mu^+\to e^+_1 e^- e^+_2$ where $e^+_1$ is defined to be the more energetic positron. We show that transversely polarized $e_1^+$ can provide information on a certain combination of the CP-violating phases of the underlying theory which would be lost by averaging over the spin of $e^+_1$.Comment: 6 pages, 2 figure

The MEGA Advanced Compton Telescope Project

The goal of the Medium Energy Gamma-ray Astronomy (MEGA) telescope is to improve sensitivity at medium gamma-ray energies (0.4-50 MeV) by at least an order of magnitude over that of COMPTEL. This will be achieved with a new compact design that allows for a very wide field of view, permitting a sensitive all-sky survey and the monitoring of transient and variable sources. The key science objectives for MEGA include the investigation of cosmic high-energy particle accelerators, studies of nucleosynthesis sites using gamma-ray lines, and determination of the large-scale structure of galactic and cosmic diffuse background emission. MEGA records and images gamma-ray events by completely tracking both Compton and pair creation interactions in a tracker of double-sided silicon strip detectors and a calorimeter of CsI crystals able to resolve in three dimensions. We present initial laboratory calibration results from a small prototype MEGA telescope.Comment: 7 pages LaTeX, 5 figures, to appear in New Astronomy Reviews (Proceedings of the Ringberg Workshop "Astronomy with Radioactivities III"

Development of an XSPEC-Based Spectral Analysis System for the Coded-Aperture Hard X-ray Balloon Payload EXITE2

We present the spectral analysis system for the second-generation Energetic X-ray Imaging Telescope Experiment (EXITE2) balloon payload. EXITE2 is an imaging hard X-ray telescope using a coded-aperture mask and a NaI/CsI phoswich detector operating in the energy range 20--600 keV. The instrument was flown on a high-altitude scientific balloon from Ft. Sumner, NM on 1997 May 7-8. We describe the details of the EXITE2 spectral analysis system, with emphasis on those aspects peculiar to coded-aperture instruments. In particular, we have made our analysis compatible with the standard X-ray spectral fitting package XSPEC by generating a response matrix in the appropriate format including all the effects of a coded-aperture system. The use of XSPEC, which may be a first for coded-aperture data, permits great flexibility in the fitting of spectral models. The additional effects of our phoswich system, or any other detector-specific considerations, may be easily included as well. We test our spectral analysis using observations of the Crab Nebula, and find that the EXITE2 Crab spectrum is consistent with those recorded by previous instruments operating in this energy range.Comment: 17 pages LaTeX, 6 figures, accepted to Astroparticle Physic

A Window on the CP-violating Phases of MSSM from Lepton Flavor Violating Processes

It has recently been shown that by measuring the transverse polarization of the final particles in the LFV processes $\mu \to e\gamma$, $\mu \to eee$ and $\mu N\to e N$, one can derive information on the CP-violating phases of the underlying theory. We derive formulas for the transverse polarization of the final particles in terms of the couplings of the effective potential leading to these processes. We then study the dependence of the polarizations of $e$ and $\gamma$ in the $\mu \to e \gamma$ and $\mu N \to e N$ on the parameters of the Minimal Supersymmetric Standard Model (MSSM). We show that combining the information on various observables in the $\mu \to e\gamma$ and $\mu N\to e N$ search experiments with the information on the electric dipole moment of the electron can help us to solve the degeneracies in parameter space and to determine the values of certain phases.Comment: 16 pages, 8 figure

Calibration of the Gamma-RAy Polarimeter Experiment (GRAPE) at a Polarized Hard X-Ray Beam

The Gamma-RAy Polarimeter Experiment (GRAPE) is a concept for an astronomical hard X-ray Compton polarimeter operating in the 50 - 500 keV energy band. The instrument has been optimized for wide-field polarization measurements of transient outbursts from energetic astrophysical objects such as gamma-ray bursts and solar flares. The GRAPE instrument is composed of identical modules, each of which consists of an array of scintillator elements read out by a multi-anode photomultiplier tube (MAPMT). Incident photons Compton scatter in plastic scintillator elements and are subsequently absorbed in inorganic scintillator elements; a net polarization signal is revealed by a characteristic asymmetry in the azimuthal scattering angles. We have constructed a prototype GRAPE module containing a single CsI(Na) calorimeter element, at the center of the MAPMT, surrounded by 60 plastic elements. The prototype has been combined with custom readout electronics and software to create a complete "engineering model" of the GRAPE instrument. This engineering model has been calibrated using a nearly 100% polarized hard X-ray beam at the Advanced Photon Source at Argonne National Laboratory. We find modulation factors of 0.46 +/- 0.06 and 0.48 +/- 0.03 at 69.5 keV and 129.5 keV, respectively, in good agreement with Monte Carlo simulations. In this paper we present details of the beam test, data analysis, and simulations, and discuss the implications of our results for the further development of the GRAPE concept.Comment: 35 pages, 14 figures, accepted for publication in NIM-

Testing and simulation of silicon photomultiplier readouts for scintillators in high-energy astronomy and solar physics

Space-based gamma-ray detectors for high-energy astronomy and solar physics face severe constraints on mass, volume, and power, and must endure harsh launch conditions and operating environments. Historically, such instruments have usually been based on scintillator materials due to their relatively low cost, inherent ruggedness, high stopping power, and radiation hardness. New scintillator materials, such as LaBr3:Ce, feature improved energy and timing performance, making them attractive for future astronomy and solar physics space missions in an era of tightly constrained budgets. Despite this promise, the use of scintillators in space remains constrained by the volume, mass, power, and fragility of the associated light readout device, typically a vacuum photomultiplier tube (PMT). In recent years, silicon photomultipliers (SiPMs) have emerged as promising alternative light readout devices that offer gains and quantum efficiencies similar to those of PMTs, but with greatly reduced mass and volume, high ruggedness, low voltage requirements, and no sensitivity to magnetic fields. In order for SiPMs to replace PMTs in space-based instruments, however, it must be shown that they can provide comparable performance, and that their inherent temperature sensitivity can be corrected for. To this end, we have performed extensive testing and modeling of a small gamma-ray spectrometer composed of a 6 mm×6 mm SiPM coupled to a 6 mm×6 mm ×10 mm LaBr3:Ce crystal. A custom readout board monitors the temperature and adjusts the bias voltage to compensate for gain variations. We record an energy resolution of 5.7% (FWHM) at 662 keV at room temperature. We have also performed simulations of the scintillation process and optical light collection using Geant4, and of the SiPM response using the GosSiP package. The simulated energy resolution is in good agreement with the data from 22 keV to 662 keV. Above ~1 MeV, however, the measured energy resolution is systematically worse than the simulations. This discrepancy is likely due to the high input impedance of the readout board front-end electronics, which introduces a non-linear saturation effect in the SiPM for large light pulses. Analysis of the simulations indicates several additional steps that must be taken to optimize the energy resolution of SiPM-based scintillator detectors