Performance of an LYSO-Based Active Converter for a Photon Pair-Spectrometer aiming for 52.8 MeV photon detection in Future $μ^+ \to e^+ γ$ Search Experiments

TL;DR

This work addresses the challenge of detecting 52.8 MeV photons with high timing ($\Delta t_\gamma$) and energy ($\Delta E_\gamma$) precision in next-generation muon-to-electron-gamma searches. It presents a photon pair-spectrometer concept employing an active LYSO converter, optimized via Geant4 simulations to maximize signal efficiency while controlling energy leakage and topology effects, and demonstrates that LYSO offers sufficient light yield and fast response. Beam tests at KEK PF-AR validate timing performance near $\sim$25 ps for single-MIP events and confirm light yields around $10^4$ photoelectrons, exceeding the required $\sim$700–2500 p.e. targets across tested conditions. The results indicate that the LYSO-based active converter can meet the stringent timing and energy-resolution goals ($\Delta t_\gamma<30$ ps, $\Delta E_\gamma<200$ keV) for a scalable photon-pair spectrometer, though practical deployment will require careful calibration, assembly control, and channel-count management. Overall, the study establishes LYSO as a viable active-converter material for high-precision photon detection in future $\mu^+ \to e^+ \gamma$ experiments.

Abstract

For future $μ^+ \to e^+ γ$ search experiments with a branching-ratio sensitivity of $10^{-15}$, we are developing a photon pair-spectrometer employing an active LYSO converter, aiming at target resolutions of 30 ps in timing and 200 keV in energy measurement for detecting 52.8 MeV photons. The converter generates electron-positron pairs from incident photons while simultaneously measuring their energy deposition and timing. On the basis of simulation studies, we optimized the converter thickness and segment dimensions, and accordingly fabricated prototype LYSO segments. Their single-MIP detection performance was evaluated using an electron beam at the KEK PF-AR test beamline. The prototypes exhibited excellent performance, achieving a time resolution of 25 ps and a light yield of $10^4$ photoelectrons, both substantially surpassing the design requirements.

Figures (35)

• Figure 1: Principle of energy and timing measurement by photon pair-spectrometer with active converter.
• Figure 2: A schematic overview of a possible future $\mu^+ \to e^+ \gamma{$μ^+ e^+ γ $}$ experiment detector layout. A magnetic field is applied in the $z$-axis, defined in the figure.
• Figure 3: Simulated spectrum of the reconstructed signal photon energy, assuming 3mm-thick LYSO converter. The blue dotted line shows the momentum sum of the conversion pair in the pair-tracker. The red line represents the reconstructed energy obtained by adding the energy deposited in the converter to the momentum sum.
• Figure 4: Simulated $E_\gamma$ spectra for signal photons converted in LYSO at different depths $x$, defined as the distance from the conversion point to the surface on the pair-tracker side. The red line corresponds to events with $0mm < x < 1mm$, while the blue line corresponds to events with $5mm < x < 6mm$.
• Figure 5: Simulated $\varepsilon_{\mathrm{phys}}$ value as a function of converter thickness for various materials.