Speaker
Description
The process $\mu \to e\gamma$ is a charged lepton flavor violating (CLFV) decay that is forbidden in the Standard Model, but its measurable branching ratio is predicted by several new physics models. The current experimental limit on this decay has been set by the MEG II experiment at PSI, which will continue data collection until the end of 2026 with the target sensitivity of $Br < 6 \times 10^{-14}$ (90% C.L.). With the increased muon beam rate available at PSI starting in 2028, discussions are underway for a future experiment aiming to search for $\mu \to e\gamma$ with a sensitivity of $\mathcal{O}(10^{-15})$.
Achieving this level of sensitivity requires the development of photon detectors with excellent resolution and high rate capability. The number of background events increases with decreasing photon energy resolution, making this a critical factor. Therefore, the use of a pair spectrometer for photon detection, which offers better resolution and higher rate capability than traditional calorimeters, is under consideration.
The photon pair spectrometer works by converting photons into electron-positron pairs in a converter, followed by measurement of their momentum, position, and timing. To achieve optimal resolution with reasonable conversion efficiency, an active material for the converter is essential to allow energy deposit measurement. LYSO:Ce is a promising candidate due to its excellent properties, such as large light yield and fast response.
To evaluate the performance of a converter prototype consisting of LYSO and SiPM readout, a 3 GeV electron beam test was conducted at the KEK PF-AR Test Beam Line. Furthermore, to optimize the design of the active converter, comparisons were made between different LYSO sizes and SiPM readout methods, focusing on light yield and timing resolution.
The results show that the active converter using LYSO achieved excellent timing resolution (20–25 ps) across the entire crystal region, as well as detection of several thousand photoelectrons, exceeding the requirements for an active converter (40 ps and 700 photoelectrons). Therefore, LYSO is identified as a suitable material for the active converter of the photon pair spectrometer in the next-generation $\mu \to e\gamma$ search experiment.
