Development of an active converter pair spectrometer for the future search for μ+→e+γ

Plans for a future experiment to search for $\mu +\to e+\gamma$after the current MEG II experiment are underway. To achieve an order of magnitude higher sensitivity than the MEG II, further improvements in detector resolution, particularly for gamma-rays, are required. To address this, we introduce a pair spectrometer consisting of a converter layer, which converts gamma-ray into electron–positron pairs, and a tracker layer, which measures their momenta. Additionally, we plan to use active material for the converter layer, allowing it to measure both the energy deposition and timing of the converted particles.

As a prototype of the active converter, we developed LYSO crystals with dimensions of $3\times 5\times \text{50}\text{mm}{}^3$, coupled with silicon photomultipliers on both edges. We evaluated their light yield and timing resolution using a 3 GeV electron test beam. The results demonstrated an excellent timing resolution of 25–30 ps and a sufficient light yield of 5000–7000 photoelectrons, meeting the requirements for the future $\mu +\to e+\gamma$experiment.