Background identification system in MEG II experiment based on high-rate scintillation detector with SiPM readout

The MEG experiment has been searching for the lepton flavor violating process, μ⁺ arrow e ⁺γ, which is a clear evidence of new physics models beyond the Standard Model. The upgrade experiment (MEG II) is currently being prepared to obtain one order higher branching ratio sensitivity B < 5.0 × 10⁻¹⁴ by using the world's most intense muon beam up to ∼10⁸ μ⁺/ s and upgraded detectors with considerably improved performance. One of the keys for the upgrade is to suppress the background rate which is significantly increased with the higher muon decay rate. In the MEG II experiment, the Radiative Decay Counter (RDC) will be newly introduced for active background identification. The RDC is able to identify the most dominant background due to photons from Radiative Muon Decay and improve the sensitivity by 22%. In this paper, the concept of the RDC and its development are described.