Development of Electromagnetic Calorimeter Using LYSO Crystals for the COMET Experiment at J-PARC

An electromagnetic calorimeter (ECAL) has been developed for the COMET experiment at J-PARC in Japan, which searches for muon-to-electron conversion of a charged lepton flavor violating process. The observation of this process forbidden in the standard model (SM) gives a clear evidence of new physics beyond the SM.
The experiment aims at achieving a single event sensitivity of 10−17 by using a very intense pulsed proton beam and a dedicated beam line.
The detector system consists of an upstream tracker and the ECAL with an active area of about 1 m diameter. Aluminum is used for the muon stopping target and signal electrons are emitted with a fixed energy of 105 MeV. Decay-in-orbit (DIO) process, in which the electron energy reaches the signal region, also occurs as an irre- ducible background.
The ECAL plays an important role in the trigger, so required to have an excellent energy resolution of < 5% at 105 MeV to suppress contamination from the DIO electrons. In addition, a position resolution of < 1 cm and time response of < 100 nsec are required to distinguish each of pile-up events.
To meet those requirements, the ECAL consists of LYSO (Lu2−xYxSiO5) scintillating crystals with a dimension of 20 × 20 × 120 mm3. They have a high light yield and short decay constant compared to conventional inorganic scintillators.
Since the detector system is operated in a vacuum of < 100 Pa and magnetic field of 1 T, Avalanche Photo Diode with a sensitive area of 10 × 10 mm2 is adopted as the photodetector and its signals are processed by a fast low-noise preamplifier.
We constructed the final prototype before its construction and its performance was studied with an electron beam. The result showed an excellent linearity within 0.5%, energy resolution of 4.2%, and position resolution of 7.6 cm at 105 MeV. Details of the experimental result and the ECAL design including its whole system will be reported in this talk.