Development of the StrECal system for COMET experiment at J-PARC

The COMET experiment at J-PARC aims to search for the charged lepton flavor violating process of neutrinoless μ-e conversion with an improvement of a sensitivity by a factor of 10000 to the current limit, in order to explore the parameter region predicted by most of well-motivated theoretical models beyond the Standard Model. When the μ-e conversion occurs, almost all the energy of the muon mass is carried out by the electron which is expected to have the monochromatic energy of about 105 MeV. The experiment requires to detect such electron with an excellent momentum resolution, better than 200 keV/c, in order to achieve the goal sensitivity. On the basis of these requirements, we plan to construct a StrECal system which consists of a straw tube tracker and an electromagnetic calorimeter (ECal). The system is put in the detector solenoid with a field strength of 1 T. The straw tube tracker detects the particle momentum with the operation in the vacuum. The tracker is constructed by the extremely light material, which can achieve the high momentum resolution. The ECal consists of segmented scintillating crystals, which is placed downstream of the straw tube tracker. The ECal measures the energy of electron with good resolution to add redundancy to the momentum measurement, and also provides the trigger signals, carrying the timing with respect to which the events are referenced. Furthermore, particle identification is done by using both information of straw tracker and ECal. The StrECal system has been developed step by step. In this paper, we report the current status of the development of the StrECal system. (author)