Construction of Vacuum-compatible Straw Tracker for COMET Phase-I

The COMET experiment at J-PARC aims to search for a lepton-flavour violating process of muon to electron conversion in a muonic atom, µ-e conversion, with a branching-ratio sensitivity of <10−16, 4 orders of magnitude better than the present limit, in order to explore the parameter region predicted by most of well-motivated theoretical models beyond the Standard Model.
The need for such an excellent sensitivity places several stringent requirements on the detector;
i) good momentum resolution, <2%, for 100 MeV/c electron, which is primarily limited by multiple scattering effect for this momentum region,
and
ii) high rate capability, up to 5 × 109µ−/s muon beam by J-PARC.
In order to fulfill such requirements, we decided to develop the straw-base planar tracker which is operational in vacuum and made of an extremely light material. The COMET straw tracker consists of 10 mm diameter straw tube, longer than 1 m length, with 20µm-thick Mylar foil and 70nm-thick aluminum cathode. In the previous conference VCI2016, the R&D for this vacuum-compatible straw tracker was reported. After that, two big milestones, detector-performance verification by the full-scale prototype with 100 MeV/c e−beam, and start the assembly of final straw tracker for COMET Phase-I, were achieved.
In VCI2019, we report these two big milestones, result of beam test and status of final assembly, and some prospects towards the COMET Phase-II.