Construction and performance tests of the COMET CDC

A cylindrical drift chamber (CDC) has been constructed for the COMET Phase-I experiment where a lepton flavor violating process, the muon-to-electron conversion, is searched for with a sensitivity of 3×10−15 at J-PARC. The CDC design was optimized to measure 105-MeV/c electrons as well as suppressing unwanted low-energy particles. In order to achieve required momentum resolution of 200 keV/c, the chamber is operated with a low-Z gas mixture of He (90\%) and i-C4H10 (10\%) to mitigate the multiple scattering effect. An alternated all stereo layer configuration is also adopted to enhance a good position resolution in the longitudinal direction. \\
The 4,986 sense and 14,562 field wires were strung using gold-plated tungsten wires and unplated aluminum wires, respectively. Proper tensions were applied to control the wire gravitational sag within 55±7 μm for the sense wires, and the sag differences between a given sense wire and surrounding field wires within less than 100 μm. CDC performance tests using cosmic rays are successfully in progress. We have confirmed decent hit efficiency as well as spatial resolution of 170 μm including a tracking uncertainty.