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 $3 \times 10^{-15}$ 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 $i$-C$_{4}$H$_{10}$ (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 $55 \pm 7$ $\mu$m for the sense wires, and the sag differences between a given sense wire and surrounding field wires within less than 100 $\mu$m. CDC performance tests using cosmic rays are successfully in progress. We have confirmed decent hit efficiency as well as spatial resolution of 170 $\mu$m including a tracking uncertainty.