Vacuum-Compatible, Ultra-Thin-Wall Straw Tracker; Detector construction, Thinner straw R&D;, and the brand-new graphite-straw development

The COMET experiment at J-PARC aims to search for a lepton-flavour violating process of muon to electron conversion in a muonic atom, l. 174 μ-e conversion, with a branching-ratio sensitivity better than l. 175 10−16, 4 orders of magnitude better than the present limit, in order to explore the parameter region predicted by most 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, l. 179 <2%, for 100 MeV/l. 179 c electron, which is primarily limited by multiple scattering effect for this momentum region; and (ii) high rate capability, up to l. 179 5×109μ−/s muon beam enabled by J-PARC. In order to fulfil such requirements a vacuum-compatible, ultra-thin-wall straw tracker has been designed, and a l. 182 20μm-thick Mylar straw with 70 nm Al cathode has been developed employing an ultrasonic-welding technique. The detector performances such as detection efficiency and intrinsic spatial resolutions were investigated with test-beam experiments and confirmed to be acceptable for the COMET experiment. The construction of the straw tracker for COMET Phase-I has been completed. In parallel to the construction of present tracker a thinner l. 187 12μm-thick straw has been developed with joint collaboration among KEK, JINR and CERN. During this R&D, it was noticed that the current technology cannot achieve tubes much smaller than 5 mm in diameter or walls much thinner than l. 188 12μm. We also launched a brand-new project to realize the graphite-textile straw which has a potential to realize an extremely low material tracker. In this article, a brief report on detector construction with a present l. 191 20μm-thick straw, R&D on a new l. 191 12μm-thick straw, and a brand-new graphite straw, is provided.