Development and property study of the extremely thin 12 {μ m} C-type straw tubes with 5-mm diameter for a Straw Tracker System of COMET

The COMET experiment focuses on searching for the direct conversion of a muon into an electron with aluminum nuclei, without emitting a neutrino (so called μ→ e conversion). This conversion violates charged lepton flavor conservation law, a fundamental principle in the Standard Model. The COMET experiment requirement is to achieve the muon-to-electron conversation sensitivity better than 10⁻¹⁷. The Straw Tracker System (STS) based on straw tubes could provide the necessary spatial resolution of 100 μm and the electron momentum resolution better than 100 MeV/c The COMET phase-II will operate with 56 kW 8 GeV proton beam increasing from 3.2 kW 8GeV proton beam proposed for COMET phase-I. The 10-mm straws system was developed for COMET-phase-I and wouldn't perform with full efficiency with 20 times increased intensity. New STT design based on 5-mm 12-μm thick straws could satisfy the requirement of increased intensity. This detector should also operate in vacuum with 1 bar internal pressure. The mechanical properties of these straws such as sagging, elongation, dependence of the diameter on over-pressure, etc are discussed in this article.