Development and property study of the $12 μ$m thick straw tubes with a diameter of 5 mm for the COMET Straw Tracker System

The COMET experiment focuses on searching for the direct conversion of a muon into an electron on an aluminum nucleus without emitting a neutrino (so-called $μ\rightarrow e$ conversion). This conversion violates the lepton flavor conservation law, a fundamental principle in the Standard Model. The COMET experiment aims to achieve the muon-to-electron conversion sensitivity on a level of $10^{-17}$. The Straw Tracker System (STS) based on straw tubes could provide the necessary spatial resolution of 150 $μ$m to achieve an momentum resolution for 105 MeV/c electrons better than 200 keV/c. The COMET experiment will be separated into two phases. Phase-I will operate with the 3.2-kW 8-GeV-proton beam, and Phase-II will operate with the beam intensity increased to 56 kW. The STS must operate in a vacuum with the inner pressure of 1 bar applied to straws. The initial design of 10-mm-diameter straws developed for Phase-I will not be as efficient with the 20 times higher beam intensity of Phase II, but the new STS design based on 5-mm-diameter 12-$μ$m-thick straws could fully satisfy the required efficiency. The mechanical properties of these straws, such as sagging, displacement, and dependence of the diameter on overpressure, are discussed in this article.