Speaker
Description
The Standard Model of particle physics is not a perfect theory. Many experiments seek to observe phenomena beyond the Standard Model to fix it.
COMET experiment (COherent Muon to Electron Transition) is planned to be conducted at J-PARC. COMET aims to find the neutrinoless transition of muon to electron, known as “µ-e conversion.” To find this process, the first stage of COMET, COMET Phase-I, plans to produce approximately $10^{15}$ muons by a strong proton beam from J-PARC. This proton beam first enters the Pion Capture Solenoid(PCS), colliding with a carbon target to produce pions. These pions decay into muons, which are transported through a 90-degree curved Transport Solenoid to the Detector Solenoid (DS). In the DS, there is an aluminum target to capture a muon in its atomic nucleus, and the tracker detector inside the DS is designed to detect trajectories of electrons produced from a muon captured in the aluminum nucleus via µ-e conversion ( $\mu^-+N(A,Z)\rightarrow e^-+N(A,Z)$ ). This process is extremely suppressed in the Standard Model with a predicted branching ratio of $O(10^{-54})$. Detecting this process with the sensitivity of COMET Phase-I ($\sim10^{-15}$) would provide evidence for new physics beyond the Standard Model.
Due to the mass difference between the muon and the electron, the electron's momentum converted from the muon in the aluminum nucleus is estimated to be $105\;\mathrm{MeV/}c$. COMET has to distinguish this electron by the momentum. COMET Phase-I plans to measure this momentum using the magnetic field's value and the signal electron's track curvature.
Therefore, the magnetic field map around the detector region is required to be measured with a relative accuracy of less than $0.5\;\%$ to realize a sufficient sensitivity for the outgoing mono-energetic conversion electrons. COMET Phase-I requires the superconducting solenoid magnet as the DS, which can generate a $1\;\rm{T}$ magnetic field at the center of the magnet The DS was fabricated in JFY 2023 and tested in March and June 2024 without an iron yoke. The DS was cooled by conduction cooling by 3 Gifford-MacMahon cryo coolers and could be successfully cooled down to about 4 K for two weeks, including liquid nitrogen-precooling for the first week. The DS has also been successfully excited to the rated current $189\;\rm{A}$ without any training quenches. The magnetic field map of DS was measured using a 3-axis Hall probe, and the analysis of the result is in progress. The detailed results of the analysis will be presented at this conference.
