DeeMe Experiment Started
Figure 1：Bird-eye view of the J-PARC. MLF is placed in the center of the picture.
Figure 2：The phenomenon of lepton flavor conversion. The conversion of neutral lepton flavors has been confirmed through neutrino oscillations. The process of charged lepton flavor conversion has not yet been discovered.
The DeeMe experiment, in which the Osaka Metropolitan University High Energy Physics Laboratory is participating, has officially started, and we would like to talk about its current status. The DeeMe experiment is conducted at the Materials and Life Science Experimental Facility (MLF) (Fig. 1) of the Japan Proton Accelerator Research Complex (J-PARC) located in Tokai Village, Naka District, Ibaraki Prefecture. It is an experiment to search for the muon-to-electron conversion process, which is forbidden by the Standard Model of particle physics, and thus is an exploration of charged lepton flavor violation. Figure 2 illustrates the flavor conversion of leptons. While neutral lepton flavor conversion has been confirmed through neutrino oscillations, charged lepton flavor conversion processes have not yet been observed. In fact, although charged lepton flavor conversion via neutrino oscillations is possible, theoretical calculations show that the branching fraction is 10-54, making it practically unobservable. Therefore, if the muon-to-electron conversion process can be experimentally discovered, it would directly serve as evidence for new physics. In reality, many theories beyond the Standard Model predict a much larger branching fraction for muon-to-electron conversion, such as 10-13 to 10-14. The goal sensitivity of the DeeMe experiment is 10-13, which is 10 to 100 times more sensitive than previous studies in searching for the muon-to-electron conversion process. The method used in the DeeMe experiment to search for the muon-to-electron conversion process utilizes the pulsed proton beam extracted from the Rapid Cycle Synchrotron (RCS) at J-PARC. When the proton beam irradiates a carbon target, a large number of pions are produced inside the target. These pions decay into muons. The muons lose energy and come to rest within the target. The stationary muons are then captured by carbon nuclei to form muonic atoms. Within the scope of the Standard Model, there are two possible outcomes for the muons in muonic atoms: First, the muon decays in orbit of the muonic atom \((\mu^- \rightarrow e^- \nu_{\mu}\bar{\nu}_e)\), and second, the muon is absorbed by the nucleus \((\mu^-+(A,Z) \rightarrow \nu_{\mu} +(A,Z-1))\).
Figure 3：Conceptual Diagram of the DeeMe Experiment: From the generation of pions to the formation of muon atoms, everything takes place within a single target. The electrons produced by the muon-to-electron conversion are transported and measured in a secondary beamline.
Thirdly, there is the possibility of a new physics process known as muon-electron conversion \((\mu^- + (A,Z) \rightarrow e^- + (A,Z))\). In this process, the atomic nucleus remains unchanged, and the muon directly converts into an electron. As a result, most of the muon’s mass is converted into the electron’s kinetic energy. Therefore, an electron with a monochromatic energy of 105 MeV is emitted. Additionally, since the muon is captured by a carbon nucleus, the electron is emitted with a delay of about 2 μs. Consequently, a delayed electron with a monochromatic energy of 105 MeV, delayed by 2 μs from the pulsed proton, becomes the signal event. This electron is transported and measured using a secondary beamline (Figure 3).
Figure 4：MWPC used in DeeMe. It utilizes HV switching methods.
The search for the muon-electron conversion process is conducted through large-scale experiments such as the COMET and Mu2e experiments. However, the DeeMe experiment has the advantage of conducting the entire process from pion production to muon atom formation within a single carbon target, making it a more compact experiment. The DeeMe experiment is being carried out using the newly constructed H-Line, a large solid-angle beamline at J-PARC MLF. The H-Line was built for fundamental physics research and was completed in January 2022. Since then, it has been used for the DeeMe experiment. The DeeMe experiment’s detector constructs a spectrometer using four MWPCs (Fig. 4) and a dipole magnet called PACMAN (Fig. 5). To minimize the dead time caused by prompt bursts of pulsed protons, the MWPC employs an HV switching technique, which sets the potential difference between the anode wires and the potential wires to 1500 V only between 300 ns and 2 μs after the prompt burst, and 0 V otherwise.
Figure 5：PACMAN dipole electromagnet. It can generate a magnetic field of 0.4T at the center.
Two MWPCs are placed upstream of PACMAN, and another two MWPCs are placed downstream of PACMAN to measure the momentum of electrons. The DeeMe experiment is currently in the data acquisition phase, and results are expected in the future.