The proposed research project is to search for a process of charged lepton flavor violation (CLFV) of muon to electron (μ-e) conversion by the first phase of the J-PARC E21 COMET experiment (COMET Phase-I), with an improved experimental sensitivity by more than hundreds.

Background of Research Project

It has been confirmed that neutrinos are massive and mixed among different neutrino flavor species, by the observation of neutrino oscillation. Therefore, lepton flavor for neutrinos is known to be violated. However, lepton flavor violation of charged leptons (CLFV) has been yet to be observed, and a discovery of CLFV is considered to be one of the most important subjects and potentially leading search in particle physics.

In the framework of the Standard Model (SM) with massive neutrinos and their mixing, the branching ratio of μ → eγ decay, which is one of the muon CLFV processes, can be estimated to be about O(10-54), being negligibly small. As a result, it can be concluded that observation of CLFV would indicate a “clean” signal of new physics beyond the SM.

CLFV is known to be sensitive to various extension of new physics beyond the SM. It could have visible rates if there are new particles carrying lepton flavor not too far from the TeV energy scale. Among them, one of the well-motivated physics models is supersymmetric (SUSY) models, in particular, SUSY GUT models and SUSY seesaw models. Other models to predict sizable rates are extra-dimension models and little-Higgs models and so on.

The Past Achievements

To improve an experimental sensitivity of searches for μ-e conversion, it would be critical to increase a number of muons significantly. We have developed and constructed a new highly intense muon source at Osaka University, which is called “MuSIC”, shown in Fig. 2. It has a novel pion capture system, where a pion production target is placed inside superconducting solenoidal magnets. At MuSIC, we have observed about 108 muons/s produced with a 400 W proton beam power, in contrast to 108 muons/s produced at PSI with 1.2 MW proton beam power. We have demonstrated an improvement of the muon production efficiency of about a factor of thousands. From this notable experimental demonstration, one of the important milestones of COMET has been reached.

The SM of particle physics is known to be incomplete since it has many self-undetermined parameters, although we have not seen any striking experimental phenomena that the SM cannot explain. The major goal of particle physics is to find new physics beyond the SM. To achieve this goal, there are two frontier areas; high energy frontier and high intensity frontier. At the high energy frontier, the Large Hadron Collider (LHC) has made magnificent progress such as discovery a Higgs-like particle, but the LHC cannot find any new particles in their energy region so far. Therefore, the other approach of searching for rare process has attracted much attention recently, in particular on CLFV.

The research objectives of this project are to search for μ-e conversion at the Japan Proton Accelerator Research Complex (J-PARC), by the “COMET Phase-I” experiment, where COMET stands for “COherent Muon to Electron Transition”. A new initiative of the staged approach has been taken to achieve an early and timely start of a series of searches for μ-e conversion. The COMET Phase-I presents the first part of the staged construction of the COMET experiment. It would include the COMET proton beam line and the COMET muon beam line up to the end of the first 90° bend, as shown in Fig. 3. The targeted goal of COMET Phase-I is to achieve an experimental single-event sensitivity of 3×10-15 or better, which is an improvement of a factor of 100 or more over the current limit of 7×10-13 (by the SINDRUM at PSI), whereas the proposed single-event sensitivity of COMET Phase-II is 3×10-17.

The COMET experiment was approved in the J-PARC program advisory committee (PAC) in 2009, and the staged approach was endorsed in 2012. The J-PARC PAC reported that “the COMET experiment (E21) will become one of the flagship experiments for JPARC and Japanese physics later in the decade”. The IPNS/KEK has submitted the budget request for the construction of the new proton beam line and muon beam line for COMET Phase-I for JFY2013. This mid-term plan was also endorsed by the J-PARC review committee organized by the MEXT, Japan.

This research proposal is to construct detectors for COMET Phase-I that will not be covered by the KEK budget. Since the electron transport of curved solenoids will not be constructed, a different detector should be chosen in COMET Phase-I.

The detector of COMET Phase-I is selected to be a cylindrical drift chamber (CDC) surrounding a muon-stopping target located at its center, shown in Fig. 4. Segmented trigger hodoscope counters are placed at both the upstream and downstream ends of the CDC. The CDC is placed inside a superconducting solenoid magnet of a magnetic field of 1 T. With a total J-PARC proton beam power of 3.2 kW, about 1.6 × 109 stopped muons/s are expected. With a running period of 8 × 106 s (about 90 days) and the detector acceptance of 0.043, an expected single event sensitivity is 3.1 × 10-15. The background events are estimated to be 0.03 events. The planned schedule is shown in Fig. 4. The physics case made by the COMET Phase-I experiment is extremely strong. We are hoping to start construction in 2013 and carry out measurements in 2016/2017.