Status of the COMET experiment

. The COMET (COherent Muon to Electron Transition) experiment, currently being built in Tokai, Japan, will search for the coherent neutrinoless transition of muons to electrons in the coulomb field of atomic nuclei. The process is highly suppressed in the Standard Model and therefore provides a promising channel to probe new physics. The experiment will be carried out in a staged approach. Phase-I of COMET aims to search for the process with a single event sensitivity of O (10 − 15 ). Additionally, precise measurements of muon beam dynamics and detector prototyping for Phase-II will be conducted. Utilizing a much higher intensity proton beam, a more complex and longer muon / electron transport system, and gained experience from Phase-I, an im-provement of at least four orders of magnitude over the current best branching ratio limit up to O (10 − 17 ) is envisioned for Phase-II. This article will give a status report for Phase-I of COMET.


Introduction
The Standard Model (SM) of particle physics has already prevailed against various experimental tests for five decades -past and recent discoveries, like the SM-like Higgs boson, further crown its success. We know, however, from several observations that physics beyond the Standard Model (BSM) must exist. By way of example, the observed baryon-antibaryon asymmetry, a missing candidate for a dark matter particle, or the existence of neutrino mixing give a strong motivation for BSM physics. The latter example directly motivates the existence of small but non-zero neutrino masses and the violation of lepton flavor conservation.
One prominent charged lepton flavor violating (cLFV) process is µ − e conversion. This process could appear in the SM (including its extension with massive neutrinos); however, due to the tiny neutrino masses, it is strongly suppressed with a branching ratio of < 10 −54 . Considering two widely acknowledged and well-motivated BSM theories, supersymmetry and warped extra dimensions, the cLFV effect is enhanced naturally up to around < 10 −14 [1,2]. Consequently, discovering any cLVF process would be clear evidence for BSM physics and proves a powerful tool for BSM physics investigation.
The current best upper limit on µ−e conversion comes from the SINDRUM-II experiment, which set a limit on the conversion rate in muonic gold at 7 × 10 −13 (90 % C.L.) [3]. The COMET experiment will search for the process with a 100 and 10000 times higher sensitivity; in its Phase-I and Phase-II, respectively. This article will focus on COMET Phase-I and aims to give an outline of the experiment, the current detector development status, and facility updates.

µ − e conversion signal and backgrounds
To observe µ − e conversion, one starts by stopping negatively charged muons in material. Once the muon loses all its kinetic energy, it is captured by an atom into a high orbital momentum state. It will rapidly (∼ 10 −13 s) cascade down towards lower energy levels, ending in the 1s ground state of the muonic atom [4]. In the scope of BSM physics, the muon can now undergo the so-called neutrinoless coherent muon to electron conversion While the total lepton number L is conserved, individual lepton flavors L e and L µ are violated by one unit. The nucleus remains in its ground state and only takes up recoil in the conversion process -hence the reaction is called coherent. With the nucleus remaining unchanged, essentially only two particles are involved in the interaction. Therefore the energy of the outgoing electron has a fixed value E e , given by the equation: For lighter elements (Z < 20), the binding energy E Binding , as well as the recoil energy of the nucleus E Recoil , is small compared to the muon mass m µ . The resulting energy of the electron, for example for an aluminum stopping target (Z = 13), is at E e = 104.97 MeV [5]. Therefore, the conversion electron's energy is firmly positioned above most SM backgrounds. The remaining background contributions are small but must not be disregarded. For the sake of simplicity and to motivate experimental design decisions discussed in the next section, they are divided into three groups: • Cosmics: One of the most critical backgrounds, cosmic rays, mimics the conversion signal by decay in flight or interaction with materials in the vicinity of the muon-stopping target. A cosmic ray veto system surrounding the whole detector region will be used to mitigate this background.
• Intrinsic physics: The highest intrinsic physics background stems from muon decay in orbit (DIO) events. While the electron momentum in a free muon decay is limited to roughly half the muon mass, the nuclear recoil in DIO allows the electron to surpass this limit significantly [5]. With the endpoint of the DIO spectrum reaching up to the conversion electron energy, only an excellent momentum resolution of the detector system can suppress this contribution.
• Beam related: The last group of backgrounds is caused by particles traveling along the muon beam line, such as electrons from muon and pion in-flight decays or antiprotons from the pion production target. With the half-life of light muonic atoms lying around multiple hundred ns, this prompt background can be strongly suppressed by implementing a delayed measurement time window. The resulting experimental requirements are a bunched beam structure, a high bunch purity, and a light material for the muon-stopping target.

The COMET experiment
An experimental overview of the COMET Phase-I layout is shown in Fig. 1. A pulsed 8 GeV proton beam impinges on a fixed target and produces charged pions which subsequently decay into muons (see left side of Fig. 1). They are guided along a short C-shaped beamline into the detector region. Here, two different configurations will be utilized. The configuration on the top right of Fig. 1 depicts the primary COMET Phase-I detector used to study the µ − e conversion process. The second configuration, shown on the bottom right, will be used to precisely measure beam dynamics, verify Monte-Carlo simulation, and deepen the understanding of the production system. A more detailed description and current status of the experimental parts are discussed in the following.

Proton beam and target
COMET is built at the Japan Proton Accelerator Research Complex (J-PARC), from where it will receive its 8 GeV proton beam. The beam energy is chosen to minimize production of antiprotons, which increases rapidly for proton energies above 10 GeV and could produce a fake 105 MeV electron signal. A major milestone achieved in 2022 is the completion of all beamlines leading from the J-PARC main ring to the COMET hall.
As motivated in section 2, COMET also requires a bunched beam to suppress beamrelated backgrounds. Operation of the beam with an interval of 1.17 µs has successfully been performed. At the same time, the bunch purity was investigated by measurement of the so-called extinction factor, defined as Number of residual protons between consecutive bunches over Number of protons in a bunch. To achieve the experiment's design sensitivity, the extinction factor must be smaller than 10 −10 . It was already measured at the J-PARC main ring position and shown to be less than 6 × 10 −11 , which satisfies the requirements [6]. A second measurement at the COMET hall utilizing the finished beamlines will be performed in early 2023.
A mock-up of the graphite proton target for COMET Phase-I was produced and is shown in Fig. 2. Recent analysis hints at a larger-than-expected interference between the holding structure and the muon transport, but new design ideas are underway to mitigate this problem. The proton target will be installed inside the pion capture solenoid, designed to optimize the rate of low-energy negative pions produced in the backward direction. The outer hull is finished and also shown in Fig. 2. Construction of the whole setup is currently underway.

Cylindrical Detector system (CyDet)
The task of measuring the conversion signal in COMET Phase-I lies in the hands of the CyDet. It consists of a Cylindrical Drift Chamber (CDC), measuring the conversion signal electron, combined with a ring of scintillators used to provide the trigger and t 0 for each event.
The whole system is contained in a 1 T solenoid field, and its inner radius of about 50 cm is  designed to let most of the low momentum beam flash pass through the middle, avoiding a dangerously high hit rate in the detectors. Furthermore, a large part of the muonic atom DIO spectrum originating in the central aluminum muon-stopping target produces electrons with energies below 60 MeV which will not be able to reach the detector.
The CDC consists of 20 sense layers, each containing about 800 to 1200 wires. The CDC hull, subsequent wire stringing of all 19548 wires, tension measurements, etc., were finished in 2016, and pictures are shown in Fig. 3. Since then, various tests utilizing cosmic rays have taken place. They show an excellent performance of the CDC, with a measured spatial resolution of better than ∼ 200 µm. The result satisfies COMET design constraints, limiting the intrinsic physics background from high-energy DIO events to less than 0.01 events during the whole Phase-I runtime. The cosmic ray tests also demonstrate a successful stable operation of the whole chamber over more than two years.
Recently achieved milestones for the CDC are the complete installation and tests of the entire data acquisition system in 2019 and the design of a cooling system for the electronics. Performance tests for the latter were finished in early 2022, demonstrating that the temperature can be kept at sufficiently low 35 • C to limit FPGA heating. The second part of CyDet is the CyDet Trigger Hodoscope (CTH). CTH rings are located at both ends of CDC, as shown at the top right of Fig. 1. A conversion electron will first spiral through the CDC, allowing its high precision momentum measurement, and subsequently hit the CTH, providing COMET's primary trigger. The CTH layout is depicted in Fig. 4. Each ring is segmented into 64 scintillation counters, with each counter tilted by a specific angle and shifted roughly half its width against the previous counter. A four-fold coincidence between scintillators will be utilized to reduce fake triggers caused by γ-rays coming mostly from Bremsstrahlung of DIO Michel electrons. With about 100 kHz, the CTH trigger rate is still expected to be very high and dominated by backgrounds. An online trigger selection, including CDC information, will be utilized to allow manageable data rates.
Development of the CTH is still ongoing. The right side of Fig. 4 shows a section of the full-sized prototype of the holding structure, which was recently produced to test the installation and fixation of scintillators. Currently, several studies using 1:1 scale counters are being carried out. Tests regarding timing, resolution, and photon yield were performed during summer 2022, using 100 MeV electrons at the Australian synchrotron. A detailed discussion of results is beyond the scope of this article, but overall the tests show good performance, satisfying COMET's requirements.

Straw-Tracker ECAL Detector System (StrECAL)
Instead of CyDet, a second configuration composed of a straw tracker and a crystal electromagnetic calorimeter, StrECAL, will be used after physics data taking. Its primary purpose is to directly measure the beam to enhance understanding of beam profile, particle production rate as a function of energy, and solenoid characteristics. The measurement will lay the foundation for the upcoming Phase-II extension of the transport solenoid. Moreover, at the end of this extension, a similar StrECAL system will replace the CyDet as the conversion electron detector to archive a better momentum resolution and higher rate capabilities. Thus, Phase-I StrECAL's secondary purpose is to serve as a prototype for the upgraded detector system.
Five straw-tracker stations (see Fig. 1) are utilized in Phase-I. Each will be made of four perpendicularly arranged planes, to precisely determine the x and y positions of passing particles. They will be deployed inside a vacuum to minimize electron multiple scattering, otherwise dominating the momentum resolution. Furthermore, the material budget for the straws themselves has to be minimized. After several R&D projects, the production of straws with only 20 µm thick aluminized mylar walls and 10 mm diameter filled with evenly mixed Argon:Ethane gas succeeded. Mass production of straw tubes for Phase-I is already finished, EPJ Web of Conferences 282, 01014 (2023) SSP 2022 https://doi.org/10.1051/epjconf/202328201014 and even thinner straws with 12 µm walls for Phase-II are produced and under testing. A milestone achieved in 2022 is the completion of Straw Station #1. Prototype performance tests show a spatial resolution of 150 µm can be achieved [7].
The electromagnetic calorimeter is placed downstream of the five straw-tracker stations. Based on cost-performance evaluations, it was decided to use LYSO scintillator crystals, each 2 mm × 2 mm × 120 mm. Besides providing an additional hit position, it will measure the particle energy E independently from the particle momentum p obtained by the straw system. The measurement is crucial to calculate the ratio E/p used for particle identification. Results from ECAL prototype tests utilizing a vacuum chamber show an excellent energy resolution of 4 %, timing resolution of < 0.5 ns, and a 6 mm spatial resolution for 105 MeV electrons [8].

Conclusion
Charge lepton flavor violation is one of the most promising BSM fields currently under investigation. The combined suppression of SM and experimental background highlights the µ − e conversion search as the golden channel for discovering BSM physics in the coming decade. The COMET experiment aims to measure µ − e conversion with an unprecedented single event sensitivity of O(10 −15 ) and O(10 −17 ) in Phase-I and Phase-II, respectively. With the completion of the proton beamline leading up to the COMET experiment and successful beam tests regarding proton bunching and extinction, a first smaller test run will commence at the end of the Japanese fiscal year 2022. This so-called Phase-α will lay the foundation for the subsequent start of Phase-I. As motivated in this article, detectors for Phase-I are currently being prepared and commissioned on schedule to start the first physics data-taking in 2024.