The COMET experiment seeks to measure the neutrinoless coherent transition of a muon to an electron in the field of an aluminium nucleus. The event signature of coherent neutrinoless μ⁻→e⁻ conversion in a muonic atom is the emission of a monoenergetic single electron in a certain time interval. The energy of the signal electron for aluminium is 104.97 MeV, and the lifetime of the muonic atom is 864 ns.

This makes neutrinoless μ⁻→e⁻ conversion very attractive experimentally. Firstly, the e⁻ energy of about 105 MeV is well above the end-point energy of the muon decay spectrum (∼52.8 MeV). Secondly, since the event signature is a monoenergetic electron, no coincidence measurement is required. Thirdly, the long lifetime means that backgrounds associated with the beam flash can be eliminated. Thus, the search for this process has the potential to improve sensitivity by using a high muon rate without suffering from accidental background events.

There exist various theoretical models which predict sizable charged lepton mixing branching ratios. Among them, the best motivated models are the supersymmetric (SUSY) extensions of the SM, such as SUSY‑GUT or SUSY‑Seesaw models. Modern theoretical motivations for lepton flavor violation, data on current experimental bounds and expected improvements are reviewed by Marciano, Mori and Roney.

The COMET experiment will be carried out in two stages: Phase-I and Phase-II. The experimental sensitivity goal for this process in Phase-I is 3.1×10⁻¹⁵, or the 90% upper limit of the branching ratio of 7×10⁻¹⁵, which is a factor of 100 improvement of the existing limit B(μ⁻ + Au → e⁻ + Au) ≤ 7×10⁻¹³ from SINDRUM-II at PSI. The goal of Phase-II is a SES of 2.6×10⁻¹⁷, which is a factor of about 10 000 better than the current experimental limit. The expected number of background events is 0.032, with a proton beam inter-bunch extinction factor of 3×10⁻¹¹. To achieve the target sensitivity and background level, the 3.2 kW 8 GeV proton beam from J-PARC (Japan) will be used. Two types of detectors - CyDet (cylindrical detector system) and StrECAL (straw tracker and electron calorimeter) - will be used for detecting the µ⁻→e⁻ conversion events and for measuring beam-related background events.

Scientists from DLNP JINR successfully participate in the preparation stage of the COMET experiment. For Phase-I, JINR specialists manufactured and tested the entire set of straw tubes with a diameter of 9.8 mm and a length of 1.6 m (more than 2700 pieces), and for Phase-II, they will produce a full set of straw tubes with a diameter of 5 mm. The JINR specialists actively participate in the creation of the straw tracker, the electromagnetic calorimeter and the cosmic ray veto system (CRV) at the stages of modelling and scientific and technical activities. They will also continue to be actively involved in the assembly and maintenance of these detectors. The JINR specialists participate in the analysis of the test measurement data and will participate in the analysis of the COMET experiment data.

Expected results upon completion of the project:
Completion of assembly, testing, calibration, installation, cosmic test and maintenance of the straw detector for Phase-I.

Development and optimization of the crystal calibration method for the calorimeter with allowance for the features of the experiment: the presence of the magnetic field and the high-resolution calorimeter.

Simulation of a complex detector system (tracker, calorimeter, etc.).

Participation in the preparation, engineering and physics run, acquisition and analysis of data of Phase‑I.

Research and development for production of straw tubes with a wall thickness of 12 μm and a diameter of 5 mm. Measurement of all mechanical properties and development of quality control standards for manufactured new straw tubes 5 mm in diameter.

Production of straw tubes (about 1000 pcs) for a full-scale prototype.

Production of a full-scale straw station with new tubes (12 μm, 5 mm) at JINR, and measurements on the beam.

Preparation, mass-production and testing of straw tubes for Phase-II.

Full participation in the design, assembly, installation, cosmic test and maintenance of the calorimeter.

Participation in the assembly and maintenance of the CRV for Phase-I and Phase-II.

Participation in the beam tests of the detector components for Phase‑II.

Participation in the assembly, testing, installation and maintenance of the entire detector system for Phase‑II.

Expected results of the project this year:
Completion of assembly, testing, calibration, installation and cosmic test of the straw detector for Phase‑I.

Development and optimization of the crystal calibration method for the calorimeter with allowance for the features of the experiment: the presence of the magnetic field and the high-resolution calorimeter.

Simulation of the complex detector system (tracker, calorimeter, etc.).

Participation in the preparation of the engineering and physics run of Phase‑I.

Research and development for production of straw tubes with a wall thickness of 12 μm and a diameter of 5 mm. Measurement of all mechanical properties and development of quality control standards for manufactured new straw tubes 5 mm in diameter.

Participation in the design and assembly of the calorimeter.

Participation in the assembly of the CRV for Phase-I.