Search for New Physics in the Lepton Sector

Participating countries and international organizations:

Belarus, Bulgaria, Czech Republic, France, Georgia, Germany, Italy, Japan, Kazakhstan, Russia, Slovakia, Switzerland, Ukraine, United Kingdom, USA.

Issues addressed and main goals of research:

    The COMET experiment at J-PARC, the Mu2e experiment at Fermilab and the MEG II experiment at PSI are a dedicated search for the CLFV processes µ ^- N→e ^- N, µ ⁺→e ⁺ γ. Once neutrino masses are included, the process is allowed but still unobservable since the rate is proportional to (Δm ²_ij/M ²_W) ², where Δm ²_ij is the mass difference squared between ith and jth neutrino mass eigenstates, and M_W is the mass of the W boson. The predicted rates for the µ ^-N→e ^-N and µ ⁺→e ⁺g CLFV processes are less than 10 ^-50 each. This makes this process a very theoretically clean place to search for NP effects. In many NP models that include a description of neutrino mass, the rates for these processes are enormously enhanced so that they occur at a level to which the COMET and Mu2e experiment will have sensitivity. The conversion measurement at the level of 10 ^-17, which is COMETs goal, will be 10000 times better than the current experimental limit B(µ ^-+ Au→µ e ^- + Au) < 7·10 ^-13 from SINDRUM-II at PSI.

    The T2K experiment is the first to study the mechanism of CP-symmetry breaking in the lepton sector, which is experimentally manifested in the difference between oscillation probabilities of neutrinos and antineutrinos. The observation of CP-symmetry breaking in neutrino oscillations together with nonconservation of the lepton number can serve as an argument in favor of explaining the baryon asymmetry of the Universe through the mechanism of leptogenesis (leptogenesis is a process of the appearance of lepton-antilepton asymmetry (nonzero lepton number) in the early stages of the formation of the Universe). On the basis of data of the T2K experiment, the observation of CP violation with a significance of 3σ or higher is expected in the case of large CP violation, as well as the measurement of neutrino mixing parameters, θ₂₃ and Δm²₃₂, with an accuracy of 1.7º or better and 1%, respectively.

Expected results in the current year:

1. Participation in preparation, engineering, and physics run, data acquisition and analysis of Phase-a.

2. Finalization of assembling, testing, calibration, installation, cosmic test and maintenance of the straw detector for Phase-I.

3. R&D program for production of straw tubes with a 12-mm wall thickness and 5 mm in diameter. Creating a straw prototype (64 channels) with new tubes (12 mm, 5 mm) and measurements with the beam.

4. Test (certification) of LYSO crystals to be used in the calorimeter. Development and optimization of the crystal calibration method for the COMET calorimeter. Participation in calorimeter designing, assembling, installation, cosmic test, and maintenance.

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

6. Participation in assembling, testing, installation, and maintenance of the whole detector system for Phase-I.

7. Complex detector system (tracker, calorimeter etc.) simulation.

8. Participation in engineering and physics run, data acquisition and analysis.

9. Tests of CsI and BaF₂ electromagnetic calorimeter elements with gamma sources and the electron beam.

10. Manufacture and installation of electronics of the Super FGD photodetector calibration system.

11. Assembly of the SuperFGD as part of the Near Detector of the T2K experiment using a unique box support and top access systems.

12. Preparation of the launch of the Near Detector of the T2K experiment, participation in engineering and operation runs, taking of new data and analysis.

13. Investigation of systematic uncertainties for measuring δ_CP.

14. Search for manifestations of new physics in the T2K data including the search for light dark matter.

15. Filling the batch of Mu2e scintillation counters with CKTN and testing them for leakage.

16. Maintenance of the visualization and control software.

17. Development and tests of Mu2e electromagnetic calorimeter preamplifiers at JINR.

18. Participation in radiation hardness tests of detector elements.

19. Participation in development of a positron tracker for the MEG-II experiment, DAQ, data analysis.

20. Participation in data taking and analysis of experimental data obtained with CERN hadron beams.