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 (m ²_ij/M ²_W) ², where m 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⁺γ 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. Conversion measurement at the level of 10 ^-17, which is COMET goal, is a factor of 10000 better than the current experimental limit B(µ ^-+ Au→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 the process of the appearance of lepton-antilepton asymmetry (nonzero lepton number) in the early stages of the formation of Universe). On the data of the T2K experiment, observation of CP violation with a significance of 3 or higher is expected in case of large CP violation and the measurement of neutrino mixing parameters, ₂₃ and m²₃₂, with an accuracy of 1.7º or better and 1%, respectively.

Expected results in the current year:

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

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

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

• Test (certification) of the 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.

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

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

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

• Participation in the engineering and physics run, the data acquisition and analysis.

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

• Design of the platform and equipment for assembling the unique SuperFGD target with a volume of about 2 cubic meters.

• Creation of the unique box support system to ensure target assembly.

• Creation of electronics of the Super FGD photodetector calibration system.

• Assembly of the Super FGD as part of the near detector of the T2K experiment.

• Inverstigation of systematic uncertainties for measuring _ср.

• Search for manifestations of new physics in the T2K data.

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

• Maintenance of the visualization and control software.

• Development and tests of the Mu2e electromagnetic calorimeter preamplifiers at JINR.

• Participation in radiation hardness tests of detector elements.

• Participation in evelopment of the positron tracker for the MEG-II experiment, DAQ, data analysis.

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

Collaboration