J-PARC Experiment E36 at the Nuclear and Particle Physics Facillity

Precise Measurement of Γ (K+→ e+ν)/Γ(K+→ μ+ν)
using Stopped Positive Kaons

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What is J-PARC E36 experiment?

Precise measurement of RK =Γ (K+→ e+ν)/Γ(K+→ μ+ν)

We are going to precisely measure a ratio of the decay width RK= Γ (K+→ e+ν)/Γ(K+→ μ+ν) to test lepton universality using a sub-system of the TREK experiment. The Standard Model (SM) prediction for the RK value is highly precise (ΔRK/RK = 0.4×10-3) and a deviation from this value would very clearly indicate the existence of New Physics beyond the SM. Recently, a minimal supersymmetric (SUSY) extension of the SM (MSSM) has been considered as a candidate for the new physics to be tested by RK. A possible mechanism to detect the SUSY Lepton Flavor Violation effect through a deviation from the lepton universality has been discussed. The non-vanishing e-τ lepton mixing can change the RK value from the SM prediction at the percent level. In this proposed experiment, we aim at achieving an uncertainty for RK of better than Δ RK/RK = 2.5×10-3, which is about half of the current world record.

Search for heavy sterile neutrinos

We are also going to perform a new experiment to search for heavy sterile neutrinos (N) in the K+ → μ+ N decay using a stopped K+ beam. The neutrino minimal Standard Model predicts this branching ratio to be up to 10−6 which is accessible by using the TREK experimental apparatus. This decay has some important features such as,
(1) a narrow peak structure would be formed in the momentum spectrum;
(2) the μ+ polarization depends on neutrino masses.
In the E36 experiment, the μ+ momentum will be measured, and the sensitivity of the branching ratio to better than 10−8 can be achieved, which is about 100 times more sensitive than the previous experiment. If we find a peak indication in the momentum spectrum, the μ+ polarization will be measured in addition to the μ+ momentum.