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Physics motivation for R_K

Physics motivation for the heavy sterile neutrino search

Search for sterile ν in K→μν

In the TREK experiment, we will search for the transverse muon polarization in K⁺ → π⁰μ⁺ν by measuring the μ⁺ momentum, π⁰ momentum, and μ⁺ polarization. Therefore, the TREK apparatus is very suited for the heavy neutrino search because

• The μ⁺ momentum generated from the K⁺ → μ⁺N decay is monochromatic. A peak structure is formed in the momentum spectrum.

• The μ⁺ polarization in the direction of μ⁺ motion depends on neutrino masses and N production mechanism.

In addition to the detector system for the Γ(Ke2)/Γ(Kμ2) measurement, which consist of the toroidal spectrometer and the CsI(Tl) calorimeter, the muon polarimeter will be used for the measurement of the longitudinal muon polarization. Two settings of the magnetic fields B = 1.4 T and 0.65 T will be chosen to cover a wide μ⁺ momentum region. The μ⁺ momentum is analyzed by the spectrometer and the μ⁺ is identified by the TOF measurement. After passing through the spectrometer, the muons are slowed down by a copper degrader and stop in the polarimeter. The magnetic field from the spectrometer magnet is shielded by adding new iron plates and an external decouping field of ≈10 Gauss is applied to preserve the muon polarization in longitudinal direction. The muon polarization in the longitudinal direction is measured as position asymmetry from μ⁺ → e⁺νν in forward and backward directions.