Measurement of muon spin relaxation time in scintillating materials

new experimental method to search for T -violating transverse muon polarization ( P_T ) in the K^+ →π ^0 μ ^+ ν decay has been proposed. In this new experiment, the measurements of the μ ^+ momentum vector, the π ^0 momentum vector, and the μ ^+ polarization will be performed by the same electro-magnetic calorimeter. One of main issues is the choice of a scintillation material which can preserve the μ ^+ spin polarization for several μ ^+ lifetimes. A test experiment to measure the μ ^+ polarization in various scintillating crystals was performed at J-PARC Material and Life Science Facility (MLF). We concluded that the μ ^+ polarization in CeF _3 is high enough to perform the new T -violation experiment.