Measurement of residual μ+ polarization in various scintillating materials to search for T-violating μ+ polarization in K+→π0μ+ν decay

A new experimental method to search for T-violating transverse muon polarization (PT) in the K+→π0μ+ν (Kμ3) decay using a segmented CeF3 calorimeter has been proposed. One of the key issues for this experiment is the choice of a scintillation material which can maintain the μ+ spin polarization for several μ+ lifetimes. The residual μ+ polarization in a CeF3 scintillating crystal was previously measured at the J-PARC Material and Life Science Facility (MLF). Unfortunately, due to the small size of the CeF3 material, beam backgrounds contributed significantly to the μSR spectra, and the absolute μ+ polarization could not be determined accurately. In order to solve this problem, a new experiment was performed at the MLF using a large CeF3 crystal, which successfully reduced the beam backgrounds to a negligible level. The residual μ+ polarization in CeF3 was measured by changing the strength of the applied longitudinal field (LF). The polarization was obtained to be 90 ± 2% with LF = 140 Gauss, which is high enough to perform the new T-violation experiment.