Measurement of \(\mu \)SR spectra in a scintillating LaF\(_3\) material

A new experimental method to search for T-violating transverse muon polarization ( \(P_T\) $P_T$ ) in the \(K^+ \rightarrow \pi ^0 \mu ^+ \nu \) $K^{+}\to {\pi }^0{\mu }^{+}\nu$ decay using an electro-magnetic calorimeter has been proposed. The test experiment to find good scintillating materials was performed at the J-PARC Material and Life Science Facility (MLF), and the residual \(\mu ^+\) ${\mu }^{+}$ polarization in a scintillating CeF \(_3\) ${}_3$ material was found to be higher than 90%. It is highly preferable to find other scintillating materials whose performance is nearly equivalent to CeF \(_3\) ${}_3$ to increase effectiveness of the new polarimeter system in various experiments. A muon spin relaxation/rotation ( \(\mu \) $\mu$ SR) experiment was performed using a 2% Ce-doped LaF \(_3\) ${}_3$ crystal. The time-integrated residual polarization in LaF \(_3\) ${}_3$ within the time region of 10 \(\mu \) $\mu$ s was obtained to be 83% and 87% for LF=100 and 140 Gauss, respectively, which is high enough to perform the new T-violation experiment at J-PARC. The ZF \(\mu \) $\mu$ SR spectra in the temperature region from room temperature to 328 K were taken to check the reproducibility of the \(\mu \) $\mu$ SR spectra reported in the previous experiment, in which the rapid transition of the zero-field \(\mu \) $\mu$ SR spectra was observed at 323 K. The ZF \(\mu \) $\mu$ SR spectrum obtained at 328 K is in good agreement with that at room temperature, and an obvious change reported in the previous study [8] was not observed in the present work.