Speaker
Description
Time reversal symmetry has long been a subject of interest from pre-modern physics time, since it implies the reversibility of motion. In the $K^+ \to \pi^o\mu^+ \nu$ ($K_{\mu3}$) decay, the transverse muon polarization (𝑃𝑇) is defined as the polarization component perpendicular to the decay plane. A non-vanishing value of 𝑃𝑇 provides clear evidence for T-violation under the condition that spurious effects from final state interactions are negligibly small. We are now proposing a new T-violation experiment to achieve $\Delta PT \sim 10^{-5}$ at the J-PARC Hadron Hall without using a magnetic spectrometer. The most important characteristic of the new experiment is the measurements of the muon momentum vector, the $\pi^o$ momentum vector, and the muon polarization by the same highly segmented sequential electro-magnetic calorimeter surrounding the $K^+$ stopping target. Here it should be noted that one of key issues in the experiment is the choice of a scintillation material which can preserve the muon spin polarization for a reasonably long time [1-3].
The $\mu^+$ polarization can be determined by the delayed $e^+$ signals from the $\mu^+$ decay detected by the calorimeter module around the muon stop, as shown in Fig.1. The experimental method to measure the $e^+$ asymmetry by selecting events with $\pi^o$ going forward and backward direction is adopted to suppress systematic uncertainties.
Furthermore, the analyzing power in the polarization measurement should be improved by measuring the $e^+$ energy using the calorimeter [3] because the magnitude of the $e^+$ asymmetry depends on the $e^+$ energy, as shown in Fig.2, while only the energy-integrated asymmetry is obtained in the standard polarization measurement. The dedicated analysis method has been developed by separating events into partial energy regions and optimizing weight parameters to averaging the $e^+$ asymmetry in each bin. We will apply this method in practice for a muon spin imaging experiment which will be conducted at TRIUMF in Canada this November, and we will also present details of this application as an application of the present method. In this talk, some details of the future T-violation experiment, a new polarimeter system, and significance of the present method for improving statistical uncertainties in the polarization measurement will be reported.
References
[1] S. Shimizu et al., Nucl. Instrum. Methods A 945 (2019) 162587.
[2] K. Horie et al., Nucl. Instrum. Methods A 1037 (2022) 166932.
[3] K. Horie et al., Nucl. Instrum. Methods A 1066 (2024) 169606.