Search for T-violating transverse muon polarization in the K+→π0μ+ν decay

At the 12-GeV proton synchrotron of KEK, an experiment (KEK-PS-E246) was performed to measure the transverse muon polarization (P-T) in K+->pi(0)mu(+)nu decays as a direct search of violation of time reversal invariance (T-violation). P-T is a very sensitive probe of CP violation beyond the standard model. A stopped positive kaon beam was used. An elaborate high-acceptance detector, in conjunction with a 12-sector superconducting toroidal spectrometer, was utilized. The stopped kaon method provided several advantages in performing a high-precision experiment. The decay was identified by detecting a mu(+) with the spectrometer and a pi(0) with a CsI(Tl) calorimeter surrounding the target as two photons as well as one photon with high energy. For the P-T measurement, pi(0) events in the forward (fwd) and backward (bwd) regions in the calorimeter were relevant. Systematic errors were greatly suppressed by exploiting the rotational and fwd-bwd symmetry of the system. Polarization measurement was done in terms of asymmetry measurement of decay positrons of stopped muons in the polarimeter. A longitudinal field method was applied for the P-T polarization component. In the data analysis a novel technique of the two-analysis method was employed. Two independent analyses were performed following their own policy and criteria and good events were selected. Then, the two analyses were combined. A data quality check was carefully done by looking at the null asymmetry A(0), the sensitivity function A(N), the kinematical attenuation coefficient, and the decay-plane distributions. The asymmetry measurement for finite-size muon stoppers was performed "differentially" using the position information by the chamber in front of the polarimeter. From the measurements during 1996-2000, we accumulated 11.8x10(6) good events after the two-analysis combination, and deduced P-T=-0.0017 +/- 0.0023(stat)+/- 0.0011(syst), corresponding to the T-violating parameter Im xi=-0.0053 +/- 0.0071(stat)+/- 0.0036(syst). From these results the upper limits of parallel to P-T parallel to < 0.0050 (90% C.L.) and parallel to Im xi parallel to < 0.016 (90% C.L.) were deduced. Systematic errors were carefully studied and estimated. Almost all of the errors were canceled by the 12-sector summation and/or fwd-bwd subtraction, and the total systematic error was concluded to be as a half of the statistical error. The present result set constraints on model parameters of several theoretical models. For the three-Higgs-doublet model, e.g., a limit parallel to Im(alpha(1)gamma(*)(1))parallel to < 544(M-H1/GeV)(2) was obtained. Implications for several other models are discussed.