Search for T-violating transverse muon polarization in the K+ ---> pi0 mu+ nu decay

At the 12-GeV proton synchrotron of KEK, an experiment (KEK-PS-E246) was performed to measure the transverse muon polarization (P{sub T}) in K{sup +}{yields}{pi}{sup 0}{mu}{sup +}{nu} decays as a direct search of violation of time reversal invariance (T-violation). P{sub 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}{sup +} with the spectrometer and a {pi}{sup 0} with a CsI(Tl) calorimeter surrounding the target as two photons as well as one photon with high energy. For the P{sub T} measurement, {pi}{sup 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{sub T} polarization component. In the data analysis a novel technique of the two-analysis method was employed. Two independent analyses more » 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{sub 0}, the sensitivity function A{sub 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{sup 6} good events after the two-analysis combination, and deduced P{sub 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 |P{sub T}|<0.0050 (90% C.L.) and vertical bar Im{xi} vertical bar <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 |Im({alpha}{sub 1}{gamma}{sub 1}*)|<544(M{sub H{sub 1}}/GeV){sup 2} was obtained. Implications for several other models are discussed. « less