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

Apr 1, 2006
34 pages
Published in:
  • Phys.Rev.D 73 (2006) 072005

Citations per year

2006201120162021202401234567
Abstract: (APS)
At the 12-GeV proton synchrotron of KEK, an experiment (KEK-PS-E246) was performed to measure the transverse muon polarization (PT) in K+→π0μ+ν decays as a direct search of violation of time reversal invariance (T-violation). PT 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 μ+ with the spectrometer and a π0 with a CsI(Tl) calorimeter surrounding the target as two photons as well as one photon with high energy. For the PT measurement, π0 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 PT 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 A0, the sensitivity function AN, 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 11.8×106 good events after the two-analysis combination, and deduced PT=−0.0017±0.0023(stat)±0.0011(syst), corresponding to the T-violating parameter Imξ=−0.0053±0.0071(stat)±0.0036(syst). From these results the upper limits of |PT|<0.0050 (90% C.L.) and |Imξ|<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(α1γ1*)|<544(MH1/GeV)2 was obtained. Implications for several other models are discussed.
  • 12.60.Fr
  • 11.30.Er
  • 13.20.Eb
  • K+: secondary beam
  • K+: semileptonic decay
  • at rest
  • muon+: polarization
  • polarization: transverse
  • time reversal: violation
  • upper limit
Loading ...