Study of Symmetry in Kaon Decays

在KEK-PS的E246国际合作下，通过高精度测量K^+*pi^0mu^+nu衰变中的横向介子极化P_T，对时间反转不变性(T)破坏进行了研究。在这种带电介子衰变中，来自末态相互作用的虚假效应很小。此外，标准模型对P_T的贡献可以忽略不计。因此，P_T对标准模型以外的CP破坏机制非常敏感。多希格斯双重态或轻夸克和超对称模型是目前比较感兴趣的模型。实验使用了一束停止的K^+束流和一台超导环面光谱仪。停止K^+可以覆盖整个衰变运动学区域。结合环面30ﾟ的旋转对称性，几乎消除了所有的系统误差。该探测器包括：1)一个有256根闪烁光纤的有源靶和一个环形计数器系统；2)一个有768个CsI(TI)晶体的pi^0探测器；3)一个四线室的Muon跟踪系统；4)TOF计数器系统；5)Muon偏振器。P_T是指极化元件中衰变正电子的顺时针/逆时针不对称性。数据采集始于1996年。在数据采集的早期阶段，验证了良好的探测器性能。正电子时间谱包含一个非常小的与时间无关的背景。对于pi^0的识别，可以分析双光子和单光子事件。1996年和1997年的数据正在分析中。根据初步结果，违反T的参数Imxi与零一致，统计精度为2×10~(-2)~(-2)，取代了BNL-AGS以前的限制。系统误差比实验数据的统计误差小得多。

A Search for time-reversal invariance (T) violation, via a high precision measurement of the transverse muon polarization, P_T in the K^+*pi^0mu^+nu decay, was conducted by the E246 international collaboration at KEK-PS.A non-zero value of P_T would be clear evidence for T-violation. The spurious effect from final state interactions is small in this charged kaon decay. In addition, the standard model contribution to P_T is negligibly small. Hence P_T is very sensitive to mechanismes of CP-violation beyound the standard model. The multi-Higgs doublet or leptoquark and supersymmetric models are currently of interest.The experiment used a stopped K^+ beam and a Superconducting Toroidal Spectromer. With stopped K^+ the entire decay kinematics region could be covered. Combined with the 30ﾟ rotational symmetry of the toroid almost all systematic errors were cancelled. The detector consisted of 1) an active target with 256 scintillating fibers and a ring counter system, 2) a pi^0 detector with 768 CsI (TI) crystals, 3) a muon tracking system with four wire chambers, 4) TOF counter system, and 5) muon polarimeters. P_T was measured as the clockwise/counterclockwise asymmetry of decay positrons in the polarimeter.Data taking started in 1996. At an early stage of data taking, good detector performance was verified. The positron time spectra contained a very small level of time independent background. For pi^0 identification both two photon and single photon events could be analyzed. The data taken in 1996 and 1997 are under analysis. According to a preliminary result, the T-violating parameter Imxi is consistent with zero with a statistical accuracy of 2x10^<-2> superceding the previous limit from BNL-AGS.Systematic errors have been shown to be much smaller than the statistical ones from the experimental data.