Study of CP Violation in the Neutral Kaon System

1996年至1997年，我们在费米实验室进行了KTeV实验，得到了以下结果：通过测量K_L分支分数的两倍比，我们发现在衰变本身中CP被违背了（直接CP违背），Ks * <产物> + <产物>和<产物>^0 <产物>^0衰变。根据20%的数据，我们得到了初步结果Re(e′/e) * (28 0*4.1) x 10^<-4>，距离0有6个标准差以上。这一结果否定了解释CP破坏机制的理论之一的超弱模型。在K_L * <product> + <product> e+e-衰变中，我们观察到第一个CP违逆，即<product> + <product>-平面和e+e-平面之间的角度分布不对称。我们搜索了以直接CP违背为主的衰变K_L * <产物>^Onunu，并确定了分支比的最佳上限<5.9x10^7 (90%CL)（初步）。我们还利用标记光子束在KEK Tanashi测量了由于光核相互作用导致的光子探测器的检测效率低下。我们发现纯CsI和铅/闪烁体夹层探测器满足未来K_<1L> * _<1@: s10> nunu实验所需的规格。我们召开了一个关于K中CP违背的国际研讨会，并对未来测量K_L * ^Onunu衰变分支比的实验进行了广泛的讨论。

We ran KTeV Experiment at Fermilab from 1996 to 1997, and obtained following results.1. We found that the CP is violated in the decay itself (direct CP violation), by measuring the double ratio of the branching fractions of K_L, Ks * <product> + <product> -and <product>^0 <product>^0 decays. Based on the 20% of the data, we obtained a preliminary result, Re(e'/e) * (28 0*4.1) x 10^<-4> which is more than 6 standard deviations away from 0. This result rejects the Superweak model which was one of the theories explaining the CP violation mechanism.2. We observed the first CP violation in a kinematical variable, the asymmetry in the distribution of the angle between <product> + <product>- and e+e-planes, in the K_L * <product> + <product> e+e- decay.3. We searched for the decay K_L * <product>^Onunu, which is dominated by direct CP violation, and set the best upper limit on the branching ratio, <5.9x10^7 (90%CL) (preliminary).We also measured the detection inefficiency of photon detectors due to photo-nuclear interactions by using tagged photon beam at KEK Tanashi. We found that pure CsI and lead/scintillator sandwich detectors satisfy the specs necessary for the future K_<1L> * _<1@ : S1O>nunu experiments.We had an international workshop on the CP violation in K, and had an extensive discussion on the future experiments to measure the branching ratio of K_L * ^Onunu decay.