[论文解读] Search for the CP symmetry violation in the decays of Ks mesons using the KLOE detector

M. Silarski|arXiv (Cornell University)|Feb 19, 2013

Particle physics theoretical and experimental studies参考文献 3被引用 6

一句话总结

本研究利用DAFNE上的KLOE探测器，通过运动学拟合与判别分析方法，在K_S → 3π⁰衰变中搜索CP破坏，识别六光子末态。实验在90%置信水平下设定了新的分支比上限：BR(K_S → 3π⁰) < 2.7 × 10⁻⁸，对应|η₀₀₀| < 0.009，为迄今最严格的约束。

ABSTRACT

The aim of this work was to determine the KS -&gt; 3pi0 decay branching ratio and a modulus of the eta000 parameter, defined as the ratio of amplitudes for KS -&gt; 3 pi0 to KL -&gt; 3pi0 decays, which characterizes the CP symmetry violation in this decay. The measurement has been carried out with the KLOE detector operating at the phi-factory DAFNE in the Italian National Center for Nuclear Physics in Frascati. The KS mesons were identified with high efficiency via registration of these KL mesons which crossed the drift chamber without decaying and then interacted with the KLOE electromagnetic calorimeter. The KS four-momentum vector was then determined using the registered position of the KL meson and the known momentum of the phi meson. Next, the search for the KS -&gt; 3pi0 -&gt; 6gamma decay was carried out by the selection of events with six gamma quanta which momenta were reconstructed using time and energy measured by the electromagnetic calorimeter. To increase the signal over background ratio after identification of the KS meson and requiring six reconstructed photons a discriminant analysis is performed. It is based on kinematical fit, testing of the signal and background hypotheses and exploiting of the differences in kinematics of the KS decays into 2pi0 and 3pi0. Hence, we have obtained the upper limit on the KS -&gt; 3pi0 branching ratio at the 90% confidence level BR(KS -&gt; 3pi0)&lt; 2.7 x 10^-8, which is almost five times lower than the latest published result. The search for the KS -&gt; 3pi0 decay presented in this work failed to detect a signal of sufficient statistical significance. This upper limit can be translated into a limit on the modulus of the eta000 parameter amounting to: |eta000| &lt; 0.009 at the 90% confidence level.

研究动机与目标

• 测量稀有衰变K_S → 3π⁰的分支比，该过程对CP破坏敏感。
• 确定η₀₀₀参数的模值，该参数量化了K_S → 3π⁰相对于K_L → 3π⁰的CP破坏程度。
• 在先前实验极限的基础上，提升对K_S衰变中CP破坏搜索的灵敏度。
• 应用先进的事件选择技术，抑制六光子末态中的本底

提出的方法

• 通过探测未在漂移室中衰变的K_L介子，识别K_S介子。
• 利用已知的φ介子四momentum和K_L在量能器中的测量位置，重建K_S的四momentum。
• 选择六条重建光子的事件，作为K_S → 3π⁰ → 6γ衰变的候选事件。
• 应用判别分析，结合运动学拟合与假设检验，区分信号与本底。
• 利用K_S → 2π⁰与K_S → 3π⁰衰变之间运动学差异，提升信号灵敏度。
• 使用90%置信水平区间对分支比和η₀₀₀参数设定统计上限。

实验结果

研究问题

• RQ1 在90%置信水平下，K_S → 3π⁰衰变的分支比上限是多少？
• RQ2 利用K_S → 3π⁰衰变，CP破坏参数|η₀₀₀|能被多精确地约束？
• RQ3 在当前探测器能力下，能否在本底之上探测到K_S → 3π⁰的信号？
• RQ4 新上限与先前实验结果相比如何？

主要发现

• 在90%置信水平下，K_S → 3π⁰衰变的分支比上限为BR(K_S → 3π⁰) < 2.7 × 10⁻⁸。
• 该结果比先前公布的极限低近五倍，显著提升了灵敏度。
• CP破坏参数的模值在90%置信水平下被约束为|η₀₀₀| < 0.009。
• 未观测到K_S → 3π⁰衰变的显著信号，与上限一致。
• 判别分析有效抑制了本底，增强了六光子末态中的信号识别能力。
• 该测量展示了KLOE探测器探测中性K介子衰变中稀有CP破坏过程的能力。