中性K介子之稀有衰變及電荷共軛破壞衰變的探尋

Abstract

本論文研究中性K介子至二個中性π介子以及假想X中性粒子的衰變。X粒子為HyperCP 實驗所提出的未知新粒子。理論計算推測此X粒子可能出現於中性K介子的衰變中。因此我們於KEK-E391a 實驗的數據中探詢X粒子。我們的研究結果顯示並未有X粒子存在的跡象，因此我們給予90%信心水準之衰變上限為 2.4x10-7。 本論文並呈現中性K介子至三個光子的衰變研究。此衰變為電荷共軛破壞，因此衰變分歧率極為小。在KEK-E391a的數據中，此衰變的事件並未發生。我們給予90%信心水準之衰變上限為 7.4x10-8。

We perform a search for a light pseudoscalar particle X in the decay K0L → π0π0X, X → γγ with the E391a detector at KEK-PS. Such a particle with a mass of 214.3 MeV/c2 is suggested by the HyperCP experiment. In the E391a Run-II data, we perform a complete search of the K0L → π0π0X, X → γγ decay in the X mass region of 194.3–219.3 MeV/c2 and the X lifetime region of τX < 10−9. No evidence for X is found. An upper limit on the product branching ratio for K0L → π0π0X, X → γγ of 2.4 × 10−7 at the 90% confidence level for the 214.3-MeV/c2 X mass. Upper limits on the the branching ratios for other X mass and lifetime regions are also determined. We also perform a search for the charge-conjugation violating decay K0L → 3γ. In the data accumulated in 2005, no event is observed in the signal region. Based on the assumption of K0L → 3γ proceeding via parity-violation, we obtain the single event sensitivity to be (3.23 ± 0.14) × 10−8, and set an upper limit on the branching ratio to be 7.4 × 10−8 at the 90% confidence level. This is a factor of 3.2 improvement compared to the previous results. The results of K0L → 3γ proceeding via parity-conservation and phase-space are also presented. We also present a new method of the timing calibration and timing simulation for the main barrel detector.