在 KOTO 實驗中尋找長生命期中性 K 介子之稀有衰變K L → π 0 ν ν ̄ 以及 K L → π 0 X(X 至不可見粒子 )

Abstract

我們在位於日本大強度質子加速器研究綜合機構(J-Parc)的KOTO實驗中尋找長生命期中性K介子(KL)的電荷宇稱不守恆衰變KL→π0νν ̄。在標準模型中此衰變模式的分支比有明確的計算結果,且其預測值僅:Br(KL→π0νν ̄)=(3.00±0.30)×10−11。基於此特性,測量KL→π0νν ̄的分支比不僅可以直接測量CKM矩陣中的參數η,更是一個有機會發現新物理的良好途徑。 本論文使用KOTO實驗於西元2015年取得的數據,其數據量約莫2013年5月的20倍。2015年的數據依照月份及其質子束通量可分為9個時期,我們首先利用衰變模式KL→3π0、KL→2π0以及KL→γγ分別對每個時期的數據做KL於KOTO偵測器前方的通量分析,不同衰變模式的結果在系統誤差的估計範圍內。 在KL→π0νν ̄分析中,我們使用盲測實驗法,並且以得到的通量作為歸一化常數,分析可能的背景事件、系統誤差以及計算對單一訊號事件敏感度(S.E.S.)。該分析的結果為 S.E.S.=(1.27±0.01stat±0.1sys)×10−9 其預期的背景事件數量為1.32±0.33。除此之外,在尋找KL→π0X,(X→不可見粒子)衰變的分析中,我們利用KL→π0νν ̄的蒙地卡羅模擬計算出不同X質量對應到的對單一訊號事件敏感度,得到當X質量等於π0質量時,其對單一訊號事件敏感度為0.99×10−9。

A search of a direct CP-violating decay K L → π 0 ν ν ̄ is performedin KOTO experiment, which was launched in Japan Proton AcceleratorComplex (J-Parc). K L → π 0 ν ν ̄ is highly suppressed in the Standard Model (SM), and the branch ratio Br(K L → π 0 ν ν ̄ ) = (3.00 ± 0.30) ×10 −11 is given by the SM prediction. Due to its theoretically cleanness, the measurement of K L → π 0 ν ν ̄ branching ratio provide a direct measurement of η in CKM matrix, as well as a good probe to the New Physics. This thesis performs an analysis with data collected in 2015, in which the statistics of data is about 20 times larger than that taken in May 2013. There are 9 periods with different intensity of K L beam during the 4-months data taking in 2015. The analysis of K L flux has been done for each periods using 3 normalization modes: K L → 3π 0 , K L → 2π 0 , and K L → γγ, and the results are consistent with each other. Using the blind analysis method, a search for K L → π 0 ν ν ̄ , which consists of estimation of systematic uncertainty, background study, and normalization, is then performed based on 2015 data. As a preliminary result, the single event sensitivity (S.E.S.) is obtained as S.E.S. = (1.27 ± 0.01 stat ± 0.1 sys ) × 10 −9 . The expected number of background in the signal region is 1.32±0.33. In addition, for K L → π 0 X, (X → invisible) search, X mass is calculated by means of invariant mass of ν ν ̄ in K L → π 0 ν ν ̄ MC simulation, and S.E.S. for M X = M π 0 is obtained as 0.99 × 10 −9 .