| Summary: | 碩士 === 國立臺灣大學 === 物理學研究所 === 105 === 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 .
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