在KOTO實驗中尋找無質量暗光子：K0 → gamma + gamma*

Abstract

本論文展示了在J-PARC KOTO實驗中尋找質量為零的暗光子（gamma*）於 K0 -> gamma + gamma* 衰變的研究，此研究基於2020年特殊運行數據所收集的數據。 與有質量的暗光子不同，質量為零的暗光子不會直接與普通光子混合，但可以通過直接與夸克耦合來與標準模型（SM）粒子互動。一些理論模型提出 K0 -> gamma + gamma* 衰變的分支比（BR）可能達到O(−3)。 在特殊運行中收集的K0衰變數量估計為(1.29 ± 0.02) × 1010。單一事件的靈敏度計算為(2.91 ± 0.02(stat.) ± 0.30(syst.))× 10(−8)。總背景水平的預測為(12.66 ± 4.42(stat.) ± 2.13(syst.))， 並與邊帶區域的數據相符。我們揭開了盲區並觀測到13個事件。 使用Feldman-Cousins方法計算 K0 -> gamma + gamma* 衰變的分支比上限為< 3.47 × 10(−7)(90%C.L.)。

This thesis presents the search for the massless dark photon (gamma*) in the K0 -> gamma + gamma* decay at the J-PARC KOTO experiment, based on the special run data collected in 2020. Distinguished from the massive dark photon, the massless one does not directly mix with the ordinary photon but could interact with Standard Model (SM) particles through direct coupling to quarks. Some theoretical models propose that the branching ratio (BR) of the K0 -> gamma + gamma* decay could reach up to O(10−3). The number of K0 decays that had been collected in the special run is estimated to be (1.29 ± 0.02) × 1010. The single event sensitivity is calculated to be (2.91 ± 0.02(stat.) ± 0.30(syst.)) × 10−8. The total background level prediction is (12.66 ± 4.42(stat.) ± 2.13(syst.)) with the agreement from the side-band region. We uncovered the blind region and observed 13 events. The Feldman-Cousins method is used to calculate the upper limit of the K0 -> gamma + gamma* branching ratio to be < 3.47 × 10−7(90%C.L.).