Department of Physics, Korea University, Seoul 02841, Republic of Korea.
Department of Physics, University of Michigan, Ann Arbor, Michigan 48109, USA.
Phys Rev Lett. 2019 Jan 18;122(2):021802. doi: 10.1103/PhysRevLett.122.021802.
A search for the rare decay K_{L}→π^{0}νν[over ¯] was performed. With the data collected in 2015, corresponding to 2.2×10^{19} protons on target, a single event sensitivity of (1.30±0.01_{stat}±0.14_{syst})×10^{-9} was achieved and no candidate events were observed. We set an upper limit of 3.0×10^{-9} for the branching fraction of K_{L}→π^{0}νν[over ¯] at the 90% confidence level (C.L.), which improved the previous limit by almost an order of magnitude. An upper limit for K_{L}→π^{0}X^{0} was also set as 2.4×10^{-9} at the 90% C.L., where X^{0} is an invisible boson with a mass of 135 MeV/c^{2}.
进行了一项对罕见衰变 K_{L}→π^{0}νν[over ¯] 的搜索。利用 2015 年采集的对应于 2.2×10^{19} 个质子靶的数据分析，实现了单个事例灵敏度为 (1.30±0.01_{stat}±0.14_{syst})×10^{-9}，没有观察到候选事例。我们在 90%置信水平（C.L.）下对 K_{L}→π^{0}νν[over ¯]的分支比设定了一个 3.0×10^{-9}的上界，这比之前的结果提高了近一个数量级。我们还在 90%置信水平下对 K_{L}→π^{0}X^{0}设定了一个 2.4×10^{-9}的上界，其中 X^{0}是一个质量为 135 MeV/c^{2}的不可见玻色子。
