Search for the Pair Production of Dark Particles X with K_{L}

We present the first search for the pair production of dark particles X via K_{L}^{0}→XX with X decaying into two photons using the data collected by the KOTO experiment. No signal was observed in the mass range of 40-110 MeV/c^{2} and 210-240 MeV/c^{2}. This sets upper limits on the branching fractions as B(K_{L}^{0}→XX)<(1-4)×10^{-7} and B(K_{L}^{0}→XX)<(1-2)×10^{-6} at the 90% confidence level for the two mass regions, respectively.

Study of the K_{L}→π

The rare decay K_{L}→π^{0}νν[over ¯] was studied with the dataset taken at the J-PARC KOTO experiment in 2016, 2017, and 2018. With a single event sensitivity of (7.20±0.05_{stat}±0.66_{syst})×10^{-10}, three candidate events were observed in the signal region. After unveiling them, contaminations from K^{±} and scattered K_{L} decays were studied, and the total number of background events was estimated to be 1.22±0.26. We conclude that the number of observed events is statistically consistent with the background expectation. For this dataset, we set an upper limit of 4.9×10^{-9} on the branching fraction of K_{L}→π^{0}νν[over ¯] at the 90% confidence level.

Search for K_{L}→π

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}.

Well differentiated adult-type fibrosarcoma arising from the occipital subcutaneous tissue in a 17-year-old man: case report with immunohistochemical study.

We report a case of a 17-year-old man with a spindle cell tumor in the occipital subcutaneous tissue. The enucleated tumor, measuring 2.5 x 2.0 x 1.0 cm, had a broad-bean shape and well circumscribed border and was localized to the subcutis without dermal involvement. Microscopically, the tumor was composed of uniform spindle cells showing interlacing bundle formation and a herringbone pattern. The neoplastic cells were separated by collagen fibers in parallel fashion, the amount of which varied with different areas in the tumor. Mitotic figures were eight mitoses per 50 high-power fields in number. The neoplastic cells were positive for vimentin, Factor-XIIIa, alpha-smooth muscle actin and CD34, but negative for desmin, calponin, high molecular weight caldesmon, smooth muscle myosin heavy chain, collagen type IV, laminin and S-100 protein. These immunohistochemical results indicated that the neoplastic cells showed differentiation toward fibroblasts/myofibroblasts/dendritic interstitial cells. Although more than 50% of the neoplastic cells were positive for CD34, the present tumor should be diagnosed as well differentiated fibrosarcoma, adult type, rather than extrapleural solitary fibrous tumor and fibrosarcomatous areas of dermatofibrosarcoma protuberans, on the basis of routine microscopic findings mentioned above.