A pulse shape difference between photons and neutrons was observed in the output signals of scintillation light from the un-doped CsI calorimeter of the KOTO experiment for the KL→π0νν̄ decay. We developed a discrimination method to reject neutrons and to accept photons in the deposited energy range from 0.1 to 2 GeV. The method rejects 67% of neutron-induced single hadronic clusters and 86% of neutron-induced two-cluster events while keeping more than 90% of photon-induced single electromagnetic clusters and two-photon events.

J. K. Ahn, B. Beckford, M. Campbell, S. H. Chen, J. M. Choi, J. Comfort, K. Dona, M. S. Farrington, N. Hara, H. Haraguchi, Y. B. Hsiung, M. Hutcheson, T. Inagaki, M. Isoe, I. Kamiji, E. J. Kim, J. L. Kim, ∗ H. M. Kim, T. K. Komatsubara, K. Kotera, J. W. Lee, G. Y. Lim, 10 C. Lin, Q. S. Lin, Y. Luo, T. Mari, T. Matsumura, D. Mcfarland, K. Miyazaki, R. Murayama, † K. Nakagiri, ‡ H. Nanjo, H. Nishimiya, Y. Noichi, T. Nomura, T. Nunes, M. Ohsugi, H. Okuno, J. C. Redeker, K. Sato, § T. Sato, Y. Sato, N. Shimizu, T. Shimogawa, ‡ T. Shinkawa, S. Shinohara, ¶ K. Shiomi, 10 R. Shiraishi, S. Su, Y. Sugiyama, ‡ S. Suzuki, Y. Tajima, M. Taylor, M. Tecchio, M. Togawa, ‡ T. Toyoda, Y. C. Tung, ∗∗ Q. H. Vuong, Y. W. Wah, H. Watanabe, 10 T. Yamanaka, and H. Y. Yoshida

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}.

J. K. Ahn, B. Beckford, J. Beechert, K. Bryant, M. Campbell, S. H. Chen, J. Comfort, K. Dona, N. Hara, H. Haraguchi, Y. B. Hsiung, M. Hutcheson, T. Inagaki, I. Kamiji, N. Kawasaki, E. J. Kim, J. L. Kim, ∗ Y. J. Kim, J. W. Ko, T. K. Komatsubara, K. Kotera, A. S. Kurilin, † J. W. Lee, ‡ G. Y. Lim, 10 C. Lin, Q. Lin, Y. Luo, J. Ma, Y. Maeda, § T. Mari, T. Masuda, ¶ T. Matsumura, D. Mcfarland, N. McNeal, J. Micallef, K. Miyazaki, R. Murayama, ∗∗ D. Naito, ∗∗ K. Nakagiri, H. Nanjo, †† H. Nishimiya, T. Nomura, 10 M. Ohsugi, H. Okuno, M. Sasaki, N. Sasao, K. Sato, ‡‡ T. Sato, Y. Sato, H. Schamis, S. Seki, N. Shimizu, T. Shimogawa, ∗∗ T. Shinkawa, S. Shinohara, K. Shiomi, 10 S. Su, Y. Sugiyama, ∗∗ S. Suzuki, Y. Tajima, M. Taylor, M. Tecchio, M. Togawa, ∗∗ Y. C. Tung, Y. W. Wah, H. Watanabe, 10 J. K. Woo, T. Yamanaka, and H. Y. Yoshida

J-PARC KOTO Collaboration J. K. Ahn, K. Y. Baek, S. Banno, B. Beckford, B. Brubaker, T. Cai, M. Campbell, C. Carruth, S. H. Chen, S. Chu, J. Comfort, Y. T. Duh, T. Furukawa, H. Haraguchi, T. Hineno,Y. B. Hsiung, M. Hutcheson, T. Inagaki, M. Isoe, E. Iwai, T. Kamibayashi, I. Kamiji, N. Kawasaki, E. J. Kim, Y. J. Kim, J. W. Ko, T. K. Komatsubara, A. S. Kurilin, G. H. Lee, H. S. Lee, J. W. Lee, S. K. Lee, G. Y. Lim, C. Lin, J. Ma, Y. Maeda9,24,∗, T. Masuda, T. Matsumura, D. Mcfarland, J. Micallef, K. Miyazaki, K. Morgan, R. Murayama, D. Naito, K. Nakagiri, Y. Nakajima, Y. Nakaya H. Nanjo, T. Nomura, T. Nomura, Y. Odani, R. Ogata, H. Okuno, T. Ota, Y. D. Ri, M. Sasaki, N. Sasao, K. Sato, T. Sato, S. Seki, T. Shimogawa, T. Shinkawa, S. Shinohara, K. Shiomi, J. S. Son, J. Stevens, S. Su,Y. Sugiyama, S. Suzuki,Y. Tajima, G. Takahashi,Y. Takashima, M. Tecchio, I. Teo, M. Togawa, T. Toyoda, Y. C. Tung, T. Usuki, Y. W. Wah, H. Watanabe, N. Whallon, J. K. Woo, J. Xu, M. Yamaga, S. Yamamoto, T. Yamanaka, H. Yamauchi, Y. Yanagida, H. Yokota, H. Y. Yoshida, and H. Yoshimoto

We searched for the $CP$-violating rare decay of neutral kaon, $K_{L} \to \pi^0 \nu \overline{\nu}$, in data from the first 100 hours of physics running in 2013 of the J-PARC KOTO experiment. One candidate event was observed while $0.34\pm0.16$ background events were expected. We set an upper limit of $5.1\times10^{-8}$ for the branching fraction at the 90\% confidence level (C.L.). An upper limit of $3.7\times10^{-8}$ at the 90\% C.L. for the $K_{L} \to \pi^{0} X^{0}$decay was also set for the first time, where $X^{0}$ is an invisible particle with a mass of 135 MeV/$c^{2}$.

The KOTO data acquisition system (DAQ) collects detector PMT waveform signals and saves digitized events to permanent storage using frontend ADC modules, two levels of hardware triggers, and a computing farm. The KOTO DAQ system ran stably in 2013 with 24 kW beam power. To maintain high DAQ livetime with increasing beam power, we implemented lossless data compression inside the ADC modules and developed a new L3 computing farm. The upgraded KOTO DAQ system was able to maintain livetime above 80% with 42 kW beam power during the 2015 and 2016 runs. To sustain high DAQ livetime for data taking with beam power of 50 kW and above, an upgrade of our hardware trigger is proposed.

The KOTO experiment is a particle physics experiment located in J-PARC, Japan, aiming to explore physics beyond the Standard Model by measuring the branching ratio of the K_L→π^0νν̅ decay. This decay has not yet been observed. The branching ratio predicted by the Standard Model of (3.0±0.3)×10^-11 and the current experimental upper limit established by KEK E391a is 2.6×10^-8. The signal of K_L→π^0νν̅ decay has the signature of two photons on the calorimeter with no signal on the veto detectors. It also has a large transverse momentum due to missing neutrinos. Kaons that decay outside the beam line with final product of two photons, such as K_L→γγ and K_L→π^+π^-π^0, can appear to have large transverse momentum due to kaon scattering and beam interaction with the detectors. These off-axis kaon decay events can impact the upper limit of K_L→π^0νν̅ branching ratio. Aluminum targets located at the upstream of the KOTO detector and inside the decay-volume were used to study kaon beam profile, which provided off-axis kaon decay vertex information. The beam profile provided insights on background contributions to the signal. Studies on the kaon beam profile and background identification from kaon scattering were presented in this talk.

A major upgrade to the KOTO detector data acquisition system based on the ATCA standard is being considered. The ATCA standard provides a natural solution to the current KOTO constraints, including communication between boards and higher input and output bandwidth.

We searched for the CP-violating rare decay of neutral kaon, K_L→π^0 νν, in data from the first 100 hours of physics running in 2013 of the J-PARC KOTO experiment. One candidate event was observed while 0.34±0.16 background events were expected. We set an upper limit of 5.1×10^-8 for the branching fraction at the 90% confidence level (C.L.). An upper limit of 3.7×10^-8 at the 90% C.L. for the K_L→π^0 X^0decay was also set for the first time, where X^0 is an invisible particle with a mass of 135 MeV/c^2.

The KOTO experiment at J-PARC in Tokai, Ibaraki, Japan, aims to observe rare neutral kaon decay mode K L → π 0 νν. Followed by the first KOTO physics run in May 2013 with 24 kW beam power, we upgraded the KOTO data acquisition system in 2015 to accommodate efficient and reliable data collection with higher beam intensities. Lossless data compression inside the ADC modules was implemented to reduced the size of data packets. The lossless data compression enhanced the data collection rate by a factor of three. We designed a new software trigger, which consists of 47 computer nodes. It uses Infiniband hardware with MPI protocol to establish mesh network within the computer cluster and parallel data processing. The upgrades of the KOTO data acquisition system were commissioned in 2015 and used to successfully collect data with beam intensity up to 42 kW. In preparation for increasing beam intensities in 2016 runs, we are developing the hardware trigger upgrades using the RCE Platform Technology (RPT).

The goal of KOTO experiment at J-PARC is to discover and measure the rate of the rare decay K L → π 0 ν ν, for which the Standard Model predicts a branching ratio of (2.4 ± 0.4)×10 -11 .The

Quantitative adsorption structure determinations on quasicrystals are scarce because most techniques for measuring surface structures are not well suited to the complex and infinite unit cells of quasicrystals. The normal incidence standing x-ray wave field technique presents a solution to these problems because it can be made inherently surface sensitive and does not involve extensive computational effort. We describe a method for applying this technique to adsorbates on quasicrystals, with specific application to a submonolayer of Si atoms on a decagonal Al-Co-Ni surface. We demonstrate the sensitivity of the technique to both adsorption site and geometry, leading to the conclusion that the Si atoms, which form six-atom pentagonal clusters, have an average height of 1.77 +/- 0.05 angstrom above pentagonal hollow sites, with a significant height variation among the Si atoms in the cluster. In particular, the central Si atom sits more deeply than the five surrounding Si atoms, which are, on average, 2.7 angstrom away from the central Si atom. Although this study was performed on a decagonal quasicrystal that is periodic perpendicular to the surface, we describe how the technique can be applied to cases with no periodicity.

The quantitative structure determination of adsorbed species on quasicrystal surfaces has so far appeared to present insurmountable problems. The normal incidence standing x-ray wave field technique offers a simple solution, without extensive data sets or large computations. Its application to quasicrystals raises several conceptual difficulties that are related to the phase problem in x-ray diffraction. We demonstrate their solution for the case of Si atoms adsorbed on the decagonal Co-rich modification of the Al-Co-Ni quasicrystal to determine the local structure, comprising 6-atom clusters in particular hollow sites.

C${}_{60}$ monolayers grown on Ag(111) at room temperature form incommensurate lattices that convert into a commensurate (2$\sqrt{3}$ \ifmmode\times\else\texttimes\fi{} 2$\sqrt{3}$)R30\ifmmode^\circ\else\textdegree\fi{} phase upon annealing. The C${}_{60}$ molecules in the commensurate phase have been observed to exist in three different states on Ag(111), namely bright, dim, and superbright (SB). All three species are in dynamical equilibrium at 280 $T$ 440 K. The bright and dim species were the subject of an earlier low-energy electron diffraction study that determined their geometries on the surface and the dynamics of the switching between those two states. The study presented here takes a closer look at the SB molecules, which appear and disappear at temperature-dependent rates with a measured activation barrier of 1.5 eV. The SB molecules in the commensurate phase comprise about 0.5% of the molecules and have a spatially random distribution. The evidence suggests that the formation of the three different states of C${}_{60}$ on Ag(111) is a result of stress imposed by the substrate as the C${}_{60}$ adopts the commensurate spacing that is slightly smaller than its natural spacing. In the incommensurate phases, there is no bright-dim contrast, but SB C${}_{60}$ molecules form and organize into ordered arrays that appear to correspond to the moir\'e patterns that are produced by the mutually incommensurate lattices. This suggests that the substrate responds to the nonuniform forces imposed by the C${}_{60}$ molecules by producing raised islands of Ag atoms at the vertices of the moir\'e structure. A similar island structure may account for the SB molecules in the commensurate phase.

K. Pussi,1 H. I. Li,2 Heekeun Shin,2 L. N. Serkovic Loli,3 A. K. Shukla,3 J. Ledieu,3 V. Fournee,3 L. L. Wang,4 S. Y. Su,2 K. E. Marino,2 M. V. Snyder,2 and R. D. Diehl2 1Department of Mathematics and Physics, Lappeenranta University of Technology, P.O. Box 20 FIN-53851 Lappeenranta, Finland 2Department of Physics, Penn State University, University Park, Pennsylvania 16802, USA 3Institut Jean Lamour, UMR 7198 CNRS Universite de Lorraine, Parc de Saurupt, 54042 Nancy Cedex, France 4Division of Materials Science & Engineering, Ames Laboratory, Ames, Iowa 50011, USA (Received 17 August 2012; published 5 November 2012)