The dynamics of a magnetic domain wall (DW) under a transverse magnetic field Hy are investigated in two-dimensional (2D) Co/Ni microstrips, where an interfacial Dzyaloshinskii-Moriya interaction (DMI) exists with DMI vector D lying in +y direction. The DW velocity exhibits asymmetric behavior for ±Hy; that is, the DW velocity becomes faster when Hy is applied antiparallel to D. The key experimental results are reproduced in a 2D micromagnetic simulation, which reveals that the interfacial DMI suppresses the periodic change of the average DW angle φ even above the Walker breakdown and that Hy changes φ, resulting in a velocity asymmetry. This suggests that the 2D DW motion, despite its microscopic complexity, simply depends on the average angle of the DW and thus can be described using a one-dimensional soliton model. These findings provide insight into the magnetic DW dynamics in 2D systems, which are important for emerging spin-orbitronic applications.

We performed a search for the decay $K_L^0 \rightarrow 3\gamma$ with the E391a detector at KEK. In the data accumulated in 2005, no event was observed in the signal region. Based on the assumption of $K_L^0 \rightarrow 3\gamma$ proceeding via parity-violation, we obtained the single event sensitivity to be $(3.23\pm0.14)\times10^{-8}$, and set an upper limit on the branching ratio to be $7.4\times10^{-8}$ at the 90% confidence level. This is a factor of 3.2 improvement compared to the previous results. The results of $K_L^0 \rightarrow 3\gamma$ proceeding via parity-conservation were also presented in this paper.

The rare decay K0L→π0π0ν¯ν was studied with the E391a detector at the KEK 12-GeV proton synchrotron. Based on 9.4×109 K0L decays, an upper limit of 8.1×10−7 was obtained for the branching fraction at 90% confidence level. We also set a limit on the K0L→π0π0X (X→invisible particles) process; the limit on the branching fraction varied from 7.0×10−7 to 4.0×10−5 for the mass of X ranging from 50 MeV/c2 to 200 MeV/c2.Received 13 June 2011DOI:https://doi.org/10.1103/PhysRevD.84.052009© 2011 American Physical Society

The rare decay K-L(0) -> pi(0)pi(0)nu(nu) over bar was studied with the E391a detector at the KEK 12-GeV proton synchrotron. Based on 9.4 x 10(9) K-L(0) decays, an upper limit of 8.1 x 10(-7) was obtained for the branching fraction at 90% confidence level. We also set a limit on the K-L(0) -> pi(0)pi X-0 (X -> invisible particles) process; the limit on the branching fraction varied from 7.0 x 10(-7) to 4.0 x 10(-5) for the mass of X ranging from 50 MeV/c(2) to 200 MeV/c(2).

The rare decay K0(L) –> pi0 pi0 nu nu-bar was studied with the E391a detector at the KEK 12-GeV proton synchrotron. Based on 9.4 x 10^9 K0L decays, an upper limit of 8.1 x 10^-7 was obtained for the branching fraction at 90 confidence level. We also set a limit on the K0(L) –> pi0 pi0 X (X –> invisible particles) process; the limit on the branching fraction varied from 7.0 x 10^-7 to 4.0 x 10^-5 for the mass of X ranging from 50 MeV/c^2 to 200 MeV/c^2.

A measurement of the decay K+ -> pi0 mu+ nu_mu gamma has been performed with the E787 detector at Brookhaven National Laboratory. Forty events were observed in the signal region with the background expectation of (16.5 +- 2.7) events. The branching ratio was measured to be (1.58 +- 0.46(stat.) +- 0.08(syst.)) x 10^{-5}} in the kinematic region Eg >30 MeV and theta_{mu gamma} > 20degree, where Eg is the energy of the emitted photon and theta_{mu gamma} is the angle between the muon and the photon in the K+ rest frame. The results were consistent with theoretical predictions.

S. Adler, A.O. Bazarko, ∗ P.C. Bergbusch, E.W. Blackmore, D.A. Bryman, S. Chen, † I-H. Chiang, M.V. Diwan, J.S. Frank, T. Fujiwara, ‡ J.S. Haggerty, J. Hu, T. Inagaki, M.M. Ito, § D.E. Jaffe, V. Jain, ¶ S. Kabe, S.H. Kettell, P. Kitching, M. Kobayashi, T.K. Komatsubara, A. Konaka, Y. Kuno, ∗∗ M. Kuriki, †† K.K. Li, L.S. Littenberg, J.A. Macdonald, ‡‡ P.D. Meyers, J. Mildenberger, M. Miyajima, N. Muramatsu, T. Nakano, C. Ng, §§ S. Ng, T. Nomura, ¶¶ T. Numao, J.-M. Poutissou, R. Poutissou, G. Redlinger, ∗∗∗ T. Sato, K. Shimada, T. Shimoyama, T. Shinkawa, † † † F.C. Shoemaker, ‡‡ J.R. Stone, R.C. Strand, S. Sugimoto, Y. Tamagawa, T. Tsunemi, ‡ ‡ ‡ C. Witzig, and Y. Yoshimura

We performed a search for the decay K_L^0 → 3γ with the E391a detector at KEK. In the data accumulated in 2005, no event was observed in the signal region. Based on the assumption of K_L^0 → 3γ proceeding via parity-violation, we obtained the single event sensitivity to be (3.23±0.14)×10^-8, and set an upper limit on the branching ratio to be 7.4×10^-8 at the 90 improvement compared to the previous results. The results of K_L^0 → 3γ proceeding via parity-conservation were also presented in this paper.

A measurement of the decay K+ -> pi(0)mu(+)nu(mu)gamma has been performed with the E787 detector at Brookhaven National Laboratory. Forty events were observed in the signal region with the background expectation of (16.5 +/- 2.7) events. The branching ratio was measured to be (1.58 +/- 0.46(stat.) +/- 0.08(syst.)) x 10(-5) in the kinematic region E-gamma > 30 MeV and theta(mu gamma) > 20 degrees, where E-gamma is the energy of the emitted photon and theta(mu gamma) is the angle between the muon and the photon in the K+ rest frame. The results were consistent with theoretical predictions.

The first dedicated search for the rare neutral-kaon decay $K_L^0\to\pi^0\nu\bar{\nu}$ has been carried out in the E391a experiment at the KEK 12-GeV proton synchrotron. The final upper limit of 2.6 $\times 10^{-8}$ at the 90% confidence level was set on the branching ratio for the decay.

Solvophobic effects on an infinitely-thin hard needle in two-dimensional square-well fluids are studied with an analytical expansion and a Monte Carlo (MC) calculation on the chemical potential from the gaseous to liquid densities. The analytical expansion is made for a short needle with respect to the needle length. The solvophobic effects on an extremely short needle is correlated with those on the radial distribution function between solvent disks at the contact distance. The solvophobic solvation on a short needle is exothermic, i.e., the chemical potential of the needle decreases with the increase in the solvent-solvent interaction, at low densities, while it is endothermic at high densities. The solvophobic interaction between short needles can be attractive or repulsive depending on the configuration of the interacting needles as well as the density region of the fluid. The solvophobic effects on a hard needle of finite length are evaluated by a MC method through the free-path distribution of a small and light particle in the square-well fluid. The MC calculation shows that the solvophobic solvation is exothermic even at high densities, when the needle is sufficiently long. The exothermic behavior is apparently due to a long range component evolved with the elongation of the needle. The solvophobic interaction at high densities changes its characteristics with the needle length also.

We performed a search for a light pseudoscalar particle X in the decay K_{L};{0}-->pi;{0}pi;{0}X, X-->gammagamma with the E391a detector at KEK. Such a particle with a mass of 214.3 MeV/c;{2} was suggested by the HyperCP experiment. We found no evidence for X and set an upper limit on the product branching ratio for K_{L};{0}-->pi;{0}pi;{0}X, X-->gammagamma of 2.4x10;{-7} at the 90% confidence level. Upper limits on the branching ratios in the mass region of X from 194.3 to 219.3 MeV/c;{2} are also presented.

Large lead/scintillator sandwich-type photon detectors, sensitive to visible energy depositions of 1 MeV, have been built for the KL0→π0νν¯ experiment at the KEK 12-GeV proton synchrotron. The front barrel (FB) is composed of 16 modules with a length of 2.75 m, forming the 1.5 m-diameter cylinder. The main barrel (MB) is composed of 32 modules with a length of 5.5 m, forming a 3.5 m-diameter cylinder. Scintillation light is read out through wavelength-shifter (WLS) fibers. In this paper, the design, construction and performance of these detectors together with some R&D results are reported.

We performed a search for the ${K}_{L}^{0}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{0}\ensuremath{\nu}\overline{\ensuremath{\nu}}$ decay at the KEK 12-GeV proton synchrotron. No candidate events were observed. An upper limit on the branching ratio for the decay was set to be $6.7\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}8}$ at the 90% confidence level.