arXiv subjects

K. Nishimura

Publications and source records attributed to K. Nishimura.

43 records · Page 3Linked to original sources

arXiv2011

Vertical current induced domain wall motion in MgO-based magnetic tunnel junction with low current densities

Shifting electrically a magnetic domain wall (DW) by the spin transfer mechanism is one of the future ways foreseen for the switching of spintronic memories or registers. The classical geometries where the current is injected in the plane of the magnetic layers suffer from a poor efficiency of the intrinsic torques acting on the DWs. A way to circumvent this problem is to use vertical current injection. In that case, theoretical calculations attribute the microscopic origin of DW displacements to the out-of-plane (field-like) spin transfer torque. Here we report experiments in which we controllably displace a DW in the planar electrode of a magnetic tunnel junction by vertical current injection. Our measurements confirm the major role of the out-of-plane spin torque for DW motion, and allow to quantify this term precisely. The involved current densities are about 100 times smaller than the one commonly observed with in-plane currents. Step by step resistance switching of the magnetic tunnel junction opens a new way for the realization of spintronic memristive devices.

cond-mat.mtrl-sci↗

arXiv2011

Measurement of the decay $B^0\toπ^-\ell^+ν$ and determination of $|V_{ub}|$

We present a measurement of the charmless semileptonic decay $B^0\toπ^-\ell^+ν$ using a data sample containing 657$\times 10^6$ $B\bar{B}$ events collected with the Belle detector at the KEKB asymmetric-energy $e^+e^-$ collider operating near the $Υ(4S)$ resonance. We determine the total branching fraction of the decay, $\mathcal{B}(B^0\toπ^-\ell^+ν)=(1.49\pm 0.04{(\mathrm{stat})}\pm 0.07{(\mathrm{syst})})\times 10^{-4}$. We also report a new precise measurement of the differential decay rate, and extract the Cabibbo-Kobayashi-Maskawa matrix element $|V_{ub}|$ using model-independent and -dependent approaches. From a simultaneous fit to the measured differential decay rate and lattice QCD results, we obtain $|V_{ub}|=(3.43\pm 0.33)\times 10^{-3}$, where the error includes both statistical and systematic uncertainties.

hep-ex↗

arXiv2010

Measurement of CP violating asymmetries in B^0 -> K^+K^- K^0_S decays with a time-dependent Dalitz approach

We report a measurement of $CP$ violating asymmetries in $B^0(\overline{B}^0) \to K^+ K^- K^0_S$ decays with a time-dependent Dalitz approach. This analysis is based on a data sample of $657\times 10^6$ $B\overline{B}$ pairs accumulated at the $Υ(4S)$ resonance with the Belle detector at the KEKB asymmetric-energy $e^+e^-$ collider. As the result of an unbinned maximum likelihood fit to the selected candidates, the mixing-induced and direct $CP$ violation parameters, $ϕ^{\rm eff}_1$ and ${\cal A}_{CP}$ are obtained for $B^0 \to ϕ(1020) K^0_S$, $B^0 \to f_0(980) K^0_S$ and other $B^0 \to K^+ K^- K^0_S$ decays. We find four solutions that describe the data. There are \{eqnarray*} ϕ_1^{\rm eff}(B^0\to ϕ(1020) K^0_S) & = & (32.2 \pm 9.0 \pm 2.6 \pm 1.4)^{\circ}; ϕ_1^{\rm eff}(B^0\to ϕ(1020) K^0_S) & = & (26.2 \pm 8.8 \pm 2.7 \pm 1.2)^{\circ};\\ ϕ_1^{\rm eff}(B^0\to ϕ(1020) K^0_S) & = & (27.3 \pm 8.6 \pm 2.8 \pm 1.3)^{\circ}\; {\rm and}\\ ϕ_1^{\rm eff}(B^0\to ϕ(1020) K^0_S) & = & (24.3 \pm 8.0 \pm 2.9 \pm 5.2)^{\circ}.{eqnarray*}\ The values for the $CP$ violating phase in $B^0\to ϕ(1020) K^0_S$ are similar but other properties of the Dalitz plot are quite different for the four solutions. These four solutions have consistent $ϕ^{\rm eff}_1$ values for all three $B$ meson decay channels and none of them deviates significantly from the values measured in $B \to (c\bar{c}) K^0$ decays with the currently available statistics. In addition, we find no significant direct $CP$ violation.

hep-ex↗

arXiv2010

The time-of-propagation counter for Belle II

The Belle II detector operating at the future upgrade to the KEKB accelerator will perform high-statistics precision investigations into the flavor sector of the Standard Model. As charged hadron identification is a vital element of the experiment's success, the time-of-propagation (TOP) counter has been chosen as the primary particle identification device in the barrel region of Belle II. The TOP counter is a compact variant of the detection of internally reflected Cherenkov light (DIRC) technique and relies heavily on exquisite single photon timing resolution with micro-channel plate photomultiplier tubes. We discuss the general principles of TOP operation and optimization of the Belle II TOP configuration, which is expected to provide 4 sigma or better separation of kaons and pions up to momenta of approximately 4 GeV/c.

physics.ins-det↗

arXiv2010

First Measurement of Inclusive B -> X_s eta Decays

We report a first measurement of inclusive B -> X_s eta decays, where X_s is a charmless state with unit strangeness. The measurement is based on a pseudo-inclusive reconstruction technique and uses a sample of 657 x 10^6 BB-bar pairs accumulated with the Belle detector at the KEKB e^+e^- collider. For M_{X_s} < 2.6 GeV/c^2, we measure a branching fraction of (26.1 +/- 3.0 (stat) +1.9 -2.1 (syst) +4.0 -7.1 (model)) x 10^-5 and a direct CP asymmetry of A_{CP} = -0.13 +/- 0.04 +0.02 -0.03. Over half of the signal occurs in the range M_{X_s} > 1.8 GeV/c^2.

hep-ex↗

arXiv2009

An imaging time-of-propagation system for charged particle identification at a super B factory

Super B factories that will further probe the flavor sector of the Standard Model and physics beyond will demand excellent charged particle identification (PID), particularly K/pi separation, for momenta up to 4 GeV/c, as well as the ability to operate under beam backgrounds significantly higher than current B factory experiments. We describe an Imaging Time-of-Propagation (iTOP) detector which shows significant potential to meet these requirements. Photons emitted from charged particle interactions in a Cerenkov radiator bar are internally reflected to the end of the bar, where they are collected on a compact image plane using photodetectors with fine spatial segmentation in two dimensions. Precision measurements of photon arrival time are used to enhance the two dimensional imaging, allowing the system to provide excellent PID capabilities within a reduced detector envelope. Results of the ongoing optimization of the geometric and physical properties of such a detector are presented, as well as simulated PID performance. Validation of simulations is being performed using a prototype in a cosmic ray test stand at the University of Hawaii.

physics.ins-det↗

arXiv2005

Self-consistent radiative effect on relativistic electromagnetic particle acceleration

We study the radiation damping effect on the relativistic acceleration of electron-positron plasmas with two-and-half-dimensional particle-in-cell (PIC) simulation. Particles are accelerated by Poynting flux via the diamagnetic relativistic pulse accelerator (DRPA), and decelerated by the self-consistently solved radiation damping force. With $Ω_{ce}/ω_{pe}\geq 10$, the Lorentz factor of the highest energy particles reaches gamma>100, and the acceleration still continues. The emitted radiation is peaked within few degrees from the direction of Poynting flux and strongly linearly polarized, which may be detectable in gamma-ray burst(GRB) observations. We also show that the DRPA is insensitive to the initial supporting currents.

astro-ph↗