H. Kakuno

Observation of Large CP Violation and Evidence for Direct CP Violation in B0→π+π- Decays

We report the first observation of CP violation in B0-->pi(+)pi(-) decays based on 152x10(6) gamma (4S)-->BB decays collected with the Belle detector at the KEKB asymmetric-energy e(+)e(-) collider. We reconstruct a B0-->pi(+)pi(-) CP eigenstate and identify the flavor of the accompanying B meson from its decay products. From the distribution of the time intervals between the two B meson decay points, we obtain A(pipi)=+0.58+/-0.15(stat)+/-0.07(syst) and S(pipi)=-1.00+/-0.21(stat)+/-0.07(syst). We rule out the CP-conserving case, A(pipi)=S(pipi)=0, at a level of 5.2 standard deviations. We also find evidence for direct CP violation with a significance at or greater than 3.2 standard deviations for any S(pipi) value.

Confinement of nonneutral plasmas in the Prototype Ring Trap device

Recently, an internal-ring device named Proto-RT (Prototype Ring Trap) was constructed at University of Tokyo, and experiments on the device have been intensively conducted. The main goal of Proto-RT is to explore an innovative method to attain a plasma equilibrium with extremely high-β (β>1) in a toroidal geometry using non-neutral condition. At the first series of the experiments, pure electron plasmas (ne∼1013 m−3) have been successfully confined inside a separatrix. No disruption is so far observed. The confinement time of the electron plasmas is of order 0.1 ms for an X point configuration. The non-neutrality of Δne∼1013 m−3 is already beyond the critical value which is required to produce an enough self-electric field E in non-neutral plasmas with n0∼1019 m−3, causing a strong E×B flow thoroughly over the plasmas where the hydrodynamic pressure of the flow is predicted to balance with the thermal pressure of the plasmas.

R&D of residual gas beam profile monitor

We have studied the residual gas beam profile monitor (RCBPM) for the J-PARC neutrino beamline, here the micro channel plate (MCP) is used as a read out device. We carried out the beam tests at the KEK-PS beamline to study the basic performance of the monitor. To check the capability of beam-profile measurement, we studied the properties of signal timing and the effect of electric field produced by the proton beam. We found that the ion collection is effective to avoid the beam induced background in a high beam background environment, and RGBPM works under the electric field produced by the beam itself of less than 20 kV/m. In addition, the gain calibration method of MCP has been developed using thermal electrons from the glowing wire. We found the gain of MCP is calibrated with an accuracy of better than 5%.

Experiments on pure electron plasmas confined in a toroidal geometry

The toroidal magnetic trap has an advantage in achieving long orbit lengths, which allows us to apply a slow process of energy reduction to the trapped particles. On Proto-RT (Prototype Ring Trap), we have demonstrated the confinement of a pure electron plasma without the help of external electric fields. We have injected electrons with the energy of 2 keV inside a separatrix. The electrostatic potential of the electron cloud is of order 100 V. The corresponding density of the electron plasma is calculated to be of order 1013 m−3. In order to modulate the kinetic energy of the electrons we are now planning RF assisted injection of electrons.