M. Akemoto

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.

The KEKB injector linac

Abstract An 8-GeV electron/3.5-GeV positron injector for KEKB was completed in 1998 by upgrading the existing 2.5-GeV electron/positron linac. The main goals were to upgrade its accelerating energy from 2.5 to 8 GeV and to increase the positron intensity by about 20 times. This article describes not only the composition and features of the upgraded linac, but also how these goals were achieved, by focusing on an optics design and commissioning issues concerning especially high-intensity single-bunch acceleration to produce positron beams.

Design of Compact Monochromatic Tunable Hard X-Ray Source Based on X-band Linac

A compact tunable monochromatic (1 to 10 percent bandwidth rms) hard X-ray source based on laser-electron collisions for medicine is proposed. An X-band linac is introduced to realize a remarkably compact system. We have designed a compact monochromatic tunable hard X-ray source as a demonstration. An X-band (11.424 GHz) linac for the purpose is being manufactured. Numerical considerations using CAIN code and luminosity calculations have been performed to estimate the X-ray yield. An X-band thermionic-cathode RF gun and an RDS (round detuned structure) X-band accelerating structure are applied to generate a 50 MeV electron beam with 20 pC/micro-bunch, 1 µs macro-pulse. The total X-ray yield by laser-electron collision with the electron beam and Q-switch Nd:YAG laser with a pulse intensity of 2 J/10 ns is 107 photons/RF-pulse (108 photons/s in 10 pps). We will adapt the technique of laser pulse circulating to increase the X-ray yield up to 108 photons/pulse (109 photons/s). Twenty eight percent of the photons with an energy spread of 10% rms are expected to be available by collimating the scattering angles of X-ray photons.

Vidence for CP-violating asymmetries in B0→π +π- decays and constraints on the CKM angle φ2

We present an improved measurement of CP-violating asymmetries in B-0-->pi(+)pi(-) decays based on a 78 fb(-1) data sample collected at the Y(4S) resonance with the Belle detector at the KEKB asymmetric-energy e(+)e(-) collider. We reconstruct one neutral B meson as a B-0-->pi(+)pi(-) CP eigenstate and identify the flavor of the accompanying B meson from inclusive properties of its decay products. We apply an unbinned maximum likelihood fit to the distribution of the time intervals between the two B meson decay points. The fit yields the CP-violating asymmetry amplitudes A(pipi) = +0.77 +/-0.27(stat) +/-0.08(syst) and S-pipi = -1.23+/-0.41(stat) (+0.08)(-0.07)(syst), where the statistical uncertainties are determined from the Monte Carlo pseudoexperiments. We obtain confidence intervals for CP-violating asymmetry parameters A(pipi) and S-pipi based on a frequentist approach. We rule out the CP-conserving case, A(pipi) = S-pipi = 0, at the 99.93% confidence level. We discuss how these results constrain the value of the Cabibbo-Kobayashi-Maskawa (CKM) angle phi(2).

Observation of B0→π0π0

We report the observation of the decay B0-->pi(0)pi(0), using a 253 fb(-1) data sample collected at the Upsilon(4S) resonance with the Belle detector at the KEKB e(+)e(-) collider. The measured branching fraction is B(B0-->pi(0)pi(0))=(2.3(+0.4+0.2)(-0.5-0.3))x10(-6), with a significance of 5.8 standard deviations including systematic uncertainties. We also make a measurement of the direct CP violating asymmetry in this mode.We report the observation of the decay B{sup 0}{yields}{pi}{sup 0}{pi}{sup 0}, using a 253 fb{sup -1} data sample collected at the {upsilon}(4S) resonance with the Belle detector at the KEKB e{sup +}e{sup -} collider. The measured branching fraction is B(B{sup 0}{yields}{pi}{sup 0}{pi}{sup 0})=(2.3{sub -0.5-0.3}{sup +0.4+0.2})x10{sup -6}, with a significance of 5.8 standard deviations including systematic uncertainties. We also make a measurement of the direct CP violating asymmetry in this mode.

High-power switch with Sl-thyristors for klystron pulse modulators

To improve the reliability of a line-type modulator for a klystron, we have developed a solid-state switch with Sl-thyristors to replace the thyratron tubes. Fast turn-on characteristics of Sl-thyristor have been studied using a very low-inductance circuit. A maximum current of 10 kA and di/dt of 110 kA//spl mu/s with an anode voltage of 15 kV were obtained. We have designed and built a 45 kV solid-state switch that consists of 15 Sl-thyristors stacked in series. The switch has been successfully operated at 45 kV hold-off voltage, 6,000 A peak current, 6 /spl mu/s pulsewidth and 25 Hz.

High gradient experiments by the ATF

A high gradient experiment using traveling wave structures at S-band frequencies is presented. The experiments have been carried out using the ATF (Accelerator Test Facility) phase-I linac. A 0.6-m-long constant-gradient traveling-type structure was utilized to generate the maximum accelerating gradient of 100 MeV/m at 200 MW of RF peak power. The dark current dependence on the structure length and the effect of a dustless and dielectricless structure are discussed. It is concluded that the clean structure of low-dust and low-dielectric material would be suitable for decreasing the dark current one order lower than the normal structure by a standard fabrication process.<<ETX>>

High-power input coupler with a cylindrical alumina window

A high-power RF-input coupler has been installed in the TRISTAN alternating periodic structure cavity operating at 508 MHz. The structure, high power characteristics and long term performance of the coupler are described. It has a cylindrical alumina window coated with TiN of 60-AA thickness and feeds a power of 225 kW (CW) to the cavity. In an early stage of development, the multipactoring discharge of the window was a primary problem. After the coating, the window discharge was not observed. A total of 112 couplers has been installed in the TRISTAN ring and successfully operated for about 17000 h since the spring of 1985. The most serious trouble with the operation was a failure of the loop. This problem was discovered after about 5000 h of operation. Improvements to the coupler including the loop have been made.<<ETX>>

Blumlein-type X-band klystron modulator for Japan Linear Collider

A Blumlein-type X-band klystron modulator using magnetic-pulse-compression techniques was designed and constructed for the future Japan Linear Collider project. This modulator has been designed to produce pulses that are 200-ns wide, have a 600-kV peak voltage, a 1200-A peak current, a short rise time of approximately 70 ns, and a repetition rate exceeding 200 Hz. To realize a compact modulator, a spiral structure was adopted for conductors of the triaxial Blumlein. Special care was taken with the location of the magnetic switch and the charging reactor in order to eliminate any undesirable voltage during the charging process. Details concerning this X-band klystron modulator and its preliminary performance are described.<<ETX>>

Applications of the hot isostatic pressing (HIP) for high gradient accelerator structure

In order to reduce the dark current from disk edges in the traveling wave accelerator structure at field gradients up to 80 MV/m, a new type of disk was proposed which consisted of two metals, OFHC (oxygen-free-high-conductivity) metal and titanium (Ti-6Al-4V). As the secondary electron emission coefficient of titanium is less than unity, the beam hole of a disk was designed to be made of titanium. Diffusion bonding by hot isostatic pressing (HIP) was applied to join titanium to OFHC.<<ETX>>