Search Results (31 total)

We have measured and corrected chromatic X-Y coupling at an interaction point to improve the luminosity of KEKB. We have measured the beam position of betatron oscillations induced by the kicker using turn-by-turn beam position monitors. A phase space structure reconstructed by the beam position provides us not only the Twiss parameters but also information regarding X-Y coupling. We have also determined chromatic X-Y coupling using the measured X-Y coupling at each momentum deviation from the designed beam energy. Skew sextupole magnets are used to correct the chromatic X-Y coupling.

Microwave instability in the low energy ring of KEKB was studied using a broadband impedance model. The model gave excellent descriptions of longitudinal dynamics for both positive and negative momentum compactions. Moreover, it predicted that the threshold of microwave instability was a factor of 2 lower than the machine nominal operating bunch current. The prediction was confirmed by a measurement using the Belle detector. Furthermore, we integrated the longitudinal wakefield into the beam-beam simulation and applied it to study the combined effects in KEKB. As a result, the beam-beam simulation became truly three dimensional with emittance growth in all three dimensions simultaneously as the beam currents increase. In addition, an observed mystery of asymmetry in the horizontal scan could also be explained by our simulations.

An orbit feedback system around the interaction point (IP) has been developed and successfully employed at KEKB for more than 6 years. The purpose of the system is to maintain an optimum geometrical relationship of orbits of two beams at the IP and to prevent a luminosity degradation due to orbit drifts. The feedback system is based on orbit measurements around the IP rather than a direct measurement of the luminosity. Owing to the system, the luminosity degradation due to the orbit drifts is suppressed to around or less than 1%.

Optics correction is an important issue in the KEKB B-factory. Especially, the correction of beta functions is performed so as to reach near half-integer resonance of the horizontal betatron tune as closely as possible. Consequently, the luminosity can be improved during experiments of the B-meson in e⁺e^- collisions. Therefore, we have developed a correction method of the beta function not only at the designed beam energy, but also at an energy deviated from the nominal value. We present the procedures used to measure the behavior of the beta function, which depends on the momentum deviation, and the results compared with the model optics in the KEKB rings.

Incoherent emittance growth caused by a strong nonlinear interaction between beam and electron cloud is discussed. This emittance growth arises from nonlinear diffusion related to chaos and resonances, and strongly depends on the number of degrees of freedom of the interacting system. A simple model, in which beam particles interact with a fixed round charge distribution, is used to study the mechanism of the emittance growth. The same discussion can be applied to the emittance growth due to beam-beam interactions in colliders and space charge effects in low energy proton rings.

We present the first evidence of the decay B^-→τ^-ν̅ _τ, using 414  fb^-1 of data collected at the Υ(4S) resonance with the Belle detector at the KEKB asymmetric-energy e⁺e^- collider. Events are tagged by fully reconstructing one of the B mesons in hadronic modes. We detect the signal with a significance of 3.5 standard deviations including systematics and measure the branching fraction to be B(B^-→τ^-ν̅ _τ)=(1.79_-0.49^+0.56(stat)_-0.51^+0.46(syst))×10^-4. This implies that f_B=0.229_-0.031^+0.036(stat)_-0.037^+0.034(syst)  GeV and is the first direct measurement of this quantity.

We report the observation of the decay B⁰→π⁰π⁰, using a 253  fb^-1 data sample collected at the Υ(4S) resonance with the Belle detector at the KEKB e⁺e^- collider. The measured branching fraction is B(B⁰→π⁰π⁰)=(2.3_-0.5-0.3^+0.4+0.2)×10^-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 evidence for direct CP violation in the decay B⁰→K⁺π^- with 253  fb^-1 of data collected with the Belle detector at the KEKB e⁺e^- collider. Using 275×10⁶ BB̅ pairs we observe a B→K^±π^∓ signal with 2140±53 events. The measured CP violating asymmetry is A_CP(K⁺π^-)=-0.101±0.025(stat)±0.005(syst), corresponding to a significance of 3.9σ including systematics. We also search for CP violation in the decays B⁺→K⁺π⁰ and B⁺→π⁺π⁰. The measured CP violating asymmetries are A_CP(K⁺π⁰)=0.04±0.05(stat)±0.02(syst) and A_CP(π⁺π⁰)=-0.02±0.10(stat)±0.01(syst), corresponding to the intervals -0.05<A_CP(K⁺π⁰)<0.13 and -0.18<A_CP(π⁺π⁰)<0.14 at 90% confidence level.

In recent high luminosity colliders, the finite crossing angle scheme becomes popular to gain the luminosity with multibunch or long bunch operation. Success of the KEKB factory showed that the finite crossing angle scheme has no problem achieving beam-beam parameters up to 0.05. We have studied the beam-beam interactions with and without crossing angle toward higher luminosity. We discuss how the crossing angle affects the beam-beam parameter and luminosity in the present KEKB using computer simulations. The simulations showed that crab cavities, which realize the head-on collision effectively, can be expected to double the luminosity.

We report the first observation of CP violation in B⁰→π⁺π^- decays based on 152×10⁶ ϒ(4S)→BB̅ decays collected with the Belle detector at the KEKB asymmetric-energy e⁺e^- collider. We reconstruct a B⁰→π⁺π^- 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_ππ=+0.58±0.15(stat)±0.07(syst) and S_ππ=-1.00±0.21(stat)±0.07(syst). We rule out the CP-conserving case, A_ππ=S_ππ=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_ππ value.

Beam-beam effects limit the luminosity of circular colliders. Once the bunch population exceeds a threshold, the luminosity increases at a slower rate. This phenomenon is called the beam-beam limit. Onset of the beam-beam limit has been analyzed with various simulation methods based on the weak-strong and strong-strong models. We have observed that an incoherent phenomenon is mainly concerned in the beam-beam limit. The simulation have shown that equilibrium distributions of the two colliding beams are distorted from Gaussians when the luminosity is limited. The beam-beam limit is estimated to be ξ∼0.1 for a B factory with damping time of several thousand turns.

The usual formulas for the resistive-wall wake field are derived considering ultrarelativistic beams, traveling at the speed of light. This simplifies the calculation, and it leads to a cancellation between electric and magnetic fields. However, for proton beams below 10 GeV and for many heavy-ion beams, the velocities may significantly differ from the speed of light. In this paper, we compute the longitudinal and transverse wake fields for velocities smaller than c and examine under which conditions nonrelativistic effects become important. We illustrate our results by a few examples.

We report measurements of the properties of the D_sJ⁺(2317) and D_sJ⁺(2457) resonances produced in continuum e⁺e^- annihilation near sqrt[s]=10.6  GeV. The analysis is based on an 86.9  fb^-1 data sample collected with the Belle detector at KEKB. We determine the masses to be M(D_sJ⁺(2317))=2317.2±0.5(stat)±0.9(syst)  MeV/c² and M(D_sJ⁺(2457))=2456.5±1.3(stat)±1.3(syst)  MeV/c². We observe the radiative decay mode D_sJ⁺(2457)→D_s⁺γ and the dipion decay mode D_sJ⁺(2457)→D_s⁺π⁺π^- and determine their branching fractions. No corresponding decays are observed for the D_sJ(2317) state. These results are consistent with the spin-parity assignments of 0⁺ for the D_sJ(2317) and 1⁺ for the D_sJ(2457).

We report the first observation of the B→D̅ D_sJ(2317) and B→D̅ D_sJ(2457) decays based on 123.8×10⁶ BB̅ events collected with the Belle detector at KEKB. We observe the D_sJ(2317) decay to D_sπ⁰ and the D_sJ(2457) decay to the D_s^*π⁰ and D_sγ final states. We also set 90% C.L. upper limits for the decays D_sJ(2317)→D_s^*γ, D_sJ(2457)→D_s^*γ, D_sJ(2457)→D_sπ⁰, and D_sJ(2457)→D_sπ⁺π^-.

We present an improved measurement of CP-violation parameters in B⁰→ϕK_S⁰, K⁺K^-K_S⁰, and η^′K_S⁰ decays based on a 140   fb^-1 data sample collected at the Υ(4S) resonance with the Belle detector at the KEKB energy-asymmetric e⁺e^- collider. One neutral B meson is fully reconstructed in one of the specified decay channels, and the flavor of the accompanying B meson is identified from its decay products. CP-violation parameters for each of the three modes are obtained from the asymmetries in the distributions of the proper-time intervals between the two B decays. We find that the observed CP asymmetry in the B→ϕK_S⁰ decay differs from the standard model (SM) expectation by 3.5 standard deviations, while the other cases are consistent with the SM.

We report evidence for the decay B⁰→π⁰π⁰. The analysis is based on a data sample of 152×10⁶ BB̅ pairs collected at the Υ(4S) resonance with the Belle detector at the KEKB e⁺e^- storage ring. We detect a signal for B⁰→π⁰π⁰ with a significance of 3.4 standard deviations, and measure the branching fraction to be [1.7±0.6(stat)±0.2(syst)]×10^-6.

We report the observation of a narrow charmoniumlike state produced in the exclusive decay process B^±→K^±π⁺π^-J/ψ. This state, which decays into π⁺π^-J/ψ, has a mass of 3872.0±0.6(stat)±0.5(syst)  MeV, a value that is very near the M_D⁰+M_D^*0 mass threshold. The results are based on an analysis of 152M B-B̅ events collected at the Υ(4S) resonance in the Belle detector at the KEKB collider. The signal has a statistical significance that is in excess of 10σ.

A three-dimensional particle in cell simulation code has been developed to study the photoelectron cloud instabilities in KEKB LER. In this report, the program is described in detail. In particular, typical simulation results are presented for the photoelectron motion in various kinds of magnetic fields. The simulation shows that a solenoid is very effective in confining the photoelectrons to the vicinity of the vacuum chamber wall and in creating a region free of photoelectrons at the vacuum pipe center. The more uniform the solenoid field is, the more effectively does it suppress the electron-cloud buildup. Multipacting can occur both in a drift region and in a dipole magnet, and the heat load deposited on the chamber wall due to the lost electrons is important in these two cases. Electron trapping by the beam field as well as by various magnetic fields is an important phenomenon, especially inside quadrupole and sextupole magnets. Our numerical results qualitatively agree with the experimental studies.

A photoelectron-trapping phenomenon has been found in the simulation of the photoelectron cloud. It is found that the photoelectrons can be trapped in the quadrupole and sextupole magnetic fields for very long time until it longitudinally drifts out of the magnets, even a long bunch train separation is not sufficient to clear up the photoelectrons. Therefore, such a kind of long time trapped photoelectron cloud can cause coupled bunch instability. The trapping phenomenon is strongly beam dependent, especially on the bunch length. There is no such kind of trapping if the positron beam does not disturb the photoelectrons during the whole process. There is also no trapping for positron bunch with bunch length longer than the period of the photoelectron’s gyration motion at the mirror points. The trapping is a mirror field trap which is caused by beam disturbance. The trapping phenomenon and mechanism will be presented in detail.

Electron beams with the lowest, normalized transverse emittance recorded so far were produced and confirmed in single-bunch-mode operation of the Accelerator Test Facility at KEK. We established a tuning method of the damping ring which achieves a small vertical dispersion and small x-y orbit coupling. The vertical emittance was less than 1% of the horizontal emittance. At the zero-intensity limit, the vertical normalized emittance was less than 2.8×10^-8 rad m at beam energy 1.3 GeV. At high intensity, strong effects of intrabeam scattering were observed, which had been expected in view of the extremely high particle density due to the small transverse emittance.

A calculation method of emittance growth of an electron beam due to intrabeam scattering is described. The 3 degrees of freedom are equally treated in the beam rest frame, and the couplings between them are included in a natural way. This formalism is suitable for the calculation of the emittance growth with the beam-envelope method.

A three-dimensional (3D) laser cooling method of fast stored ion beams based on a linear coupling mechanism is explored. We extensively study two approaches proposed in previous publications, i.e., the dispersive coupling scheme and the coupling-cavity scheme, confirming how much one can improve the transverse cooling efficiency. A possible design of a coupling cavity is presented. We employ the tracking code SAD and the molecular dynamics code SOLID to carry out reliable numerical experiments where realistic lattice structures of storage rings and particle Coulomb interactions are taken into account. Through systematic simulations, it is demonstrated that resonant coupling remarkably enhances transverse cooling rates for any initial beams, making it feasible to reach an equilibrium temperature far below the current achievable level. We further emphasize the crucial importance of avoiding the Mathieu instability. We also discuss the minimum cooling power required for beam crystallization as well as on an interpretation of past experimental results in the TSR and ASTRID storage rings.

The KEK Accelerator Test Facility (KEK-ATF) was constructed to develop technologies for producing a low-emittance beam which will be required by future linear colliders. The KEK-ATF consists of an injector linac, a damping ring, and a beam extraction line. The basic optical structure of the damping ring is a FOBO lattice, which reduces the horizontal dispersion at the center of the bending magnets and, as a consequence, can produce an extremely small emittance beam. To verify the performance of such a unique, low-emittance lattice, it is crucial to measure the horizontal emittance. The horizontal emittance was measured using wire scanners in the beam extraction line. Since the horizontal beam position was not stable, we established a method to correct the measured beam size for position fluctuation (“jitter”) and we succeeded in the observation of the so far smallest horizontal emittance in any accelerator. The measured horizontal emittance was 1.37±0.03nm at a beam energy of 1.285 GeV and a bunch population of \(3–5\)×10⁹, in agreement with the design value of 1.27–1.34 nm at the beam energy and the bunch population.

First experimental results from the final focus test beam (FFTB) are reported. The vertical dimension of a 47-GeV electron beam from the SLAC linac has been reduced at the focal point of the FFTB by a demagnification of 320 to a beam height of approximately 70 nm.

An anomalous equilibrium emittance due to chromaticity of the focusing elements is predicted for electron storage rings. A simple model which describes the transverse beam distribution as a function of the longitudinal phase space is given to evaluate the anomalous emittance. The anomalous emittance can be a critical difficulty in achieving a very flat beam for high-luminosity electron-positron colliders.