Search Results (36 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%.

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.

A coupled-bunch instability caused by an electron cloud has been observed in the KEKB LER. The time evolution of the instabilities just after the turning-off the transverse bunch feedback was recorded with several weak solenoid-field conditions, which are used to suppress the vertical blowup of the beam size due to the electron cloud. The mode spectra and their growth rates of the coupled-bunch instabilities were compared with simulations of electrons moving in drift space, a weak solenoid field, and a strong bending field. Mode spectra without a solenoid field support the model where the instability is dominated by the electron clouds in the drift space with a lower secondary yield of photoelectron δ_2,max=1.0 rather than 1.5. With the solenoid field, the behavior of unstable modes and the growth rate with the strength of solenoid field also support the simulation with lower secondary emission yield.

An electron cloud induces a wake force on a charged particle beam which creates correlations between bunches—i.e., a small displacement of a bunch creates a perturbation of the electron cloud, which affects the motions of the following bunches, with the result that a coupled-bunch instability is caused. The coupling mode of the instability is determined by the motion of the electrons in the cloud—that is, it depends on which electrons, moving in a drift space, a weak solenoid field or a strong bending field, are dominant for the instability. We discuss the coupled-bunch instability focusing on the relation between the mode spectrum and the electron motion.

The Beijing Electron-Positron Collider (BEPC) will be upgraded to BEPCII, and the luminosity will be about 100 times higher. We developed a three-dimensional strong-strong PIC code to study the beam-beam effects in BEPCII. The transportation through the arc is the same as that in Hirata’s weak-strong code. The beam-beam force is computed directly by solving the Poisson equation using the Fourier analysis and cyclic reduction method, and the boundary potential is computed by circular convolution. The finite bunch length effect is included by longitudinal slices. An interpolation scheme is used to reduce the required slice number in simulations. The standard message passing interface is used to parallelize the code. The computing time increases linearly with (n+1), where n is the slice number. The calculated luminosity of BEPCII at the design working point is ∼50% of the design value. The best area in the tune space is near (0.505,0.57) according to the survey, where the luminosity is ∼80% of the design.

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.

The effects of electron clouds on positively charged beams have been an active area of research in recent years at particle accelerators around the world. Transverse beam-size blowup due to electron clouds has been observed in some machines and is considered to be a major limiting factor in the development of higher-current, higher-luminosity electron-positron colliders. The leading proposed mechanism for beam blowup is the excitation of a fast head-tail instability due to short-range wakes within the electron cloud. We present here observations of betatron oscillation sidebands in bunch-by-bunch spectra that may provide direct evidence of such head-tail motion in a positron beam.

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.

The energy gain and motion of electrons can quantitatively describe the mechanism of electron multipacting in a long-bunched proton machine. Strong multipacting usually happens around the bunches’ tails due to the high energy of electrons when they hit the chamber surface. We investigated several important parameters of electron multipacting, proving that it is sensitive to the beam’s intensity, the shape of its longitudinal profile, its transverse size, the secondary emission yield, and the energy at peak secondary emission yield. Our analyses, simulations, and experiments are all in agreement.

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.

We have performed a search for the lepton-flavor-violating decay τ→μγ using a data sample of 86.3   fb^-1 accumulated by the Belle detector at KEK. No evidence for a signal is seen, and we set an upper limit for the branching fraction of B(τ→μγ)<3.1×10^-7 at the 90% confidence level.

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 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 the flavor-changing neutral current decay B→K^*ℓ⁺ℓ^- and an improved measurement of the decay B→Kℓ⁺ℓ^-, where ℓ represents an electron or a muon, with a data sample of 140   fb^-1 accumulated at the Υ(4S) resonance with the Belle detector at KEKB. The results for the branching fractions are B(B→K^*ℓ⁺ℓ^-)=(11.5_-2.4^+2.6±0.8±0.2)×10^-7 and B(B→Kℓ⁺ℓ^-)=  (4.8_-0.9^+1.0±0.3±0.1)×10^-7, where the first error is statistical, the second is systematic and the third is from model dependence.

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.

We present an improved measurement of CP-violating asymmetries in B⁰→π⁺π^- decays based on a 78 fb^-1 data sample collected at the Υ(4S) resonance with the Belle detector at the KEKB asymmetric-energy e⁺e^- collider. We reconstruct one neutral B meson as a B⁰→π⁺π^- 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_ππ=+0.77 ±0.27(stat) ±0.08(syst) and S_ππ=-1.23±0.41(stat) _-0.07^+0.08(syst), where the statistical uncertainties are determined from the Monte Carlo pseudoexperiments. We obtain confidence intervals for CP-violating asymmetry parameters A_ππ and S_ππ based on a frequentist approach. We rule out the CP-conserving case, A_ππ=S_ππ=0, at the 99.93% confidence level. We discuss how these results constrain the value of the Cabibbo-Kobayashi-Maskawa (CKM) angle φ₂.

A vertical coupled-bunch instability was observed for a positron beam at the Beijing Electron Positron Collider (BEPC). The experimental results show that the instability has similar characteristics as that observed in the Photon Factory of KEK several years ago. The instability at BEPC can be explained by the effect of an electron cloud which is produced in the beam chamber by synchrotron light hitting the wall.

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.