Search Results (32 total)

We performed a search for a light pseudoscalar particle X in the decay K_L⁰→π⁰π⁰X, X→γγ with the E391a detector at KEK. Such a particle with a mass of 214.3  MeV/c² was suggested by the HyperCP experiment. We found no evidence for X and set an upper limit on the product branching ratio for K_L⁰→π⁰π⁰X, X→γγ of 2.4×10^-7 at the 90% confidence level. Upper limits on the branching ratios in the mass region of X from 194.3 to 219.3  MeV/c² are also presented.

We report new measurements of the decays B⁺→ρ⁺γ, B⁰→ρ⁰γ, and B⁰→ωγ using a data sample of 657×10⁶ B meson pairs accumulated with the Belle detector at the KEKB e⁺e^- collider. We measure branching fractions B(B⁺→ρ⁺γ)=(8.7_-2.7-1.1^+2.9+0.9)×10^-7, B(B⁰→ρ⁰γ)=(7.8_-1.6-1.0^+1.7+0.9)×10^-7, and B(B⁰→ωγ)=(4.0_-1.7^+1.9±1.3)×10^-7. We also report the isospin asymmetry Δ(ργ)=-0.48_-0.19-0.09^+0.21+0.08 and the first measurement of the direct CP-violating asymmetry A_CP(B⁺→ρ⁺γ)=-0.11±0.32±0.09, where the first and second errors are statistical and systematic, respectively.

We performed a search for the K_L⁰→π⁰νν̅ decay at the KEK 12-GeV proton synchrotron. No candidate events were observed. An upper limit on the branching ratio for the decay was set to be 6.7×10^-8 at the 90% confidence level.

The first search for the rare kaon decay K_L→π⁰π⁰νν̅ has been performed by the E391a collaboration at the KEK 12-GeV proton synchrotron. An upper limit of 4.7×10^-5 at the 90% confidence level was set for the branching ratio of the decay K_L→π⁰π⁰νν̅ using about 10% of the data collected during the first period of data taking. First limits for the decay mode K_L→π⁰π⁰P, where P is a pseudoscalar particle, were also set.

The first dedicated experiment for the rare kaon decay K_L⁰→π⁰νν̅ has been performed by the E391a Collaboration at the KEK 12-GeV proton synchrotron. A new upper limit of 2.1×10^-7 at the 90% confidence level was set for the branching ratio of the decay K_L⁰→π⁰νν̅ using about 10% of the data collected during the first period of data taking.

We present a search for electron neutrino appearance from accelerator-produced muon neutrinos in the K2K long-baseline neutrino experiment. One candidate event is found in the data corresponding to an exposure of 4.8×10¹⁹ protons on target. The expected background in the absence of neutrino oscillations is estimated to be 2.4±0.6 events and is dominated by misidentification of events from neutral current π⁰ production. We exclude the ν_μ to ν_e oscillations at 90% C.L. for the effective mixing angle in the 2-flavor approximation of sin⁡²2θ_μe(≃1 / 2sin⁡²2θ₁₃)>0.15 at Δm_μe²=2.8×10^-3   eV², the best-fit value of the ν_μ disappearance analysis in K2K. The most stringent limit of sin⁡²2θ_μe<0.09 is obtained at Δm_μe²=6×10^-3   eV².

For high luminosity in electron-positron linear colliders, it is essential to generate low vertical emittance beams. We report on the smallest vertical emittance achieved in single-bunch-mode operation of the Accelerator Test Facility, which satisfies the requirement of the x-band linear collider. The emittances were measured with a laser-wire beam-profile monitor installed in the damping ring. The bunch length and the momentum spread of the beam were also recorded under the same conditions. The smallest vertical rms emittance measured at low intensity is 4 pm at a beam energy of 1.3 GeV, which corresponds to the normalized emittance of 1.0×1.0^-8   m. It increases by a factor of 1.5 for a bunch intensity of 10¹⁰ electrons. The measured data agreed to the calculation of intrabeam scattering within much better than a factor of 2.

We present the measurement results of electron beam emittance in the Accelerator Test Facility damping ring operated in multibunch modes. The measurements were carried out with an upgraded laser wire beam profile monitor. The monitor has now a vertical wire as well as a horizontal one and is able to make much faster measurements thanks to an increased effective laser power inside the cavity. The measured emittance shows no large bunch-to-bunch dependence in either the horizontal or vertical directions. The values of the vertical emittance are similar to those obtained in the single-bunch operation. The present results are an important step toward the realization of a high-energy linear collider.

The K2K experiment observes indications of neutrino oscillation: a reduction of ν_μ flux together with a distortion of the energy spectrum. Fifty-six beam neutrino events are observed in Super-Kamiokande (SK), 250 km from the neutrino production point, with an expectation of 80.1_-5.4^+6.2. Twenty-nine one ring μ-like events are used to reconstruct the neutrino energy spectrum, which is better matched to the expected spectrum with neutrino oscillation than without. The probability that the observed flux at SK is explained by statistical fluctuation without neutrino oscillation is less than 1%.

We describe in this paper a measurement of vertical emittance in the Accelerator Test Facility (ATF) damping ring at KEK with a laser wire beam profile monitor. This monitor is based on the Compton scattering process of electrons with a laser light target which is produced by injecting a cw laser beam into a Fabry-Perot optical cavity. We installed the monitor at a straight section of the damping ring and measured the vertical emittance with three different ring conditions. In all cases, the ATF ring was operated at 1.28 GeV in a single bunch mode. When the ring was tuned for ultralow emittance, the vertical emittance of ε_y=(1.18±0.08)×10^-11   mrad was achieved. This shows that the ATF damping ring has realized its target value also vertically.

We describe the first measurement of an electron beam size in the accelerator test facility damping ring at KEK with a laser wire beam profile monitor. This monitor is based upon the Compton scattering process of electrons with a laser light target, which is produced by injecting a cw laser beam into a Fabry-Pérot optical cavity. We have observed clear signals of the Compton scattered photons and confirmed that the observed energy spectrum as well as the count rate agree with the expected ones. From the measurement, we have deduced the vertical beam size σ_b to be 9.8±1.1±0.4 μm, where the first (second) error represents statistical (systematic) uncertainty. Various improvements are in progress to enhance the signal-to-noise ratio, which is essential for the detailed study of the beam dynamics.

Experimental results on the production of dimuons by 800-GeV protons incident on a copper target are presented. The results include measurements of both the continuum of dimuons and the dimuon decays of the three lowest-mass ϒ S states. A description of the apparatus, data acquisition, and analysis techniques is included. A comparison of the results with data taken at lower incident energies indicates a scaling behavior of the continuum dimuon yields.

We have studied four-jet angular distributions using data collected with the VENUS detector at the KEK e⁺e^- collider at TRISTAN at c.m. energies between 54 and 64 GeV. The obseved angular distributions are consistent with QCD, but are inconsistent with the Abelian gluon model at the 5% significance level. We have further obtained 95%-confidence experimental bounds on the fundamental parameters of the SU(N_c) local color symmetry.

A measurement of continuum dimuon production in proton-copper collisions at 800-GeV incident energy is presented. The dimuons observed in this experiment cover the mass range from 6.5 to 18 GeV near y=0 in the proton-nucleon center-of-momentum frame. Scaling forms of the cross section for the continuum are compared with the results of other experiments in the context of the parton model and quantum chromodynamics. The present limitations of such scaling comparisons are discussed.

A search for a new charge -(1/3 quark has been carried out at the KEK e⁺e^-Collider TRISTAN under the assumption of its photonic decay through a flavor-changing neutral current (FCNC). The observed number of multihadronic events with isolated photons is consistent with that expected from the known five quark flavors. Including the usual charged-current decays, limits on the absolute photonic branching ratio and FCNC processes were obtained.

A search for a fourth-generation quark with ‖Q‖=e/3 has been made with the VENUS detector at the KEK e⁺e^- collider TRISTAN. Multihadron events with a spherical shape or containing isolated leptons were studied. There is no evidence for an excess production of such events in e⁺e^- collision at √s =56–57 GeV and a lower limit on the mass is 27.5 GeV/c² at the 95% C.L.

A sensitive search for the production of exotic hadrons with charge z=±2 and z=-(5/3 has been carried out with a 12-GeV proton beam. No candidate event was found in the mass range up to M/z=1.2 GeV/c². Upper limits are established at ∼10^-9 on the yield of such particles relative to the π yield. In particular, this experiment has ruled out the existence of R-Δ⁺⁺’s with a mass of less than ∼1.6 GeV/c² and a lifetime longer than ∼10^-7 sec.

A search for sequential charged leptons decaying into massive neutrinos has been performed at E_c.m.=56 GeV at the KEK colliding-beam accelerator TRISTAN. We have found no evidence for the production of the charged heavy leptons for an integrated luminosity of 5.3 pb^-1. A search for stable charged leptons was also conducted yielding null results. A new mass limit on the charged heavy leptons was obtained as a function of the mass of the associated neutrinos.

A search for sequential heavy leptons has been performed at the center-of-mass energy of 52 GeV at the KEK colliding-beam accelerator TRISTAN. We have found no evidence for the production of heavy leptons with the integrated luminosity of 2.9 pb^-1. A lower mass limit of 25.0 GeV/c² at 95% confidence level was obtained.

The asymmetry parameter α_γ for the Σ⁺→pγ decay has been measured with counter techniques. From a sample of about 190 Σ⁺→pγ decays, the asymmetry parameter is found to be α_γ=-0.86±0.13±0.04, where the quoted errors are statistical and systematic, respectively. The present result confirms with better accuracy the large and negative value for α_γ measured by two previous bubble-chamber experiments. The branching ratio is also found to be (1.30±0.15) ×10^-3, which is consistent with previous measurements.

A search for neutral penetrating particles has been made by use of an 800-GeV proton beam incident on the magnetized beam dump of the spectrometer of Fermilab experiment 605. Limits on the mass and couplings of such particles are presented. A 1.8-MeV axion coupled only to e⁺e^- is ruled out.

An experiment to search for the production of neutral penetrating particles decaying into electron-positron pairs was performed with a 2.5-GeV electron beam. A total of 0.027 C was injected into a tungsten target. No such particle is found. Constraints on coupling constants α_e and α_γ are given.

Spin-dependent asymmetries have been measured in inclusive deep-inelastic scattering of longitudinally polarized electrons by longitudinally polarized protons. Data were obtained at a scattering angle of 10° and for incident energies of 16.2 and 22.7 GeV, which cover the kinematic range 0.18<x<0.70 and 3.5<Q²<10.0 (GeV/c)². The present results provide a test of scaling and of the Bjorken and Ellis-Jaffe sum rules and are compared with various models of proton spin structure.