Search Results (23 total)

We report measurements of the exclusive cross section for e⁺e^-→D⁰D^*-π⁺ as a function of center-of-mass energy from the D⁰D^*-π⁺ threshold to 5.2 GeV with initial-state radiation. No evidence is found for Y(4260)→D⁰D^*-π⁺ decays. The analysis is based on a data sample collected with the Belle detector at or near a center-of-mass energy of 10.58 GeV with an integrated luminosity of 695  fb^-1 at the KEKB asymmetric-energy e⁺e^- collider.

A free relativistic electron in an electromagnetic field is a pure case of a light-matter interaction. In the laboratory environment, this interaction can be realized by colliding laser pulses with electron beams produced from particle accelerators. The process of single photon absorption and reemission by the electron, so-called linear Thomson scattering, results in radiation that is Doppler shifted into the x-ray and γ-ray regions. At elevated laser intensity, nonlinear effects should come into play when the transverse motion of the electrons induced by the laser beam is relativistic. In the present experiment, we achieved this condition and characterized the second harmonic of Thomson x-ray scattering using the counterpropagation of a 60 MeV electron beam and a subterawatt CO₂ laser beam.

We describe our studies of the generation of plasma wake fields by a relativistic electron bunch and of phasing between the longitudinal and transverse fields in the wake. The leading edge of the electron bunch excites a high-amplitude plasma wake inside the overdense plasma column, and the acceleration and focusing wake fields are probed by the bunch tail. By monitoring the dependence of the acceleration upon the plasma’s density, we approached the beam-matching condition and achieved an energy gain of 0.6 MeV over the 17 mm plasma length, corresponding to an average acceleration gradient of 35  MeV/m. Wake-induced modulation in energy and angular divergence of the electron bunch are mapped within a wide range of plasma density. We confirm a theoretical prediction about the phase offset between the accelerating and focusing components of plasma wake.

7.6×10⁶ x-ray photons per 3.5 ps pulse are detected within a 1.8–2.3 Å spectral window during a proof-of-principle laser synchrotron source experiment. A 600 MW CO₂ laser interacted in a head-on collision with a 60 MeV, 140 A, 3.5 ps electron beam. Both beams were focused to a σ  =  32 μm spot. Our next plan is to demonstrate 10¹⁰ x-ray photons per pulse using a CO₂ laser of ∼1 TW peak power.

The flux of upward through-going muons of minimum (mean) threshold energy >1.6 (3.0) GeV is measured, based on a total of 372 events observed by the Kamiokande II+III detector during 2456 detector live days. The observed muon flux was Φ_obs  =  [1.94±0.10(stat.)_-0.06^+0.07sys.)]×10^-13 cm^-2 s^-1 sr^-1, which is compared to an expected value of Φ_theo  =  [2.46±0.54(theo.)]×10^-13 cm^-2 s^-1 sr^-1. The observation is in agreement with the prediction within the errors. The zenith-angle dependence of the observed upward through-going muons supports the previous indication of neutrino oscillations made by Kamiokande using sub- and multi-GeV atmospheric neutrino events.

The large-scale anisotropy of cosmic-ray primaries in the celestial coordinate was studied using cosmic-ray muons recorded in a large water Cherenkov detector, Kamiokande. The right-ascension distribution of the muon arrival directions deviated from an isotropic distribution with a 2.8 standard deviation, and agreed well with the first harmonics with an amplitude of (5.6±1.9)×10^-4 and a phase of 8.0°±19.1°. This is the deepest underground observation of the large-scale anisotropy of cosmic rays, and agrees with observations with other underground experiments and extensive air-shower array experiments.

Results from 1036 days of solar neutrino data accumulated in the upgraded Kamiokande detector (Kamiokande III) are presented. The ⁸B solar neutrino flux observed in Kamiokande III is 2.82_-0.24^+0.25 (stat) ± 0.27 (syst) × 10⁶ cm^-2s^-1; the combined flux from Kamiokande II and III (2079 days in total) is 2.80 ± 0.19 (stat) ± 0.33 (syst) × 10⁶ cm^-2s^-1, which is 49% to 64% of the standard solar models. These combined data from January 1987 to February 1995, covering an entire period of solar cycle 22, enabled us to study a correlation between the neutrino flux and the solar activity in detail: no strong correlation of the solar neutrino flux with the sunspot numbers was found within experimental errors. The result on a search for the daytime and nighttime flux difference is also reported.

We have measured the azimuthal correlation of hadrons produced in e⁺e^- annihilations at √s =58 GeV. In perturbative QCD these correlations are sensitive to the interference between soft gluons emitted at large angles with respect to the event axis. However, Monte carlo calculations incorporating QCD-inspired hadronization models in which the correlation of interjet particles is treated in a phenomenological manner describe the data well. We also observe that angular ordering of the parton showering in the lund Monte Carlo program improves the agreement with the data.

Using 773 muons found in hadronic events from 142 pb^-1 of data at a c.m. energy of 57.8 GeV, we extract the cross section and forward-backward charge asymmetry for the e⁺e^-→bb̅ process, and the heavy quark fragmentation function parameters for the Peterson model. For the analysis of the e⁺e^-→bb̅ process, we use a method in which the behavior of the c quark and lighter quarks is assumed, with only that of the b quark left indeterminate. The cross section and asymmetry for e⁺e^-→bb̅ are found to be R_b = 0.57 ± 0.06(stat) ± 0.08(syst) and A_b = -0.59 ± 0.09 ± 0.09, respectively. They are consistent with the standard model predictions. For the study of the fragmentation function we use the variable 〈x_E〉, the fraction of the beam energy carried by the heavy hadrons. We obtain 〈x_E〉_c=0.56_-0.05-0.03^+0.04+0.03 and 〈x_E〉_b=0.65_-0.04-0.06^+0.06+0.05, respectively. These are in good agreement with previously measured values.

We report on a measurement of the forward-backward charge asymmetry in e⁺e^-→qq̅ at KEK TRISTAN, where the asymmetry is near maximum. We sum over all flavors and measure the asymmetry by determining the charge of the quark jets. In addition we exploit flavor dependencies in the jet charge determination to enhance the contributions of certain flavors. This provides a check on the asymmetries of individual flavors. The measurement agrees with the standard model expectations.

The ratio R of the total cross section for e⁺e^- annihilation into hadrons to the lowest-order QED cross section for e⁺e^-→μ⁺μ^- has been measured for center-of-mass energies ranging from 50 to 61.4 GeV. If we allow for an overall shift of —4.9%, about 1.5 times our estimated normalization error, the results are consistent with the standard-model predictions.

A search for the pair production of charged Higgs particles decaying via the H^-→τν̅ mode has been made in e⁺e^- annihilations at center-of-mass energies between 50 and 60.8 GeV using the AMY detector at the KEK collider TRISTAN. No evidence for their existence is observed and 95%-C.L. mass limits are presented. The result has been interpreted in terms of the tanβ parameter in the Higgs sector.

We present the charged-particle multiplicity distributions for e⁺e^- annihilation at center-of-mass energies from 50 to 61.4 GeV. The results are based on a data sample corresponding to a total integrated luminosity of 30 pb^-1 obtained with the AMY detector at the KEK storage ring TRISTAN. The charged-particle multiplicity distributions deviate significantly from the modified Poisson and pair Poisson distributions, but follow Koba-Nielsen-Olesen scaling and are well reproduced by the LUND parton-shower model.

We present the general properties of multihadron final states produced by e⁺e^- annihilation at center-of-mass energies from 52 to 57 GeV in the AMY detector at the KEK collider TRISTAN. Global shape, inclusive charged-particle, and particle-flow distributions are presented. Our measurements are compared with QCD+fragmentation models that use either leading-logarithmic parton-shower evolution or QCD matrix elements at the parton level, and either string or cluster fragmentation for hadronization.

The forward-backward asymmetry of quarks produced in e⁺e^- annihilations, summed over all flavors, is measured at √s between 50 and 60.8 GeV. Methods of determining the charge direction of jet pairs are discussed. The asymmetry is found to agree with the five-flavor standard model.

Using 123 multihadronic inclusive muon-production e⁺e^- annihilation events at an average c.m. energy of 55.2 GeV, we extracted the forward-backward charge asymmetry of the e⁺e^-→bb¯ process and the R ratio for bb¯ production. We used an analysis method in which the behavior of the c quark and lighter quarks is assumed, with only that of the b quark left indeterminate. The results, A_b=-0.72±0.28(stat)±0.13(syst) and R_b=0.57±0.16±0.10, are consistent with the standard model.

By studying e⁺e^- annihilations in the center-of-mass energy range between 50 and 60.8 GeV, we have established a 95%-confidence-level lower limit on the mass of a fourth-generation charge -(1/3 quark b’ of 27.2 GeV. In contrast with all previous searches, this limit has been obtained through consideration of the decay processes b’→bγ and b’→bg aswell as b’→cW^-. For the cases where any one of the three decay modes dominates, we obtain higher mass limits.

Three-jet events produced in e⁺e^- annihilations are used to provide comparisons between quark and gluon jets. Differences between quark-induced and gluon-induced jets are observed. Quark jets tend to have a more tightly collimated structure than gluon jets, which is reflected in the concentration of a larger fraction of the jet’s energy near the jet axis.

A search for unstable heavy neutral leptons has been made at center-of-mass energies from 50 to 60.8 GeV with the AMY detector at the KEK Storage ring TRISTAN. The neutral leptons are assumed to decay via mixing to electrons and muons. Events with two leptons were searched for. No evidence for their existence was found. Limits in the mass range &≤28.1 GeV/c² and mixing-parameter range 9×10^-10≤‖U_lL‖²≤1 are presented for Dirac- and Majorana-type neutrinos.

We present evidence for the non-Abelian nature of QCD from a study of multijet events produced in e⁺e^- annihilations from √s =50 to 57 GeV in the AMY detector at the KEK storage ring TRISTAN. A comparison of the three-jet event fraction at TRISTAN to the fraction of the DESY storage ring PETRA shows that the QCD coupling strength α_s decreases with increasing Q². In addition, measurements of the angular distributions of four-jet events show evidence for the triple-gluon vertex.

A search for pair production of fourth-generation sequential leptons in e⁺e^- annihilation at a center-of-mass energy sqrt[s]=56 GeV is reported. Event topologies corresponding to the cases where one particle decays leptonically while the other decays hadronically as well as those where both particles decay hadronically were explored. We set a 95%-confidence-level lower limit of 27.6 GeV/c² for the mass of a fourth-generation lepton.

Multihadronic e⁺e^- annihilation events containing final-state leptons have been investigated with a 4.7-pb^-1 data sample at c.m. energies of 50 and 52 GeV. The number of low-thrust events (T≤0.8) with isolated leptons is consistent with the expectation of the standard model with five quark flavors. Limits on the production of new quarks and leptons are reported.

The ratio R of the cross section for e⁺e^- annihilation into hadronic final states to the QED cross section for muon-pair production is measured to be 4.34±0.45±0.30 and 4.23±0.20±0.21 at c.m. energies of 50 and 52 GeV, respectively. From these values of R and an analysis of the event shapes we deduce a 95%-confidence-level upper limit for the production rate of new heavy charge +2 / 3e or -1 / 3e quarks to be 0.19 units of R.