Search Results (20 total)

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.

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 present a measurement of the standard model CP violation parameter sin2φ₁ (also known as sin2β) based on a 10.5 fb^-1 data sample collected at the ϒ(4S) resonance with the Belle detector at the KEKB asymmetric e⁺e^- collider. One neutral B meson is reconstructed in the J/ψK_S, ψ(2S)K_S, χ_c1K_S, η_cK_S, J/ψK_L, or J/ψπ⁰ CP-eigenstate decay channel and the flavor of the accompanying B meson is identified from its charged particle decay products. From the asymmetry in the distribution of the time interval between the two B-meson decay points, we determine sin2φ₁  =  0.58_-0.34^+0.32(stat)_-0.10^+0.09(syst).

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.

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.

²⁷Al(¹⁶O,X) reactions are studied at 88 MeV. The A and Z of reaction products X=¹⁶O, ^15,14,13N, ^13,12C, ^11,10B, ^9,7Be and ^7,6Li are identified. Despite the low bombarding energy, the general features of (i) energy spectra, (ii) the optimum Q values Q_eff^m and (iii) angular distributions in 5 MeV intervals in Q are very similar to those of much higher energy data of heavier systems: i.e., (i) dominance of deep inelastic reactions, (ii) Q_eff^m values well reproduced by the "universal" relations previously found for deep inelastic reactions, and (iii) σ(θ,Q) of a form A exp[-μ(Q)θ] / sinθ. The variations of Q_eff^m with θ and of μ with Q and n, the number of transferred nucleons, are discussed in connection with the gradual evolution in reaction mechanisms from quasielastic to deep inelastic to complete fusion.

NUCLEAR REACTIONS ²⁷Al(¹⁶O,X), E=88 MeV, X=¹⁶O, ^15,14,13N, ^13,12C, ^11,10B, ^9,7Be and ^7,6Li, measured energy spectra, optimum Q values, σ, σ(θ), and σ(θ,Q).

A marked difference between analyzing powers for reactions ¹²⁸Te(p, t)¹²⁶Te(2₁⁺) and ¹¹⁰Pd(p, t)¹⁰⁸Pd(2₁⁺) with polarized protons has been observed, while the ground-state transitions have shown similar analyzing powers. The difference is accounted for as a result of a phase change of the interference between direct and inelastic multistep processes in two-neutron pickup reactions. The origin of this phase change is elucidated on the basis of the microscope description of the collective quadrupole oscillation of nuclei.