Search Results (34 total)

We present the renormalization scheme used in and the characteristic features of GRACE/SUSY-loop, the package of the program for the automatic calculation of the minimal supersymmetric standard model processes including one-loop order corrections. The two-body and three-body decay widths of charginos in one-loop order evaluated by GRACE/SUSY-loop are shown.

We have demonstrated for the first time the production of highly polarized short-pulse positrons with a finite energy spread in accordance with a new scheme that consists of two-quantum processes, such as inverse Compton scattering and electron-positron pair creation. Using a circularly polarized laser beam of 532 nm scattered off a high-quality, 1.28 GeV electron beam, we have obtained polarized positrons with an intensity of 2×10⁴ e⁺/bunch. The magnitude of positron polarization has been determined to be 73±15(stat)±19(syst)% by means of a newly designed positron polarimeter.

We have developed a polarimetry of ultrashort pulse γ rays based on the fact that γ rays penetrating in the forward direction through a magnetized iron carry information on the helicity of the original γ rays. Polarized, short-pulse γ rays of (1.1±0.2)×10⁶/bunch with a time duration of 31 ps and a maximum energy of 55.9 MeV were produced via Compton scattering of a circularly polarized laser beam of 532 nm off an electron beam of 1.28 GeV. The first demonstration of asymmetry measurements of short-pulse γ rays was conducted using longitudinally magnetized iron of 15 cm length. It is found that the γ-ray intensity is in good agreement with the simulated value of 1.0×10⁶. Varying the degree of laser polarization, the asymmetry for 100% laser polarization was derived to be (1.29±0.12)%, which is also consistent with the expected value of 1.3%.

Based on the requirements from a conceptual design of a polarized positron beam for future linear colliders, we constructed a special collision system with a short focal length of 150 mm of the laser beams so as to produce γ rays through inverse Compton scattering. In order to achieve efficient laser-electron collisions, we created a special optics to produce very small e^--beam sizes of σ_ex0=7.6   μm and σ_ey0=5.4   μm in the horizontal and vertical directions at the collision point. Using laser light with a wavelength of 532 nm and an e^- beam of 1.28 GeV, provided from the ATF-damping ring at KEK, we generated 2×10⁵ γ rays with a time duration of 26 ps in rms, leading to a peak brightness of 1.7×10¹⁸/(mrad² mm² 0.1%bandwidth s) near to the maximum energy of 56 MeV.

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.

We study double Higgs boson production at future linear colliders while paying special attention to the option of high-energy and high-luminosity photon beams. The main purpose is to examine the feasibility of e⁺e^-, γe, and γγ colliders in order to establish bounds on the value of triple Higgs coupling, which could be crucial for understanding a spontaneous breaking mechanism. We consider mainly those cases of light and intermediate Higgs bosons, including an analysis of the electroweak backgrounds. The mass range M_H∼M_Z is discussed separately. It is shown that for a light Higgs boson the H³ coupling can be visible, even at a future linear e⁺e^- collider at 500 GeV. For an intermediate Higgs boson, a collider with TeV energies is suitable for investigations. We estimate the bounds on the anomalous H³ coupling which can be experimentally established at future linear colliders.

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.

If the neutrino has a finite mass and a magnetic moment, it would produce transition radiation when crossing the interface between two media of which plasma frequencies are ω₁ and ω₂ ( ω₁≫ω₂). We found that the probability of transition radiation is larger by an order of magnitude using the quantum theory than that recently reported by one of us using classical electrodynamics, and that the energy spectrum of the radiation is uniform up to ∼γω₁, where γ is the Lorentz factor of the neutrino ( γ  =  E_ν/m_ν).

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.

We have observed a large enhancement of the spin polarization of electrons extracted from an AlGaAs-GaAs superlattice illuminated by circularly polarized light. A polarization of 71.2 ±1.1(stat)±6.1(sys)% was obtained with a photon wavelength of 802 nm at room temperature. We have also confirmed the removal of the degeneracy between a heavy-hole band and a light-hole band at the Γ point from the laser-wavelength dependence of the polarization.

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.

Total and differential cross sections for ν_μe→ν_μe and ν̅ _μe→ν̅ _μe are measured. Values for the model-independent neutral-current couplings of the electron are found to be g_V=-0.107±0.035(stat)±0.028(syst) and g_A=-0.514±0.023(stat)±0.028(syst). The electroweak mixing parameter sin²θ_W is determined to be 0.195±0.018(stat)±0.013(syst). Limits are set for the charge radius and magnetic moment of the neutrino as (〈r²〉)<0.24×10^-32 cm² and f_μ<0.85×10^-9 Bohr magnetons, respectively.

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.

Measurements of the combined differential cross sections dσ/dy for ν_μ and ν¯_μ scattering by electrons yield the value sin²θ_W=0.195±0.018±0.013. This value, at Q²≃1×10^-3 (GeV/c)², is more precise than and in good agreement with our determination from the ratio of the total cross sections for the same reaction. It is consistent with, but lower than, the value determined from deep-inelastic neutrino scattering.

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.