Search Results (26 total)

We study a method of laser-Compton cooling of electron beams for future linear colliders. Using a Monte Carlo code, we evaluate the effects of the laser-electron interaction for transverse cooling. The optics with and without chromatic correction for the cooling are examined. The laser-Compton cooling for Japan Linear Collider/Next Linear Collider at E₀  =  2 GeV is considered.

Spatiotemporal patterns in rings of coupled biological oscillators of the plasmodial slime mold, Physarum polycephalum, were investigated by comparing with results analyzed by the symmetric Hopf bifurcation theory based on group theory. In three-, four-, and five-oscillator systems, all types of oscillation modes predicted by the theory were observed including a novel oscillation mode, a half period oscillation, which has not been reported anywhere in practical systems. Our results support the effectiveness of the symmetric Hopf bifurcation theory in practical systems.

Making use of the many-beam (one- and two-dimensional quantum treatment) formalism for transversely bound electrons moving through crystal lattices, we have computed planar and axial coherent bremsstrahlung (type A) spectra for 17-MeV electrons passing through a 10-μm thick diamond (C) crystal. We found that in the planar case the momentum transfer occurs in the direction perpendicular to the plane and results in a photon emission in the forward direction (electron-beam direction). In the axial case, the momentum transfer occurs in the plane perpendicular to the axis of interest. Only momentum transfers along the scan direction (electron transverse momentum direction) result in a photon emission in the forward direction. Two different scans have shown that the energies of the coherent bremsstrahlung peaks depend strongly on the direction of the electron transverse momentum but the intensities of the strongest peaks do not show any considerable change.

A living coupled oscillator system was constructed by a cell patterning method with a plasmodial slime mold, in which parameters such as coupling strength and distance between the oscillators can be systematically controlled. Rich oscillation phenomena between the two-coupled oscillators, namely, desynchronizing and antiphase/in-phase synchronization were observed according to these parameters. Both experimental and theoretical approaches showed that these phenomena are closely related to the time delay effect in interactions between the oscillators.

The efficiency of positron production by a 1.2-GeV electron beam incident on a tungsten single crystal oriented in an axially channeling condition has been experimentally studied. Positron yields from a 1.2-mm-thick crystal have been measured to be ∼2.5–3.0 times larger than those from amorphous targets with the same thickness. A simple model simulation of the positron production process shows that coherent bremsstrahlung, as well as channeling radiation, contributes to the enhancement of the positron yields.

The parametric x radiation from thick Si single crystals with 0.5–5 mm thickness was investigated at an electron energy of 900 MeV. As the crystal thickness increased, both intensity and angular spread reached a plateau after their increase in the thin crystal region, resulting in more brilliant x rays than Feranchuk and Ivashin’s prediction [J. Phys. (Paris) 46, 1981 (1985)] for thick crystals. This behavior is consistent with the incoherent model proposed in our previous paper [Phys. Rev. Lett. 70, 3247 (1993)].

Isotope shifts and hyperfine structures of twelve transitions from the high-lying metastable ⁹H_J (J=1–7) states of the 4f⁶5d6s configuration in Sm i have been measured for the stable isotopes by means of atomic-beam laser spectroscopy. Hyperfine constants A and B for the odd-mass isotopes ¹⁴⁷Sm and ¹⁴⁹Sm are determined for the ⁹H_J (J=1–7) states and six upper levels of the transitions in Sm i. $J— dependences of the isotope shifts are observed for the 4f⁶5d6s ⁹H term. Parameters z_5d of the crossed-second-order effect are derived for the 4f⁶5d6s configuration, and z_5d/λ is found to be 165(10) MHz/fm². It has been verified that the field shift is dominant in the crossed-second-order effect.

We have measured the crystal-angle dependence and the intensity of the parametric x radiation from a Si single crystal impinged upon by an electron beam with the energy ranging from 200 to 1100 MeV. It has been found that the x-ray intensity is higher and the angular spread is less broad than the predictions of the theoretical model due to Feranchuk and Ivashin. This calls for a more elaborate theoretical treatment of multiple scattering, and encourages research aiming at a new type of x-ray source based on the parametric x-radiation mechanism.

We have measured the temperature dependence of parametric x-ray radiation (PXR) from a Si single crystal bombarded by 900-MeV electrons. The radiation intensity is appreciably enhanced when the crystal is cooled to liquid-nitrogen temperature as compared with the case for room temperature. The enhancement factors are larger for the higher-order reflections and are quantitatively consistent with the assumption that the PXR intensities are proportional to the squared Debye-Waller factor.

A method has been developed by means of coherent bremsstrahlung from a single crystal for the determination of atomic form factors. This method employs a precise measurement of energy spectra of γ rays in the several-hundred-MeV region produced by 1.2-GeV electrons from single crystals as a function of the relative angle between a definite crystal axis and the incident-electron beam. It is shown that the spectra contain the electron-screening effect around the target nuclei and give information on the atomic form factor mainly at small momentum transfers. Possible problems with this method have been examined with this experiment using a silicon crystal. A method to analyze the experimental results has been established and with this method the deviation of atomic form factor from theoretical calculation is detectable at a level of down to a few percent. The present experimental data for a silicon crystal are consistent with the form factor obtained by the Pendello$uml—sung method, which shows small deviations from the Hartree-Fock model at small momentum transfers. The accuracy attained in this method has been shown not to be too much affected by the imperfections of the crystal, at least for dislocation densities of up to 10⁴ cm^-2.

The total cross section for the γd→π^-pp reaction has been measured for incident photon energies from 380 to 840 MeV in steps of 10 MeV, with the best energy resolution attained so far. A large-acceptance detector was used to observe the reaction products. Overall uncertainties in the deduced cross sections are less than 9% (∼4% statistical and ∼8% systematic). The results are in excellent agreement with previous bubble chamber measurements and do not show any statistically significant structure which can be interpreted as evidence for the formation of dibaryon resonances. An upper limit at 95% confidence level of σ_peakΓ<230 μb MeV is obtained for a resonance in the vicinity of photon energy 700 MeV (mass∼2490 MeV).

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 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.

We have performed precise measurements of the energy spectrum of bremsstrahlung from a single crystal. The results are compared with the theory of coherent bremsstrahlung and found to be sensitive to the choice of the functional form of the atomic form factor. The Si data agree with the theoretical curve which is obtained with the form factor based on the Hartree-Fock model within a systematic error of 2%. It is pointed out that measurements sensitive to the atomic form factor to the 0.5% level will become feasible for various materials by improvement of the present method.

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 differential cross section for π^--d elastic scattering at c.m. pion angles 91°, 104°, 123°, 136°, and 155° has been measured for pion momenta between 460 and 1160 MeV/c with the finest momentum steps so far employed. A pronounced dip-bump structure around 700 MeV/c observed previously at 172° is confirmed at other angles, while the dip around 1000 MeV/c is found to degrade rapidly as the angle decreases. The angular distribution at large angles (> 90°) shows rather drastic changes at these momenta.

The differential cross section for the reactions γd→pn, γd→π⁰d, and γd→pX has been measured by using a tagged photon beam in the energy range of dibaryon resonances. The most characteristic feature of the data for γd→pn is a forward nonpeaking angular distribution. This behavior is in complete disagreement with the existing predictions which take into account the dibaryon resonances. A phenomenological analysis is made by slightly modifying the model of the Tokyo group, but no satisfactory result is obtained. The data for γd→π⁰d at large angles show that the differential cross section decreases exponentially as a function of pion angle. A comparison is made with a Glauber model calculation. The result seems to be rather in favor of the existence of dibaryon resonances, but a clear conclusion is not possible because of a lack of more accurate data. In the process γd→pX, a broad peak due to quasifree pion production is observed, but the limitation of experimental sensitivity does not allow us to have a definite conclusion for the dibaryon resonance of mass 2.23 GeV conjectured by the Saclay group.

NUCLEAR REACTIONS ²H(γ, p), (γ, π⁰), (γ, p)X, E=180-600 MeV; measured σ(E, θ); tagged photons. Dibaryon resonance effect.

The backward differential cross section for π^--d elastic scattering has been measured at incident momenta between 420 and 1160 MeV/c. The data show two bumps at around 670 and 1100 MeV/c, two dips near 630 and 980 MeV/c, and a break at 550 MeV/c. The result of a phenonomenological fit is consistent with the existence of three dibaryon resonances in this energy region. A theoretical calculation of Kanai et al. agrees well with the data below 800 MeV/c, but the agreement becomes worse above 800 MeV/c.

The differential cross section for γd→pn has been measured in the energy range between 180 and 600 MeV at c.m. angles 15°, 30°, 42°, and 72°, by using tagged photons. The results, in particular at smaller angles, are in disagreement with theoretical calculations which take into account the effect of dibaryon resonances.

The angular distributions of π^- / π⁺ ratios in photoproduction from deuterium were measured for fourteen photon energies between 260 and 800 MeV. The energy dependence of σ^--σ⁺ near the Δ₃₃(1236) resonance region shows no dip structure, consistent with an absence of the isotensor term. The differential cross sections of γn→π^-p at k=350 MeV are considerably larger than the ones obtained from the inverse reaction π^-p→γn, but the two are in fair agreement at k=500 MeV.