Search Results (34 total)

A lossy data compression scheme for uniformly biased Boolean messages is investigated via statistical mechanics techniques. We utilize a treelike committee machine (committee tree) and a treelike parity machine (parity tree) whose transfer functions are nonmonotonic. The scheme performance at the infinite code length limit is analyzed using the replica method. Both committee and parity treelike networks are shown to saturate the Shannon bound. The Almeida-Thouless stability of the replica symmetric solution is analyzed, and the tuning of the nonmonotonic transfer function is also discussed.

Electron storage rings of GeV energy with laser pulse stacking cavities are promising intense sources of polarized hard photons which, via pair production, can be used to generate polarized positron beams. In this paper, the dynamics of electron bunches circulating in a storage ring and interacting with high-power laser pulses is studied both analytically and by simulation. Both the common features and the differences in the behavior of bunches interacting with an extremely high power laser pulse and with a moderate pulse are discussed. Also considerations on particular lattice designs for Compton gamma rings are presented.

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.

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.

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 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.

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.

Using modularized NaI(Tl) detectors, we carried out a high-statistics measurement of inclusive π⁰ spectra in antiproton annihilation at rest in a liquid D₂ target. At the statistical significance above 4σ, we did not see any narrow π⁰ peaks due to the production of B=baryonium or similar narrow states in p̅ d→π⁰BN_s (N_s being a spectator nucleon) or p̅ d→π⁰B. The 4σ upper limit for the yields (or branching ratios) per annihilation was 2×10^-2–2×10^-3 for the baryonia with a mass (M_B) of 1700–1100 MeV/c² and a width smaller than 9 (at M_B∼1700 MeV/c²) to 40 (M_B∼1100 MeV/c²) MeV/c².

Using modularized NaI(Tl) detectors, we carried out a high statistics measurement of inclusive γ-ray, π⁰, and η spectra, and determined the branching ratios or upper limits of Pontecorvo reactions p¯d→π⁰n, π⁰Δ⁰, ηn, and ηΔ⁰ from the corresponding monochromatic peaks in them. The obtained branching ratios are B(p¯d→π⁰n)=(1.03±0.41)×10^-5, B(p¯d→π⁰Δ⁰)=(4.67±1.66)×10^-5, B(p¯d→ηn)<8.94×10^-6 (95% C.L.), and B(p¯d→ηΔ⁰)<6.49×10^-5 (95% C.L.).

To search for baryonium states, a highly sensitive experiment with good statistics has been carried out at KEK by using an antiproton beam, deuterium target, and time-of-flight counter together with a modularized NaI(Tl) detector. At the 4σ level, no narrow peaks due to the production of baryonia in the p¯d→Bn reaction have been observed in the neutron spectra. At lower significance levels, however, we have observed four narrow peaks with 3.7σ-3.5σ, and two at an even lower significance level, in different charge-multiplicity final states. The recoil momentum distribution of the neutrons has been extracted for the channel with zero charge prongs. The energy spectra for γ rays in coincidence with the neutron have been also obtained.

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.

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.

Using modularized NaI(T1) detectors, we have carried out a high-statistics measurement of inclusive γ-ray spectra from p̅ d annihilation at rest separately for each charge multiplicity of the final state. We have not seen, at statistical significance above 4σ, any monochromatic γ-ray peaks, which may be assigned to baryonium production p̅ p or p̅ n→γB or to  N̅ NN bound-state production p̅ d→γ(N̅ NN). The 4σ upper limit for baryonium production per annihilation varied between 10^-2 and 10^-4 depending on baryonium mass of 1700 to 600 MeV/c² and on the charge multiplicity. At (2-3)σ levels, however, five peaks were observed and three of them are located at the same position with the similar (2-3)σ peaks observed in p̅ p→γB and p̅ p→π⁰B.

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.