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1.
Data were taken at the energy 2 E = 990 MeV to search for multibody events, with the same large solid angle detector which has been used for the measurement of the , ω andφ production by e +e − annilations. Assuming a π +π −π 0π 0 production by the quasi two-body process e +e − → → ωπ 0 we give the correspondi ng cross section σ(e +e − → π +π −π 0π 0) = (1.1 ± 0.5) 10 −32 cm 2. Since no events with 3 and 4 charged pions have been observed σ(e +e − → π +π −π 0π −) 1.5 × 10 −33 cm 2. 相似文献
2.
The emission of two photons above 25 MeV energy was observed in the capture of stopped pions by beryllium and carbon with a rate of (1.0 ± 0.1) × 10 −5, respectively (1.4 ± 0.2) × 10 −5 per capture. These rates are about two to four times greater than various free-nucleon estimates based on the π −+π +→γγ annihilation mechanism alone. 相似文献
3.
Angular distribution of the reaction 12C( d,τ) 11B leading to the 3/2 − ground state and the 4.44 MeV excited (5/2 − state 11B have measured at Ed = 80 MeV up to about 70° and 40°, respectively. The inclusion of second-order processes in the framework of CCBA calculations gives a good reproduction of the 3/2 − as well as of the DWBA forbidden 3/2 − angular distribution. The strong dependence of the cross section on the sign of the deformation supports a positive β 2 value for 11B. 相似文献
4.
Coherent Λπ − production on Pb of 600 GeV Σ − hyperons has been studied with the SELEX facility at Fermilab. Using the Primakoff formalism, we set a 90% CL upper limit on the radiative decay width Γ[Σ(1385) −→Σ −γ]<9.5 keV, and estimate the cross section for γΣ −→Λπ − at
GeV to be 56±16 μb. 相似文献
5.
The time-dependence of the decay rate of initially pure K 0 into the final state (π +π −π 0) has been studied in search for the decay k S0→π +π −π 0. No evidence is found in a sample of 384 observed events. The ratio of the CP -violating K S0 amplitude and the K L0 amplitude is η +−0 = (0.13 −0.20+0.17) + i(0.17 −0.26+0.27); the ratio of the CP-conserving K S0 amplitude and the K L0 amplitude is < 0.4. The energy dependence of the K 0→π +π −π 0 matrix element is found to be a+−0 = −0.31 ± 0.03. 相似文献
6.
Differential cross sections for Compton scattering by the proton have been measured in the energy interval between 200 and 500 MeV at scattering angles of θ cms = 75° and θ cms = 90° using the CATS, the CATS/TRAJAN, and the COPP setups with the Glasgow Tagger at MAMI (Mainz). The data are compared with predictions from dispersion theory using photo-meson amplitudes from the recent VPI solution SM95. The experiment and the theoretical procedure are described in detail. It is found that the experiment and predictions are in agreement as far as the energy dependence of the differential cross sections in the Δ-range is concerned. However, there is evidence that a scaling down of the resonance part of the M1+3/2 photo-meson amplitude by (2.8 ± 0.9)% is required in comparison with the VPI analysis. The deduced value of the M1+3/2-photoproduction amplitude at the resonance energy of 320 MeV is: | M1+3/2| = (39.6 ± 0.4) × 10 −3 mπ+−1. 相似文献
7.
The βγ circular polarization correlation of the 3 −(β − 621 keV)3 −(γ 1692)2 + cascade in 124Sb, the 4 +(β − 662)4 + (γ 796)2 + (γ 605)0 + in 13Z4Cs and the 6 +(β −529)6 + (γ 937)4 + (γ 885)2 + (γ 658)0 + in 110mAg have been studied by using a Compton effect polarimeter. The measured asymmetry parameters are 0.172±0.004, −0.0702 ±0.0024 and 0.0549±0.0013 respectively. 相似文献
8.
The charged current nuclear transition 12C(v e, e −) 12N g.s. has been observed in the KARMEN experiment. The flux average cross section for ve from μ + decay at rest is determined to be σ = [8.1±0.9(stat.)±0.75 (syst.)]×10 −42cm 2. For the first time also the energy dependence of the cross section has been measured for neutrino energies up to 50 MeV. 相似文献
9.
The theoretical mechanisms for the decays ψ′ → ψπ 0 and ψ′ → ψη, which violate SU 2 and SU 3, respectively discussed. It is argued that symmetry breaking in the decay amplitudes may be as important as π 0 −η −η′ mixing. The π 0 −η mechanism ψ′ → ψν → ψπ 0 leads to Γ(ψ′ → ψπ 0) = (3.3±1.0) × 10 −3 Γ(ψ′ → ψν), but this number may be enhanced by a factor as large as 12 by π 0−η′ mixing and isospin violation in the decay amplitude. The related decays ψ → ηγ and ψ → η′γ are also discussed. 相似文献
10.
The reactions π −p → 2π −π +p, π −p → 2π −π +π op and π −p → 2π −2π +n are analysed at 11 and 16 GeV/ c using longitudinal phase space (LPS) plots. The weighted LPS distributions for π −p → 2π −π +p is dominated by two well separated structures corresponding to single diffraction dissociation of the pion, π −p → (2π −π +)p, and of the proton, π −p → π −(π −π +p). The former is more abundant than the latter, and both are approximately constant with energy. In contrast, processes of type π −p → (2π)(πp) decrease with increasing energy. In the five-body reactions the weighted LPS distribution reveals especially at 16 GeV/c a maximum for single dissociation of the proton into 3πp, namely π−p → π−(π−π+πop); this process is likely to be diffractive. The neutron channel has a corresponding maximum displaced toward a multiperipheral configuration π−p → π−(π−2π+)n. Another strong maximum corresponds to the pion dissociation π−p → (2π−π+πo)p. This is interpreted to be an ω-exchange process because no analogous structure occurs in π−p → (2π−2π+)n. Finally, a broad structure reveals double dissociation of both incident particles; it occurs in the two channels π−p → (2π−π+)(πop) and π−p → (2π−π+)(π+n), being stronger in the latter. Further analysis of this process in terms of isospin exchange suggests that it is partially diffractive. Factorization is also discussed. An appendix gives general aspects of the LPS analysis for the asymptotic study of n-body collisions at very high energy. 相似文献
11.
Narrow structures in the range of a few MeV have been searched for in ppπ + and ppπ − invariant mass spectra ( Mppπ+ and Mppπ−) obtained from exclusive measurements of the pp→ ppπ +π − reaction at Tp=725,750 and 775 MeV using the PROMICE/WASA detector at CELSIUS. The selected reaction is particularly well suited for the search for dibaryon resonances decoupled from NN and/or NΔ. In the mass range 2020 MeV/ c2< mdibaryon<2085 MeV/ c2 no narrow structures could be identified on the 3σ level of statistical significance neither in Mppπ− nor in Mppπ+ giving an upper limit (95% C.L.) for dibaryon production in this reaction of σ<20 nb. 相似文献
12.
The production rate for η′ in pp → ppη′ at rest is calculated in a covariant one boson exchange model, previously applied to study π 0 and η production in NN collisions. The transition amplitudes for the elementary BN → η′ N processes with B being the meson exchanged ( B = π, σ, η, , ω and a0) are taken to be the sum of s- and u-channels with a nucleon in the intermediate states, and an a0 meson pole in a t-channel. The couplings of the η′ to hadrons are a factor 0.4 weaker than the respective η-hadron couplings, as suggested by a quark model and a singlet-octet mixing angle θ = −23°. The model reproduces near threshold cross sections for the quasielastic processes π −p → nη(η′) and pp → ppη(η′) reactions. 相似文献
13.
The lifetimes of the Cd+ 5 2P3/2 and 5 2P1/2 states have been measured by the Hanle effect. The Cd + ions are produced in a d.c. discharge in cadmium vapor, with helium as buffer gas. The results are: τ(5 2P3/2) = (2.60±0.20) ×10 −9sec, and τ(5 2P1/2) = (3.05 ± 0.13) × 10 −9sec. We measured also the cross sections for the destruction of the orientation in the 52P1/2Cd+ state (<5Å2), of the orientation (18±10Å2) and of the alignment (46±10Å2) in the 52P3/2 state due to collisions with the helium atoms. 相似文献
14.
The arguments are put forward that the many pion decays ω→2π +2π −π 0 and π +π −3π 0 provide an ideal test site for testing the predictions of chiral models of the vector meson decays into many pions. Using the approach based on the Weinberg Lagrangian or, in a new language, the Lagrangian of hidden local symmetry added with the term induced by the anomalous Lagrangian of Wess and Zumino, the partial widths of these decays are evaluated, and their excitation curves in e +e − annihilation are obtained. The discussed are the perspectives of the experimental study of the decays ω→5π in e +e − annihilation and photoproduction. 相似文献
15.
The circular polarization of the 482 keV γ-radiation of 181Ta and the 343 keV γ-radiation of 175Lu was measured with a Compton polarimeter. Spurious effects arising from higher order contributions of quantum electrodynamics to the Compton scattering cross section were discovered. The results for the polarization are P = − (4.1±1.3)10 −6 for 181Ta and P = − (1.5±6.0)10 −6 for 175Lu. These values are corrected for instrument asymmetries, bremsstrahlung and intensity of other lines. 相似文献
16.
Cross sections for the production of hypernuclei were measured in the reaction A(K −, π −) ΛA. on light and heavy nuclear targets, using a separated K − beam at the CERN Proton Synchrotron. The pions were detected in the forward direction. The results are compared with cross sections calculated under the assumption that the reaction took place on a single neutron. The good agreement between the measured and the calculated cross sections justifies the use of the (K −, π −) reaction in order to obtain spectroscopic information on hypernuclei. 相似文献
17.
β-γ(CP) correlation measurements have been carried out on the
sequence in the decay of the ground state of 49Ca to the (presumed) anti-analogue state at 3.105 MeV excitation in 49Sc. An asymmetry parameter of A = −0.132 ± 0.017 was obtained, making any spin-parity assignment other than
very unlikely for the 3.105 MeV state. The deduced Fermi matrix element for the decay is | Mv| = |1.4 ± 9.7| × 10−3, which leads to an isospin impurity (
) corresponding to the mixing of the analogue state (at 11.6 MeV) and the 3.105 MeV state of || 2 1.6 × 10 −5. This leads in turn to an effective Coulomb matrix element of | Hc| = |3.9 ± 27.4| keV, a low value compared to the value of 100 keV obtained from a theoretical estimate based on simple shell-model wave functions for the
single-particle states involved. 相似文献
18.
In a study of ∑ −π + production by stopped K − in liquid helium, (19 ± 6)% of stopped ∑ − were observed to decay. This corresponds to a “cascade time” of 28 ± 8 psec. 相似文献
19.
The π 0 spectrum in the KL0 → 3π decay was measured using a wire chambers magnetic spectrometer. In the usual approximation, the matrix element can be expressed as: M2 ≈ 1 + 2 a0( MK/ Mπ2)(2 Tπ0− Tπ0max) + a1( MK2/ Mπ4)(2 Tπ0− Tπ0max) 2. We obtained a0 = −0.282 ± 0.011 and a1 consistent with zero. 相似文献
20.
Impedance spectroscopy was used to study the oxygen reaction kinetics of La 0.8Sr 0.2MnO 3 (LSM)-based electrodes on Y 2O 3-stabilized ZrO 2 (YSZ) electrolytes. Three types of electrodes were studied: pure LSM, LSM–YSZ composites, and LSM/LSM–YSZ bilayers. The electrodes were formed by spin coating and sintering on single-crystal YSZ substrates. Measurements were taken at temperatures ranging from 550 to 850°C and oxygen partial pressures from 1×10 −3 to 1 atm. An arc whose resistance Rel had a high activation energy, Ea=1.61±0.05 eV, and a weak oxygen partial pressure dependence, ( PO2) −1/6, was observed for the LSM electrodes. A similar arc was observed for LSM–YSZ electrodes, where Rel( PO2) −0.29 and the activation energy was 1.49±0.02 eV. The combination of a high activation energy and a weak PO2 dependence was attributed to oxygen dissociation and adsorption rate-limiting steps for both types of electrodes. LSM–YSZ composite cathodes showed substantially lower overall interfacial resistance values than LSM, but exhibited an additional arc attributed to the resistance of YSZ grain boundaries within the LSM–YSZ. At 850°C and low PO2, an additional arc was observed with size varying as ( PO2) −0.80 for LSM and ( PO2) −0.57 for LSM–YSZ, suggesting that diffusion had become an additional rate limiting step. Bilayer LSM/LSM–YSZ electrodes yielded results intermediate between LSM and LSM–YSZ. The results showed that most of the improvement in electrode performance was achieved for a LSM–YSZ layer only ≈2 μm thick. However, a decrease in the grain-boundary resistance would produce much better performance in thicker LSM–YSZ electrodes. 相似文献
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