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1.
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
2.
The complete matrix elements for e +e − → 4 and e +e − → 4 fγ y are calculated in the Electroweak Standard Model for polarized massless fermions. The matrix elements for all final states are reduced to a few compact generic functions. Monte Carlo generators for e +e − → 4 f and e +e − → 4 fγ are constructed. We compare different treatments of the finite widths of the electroweak gauge bosons; in particular, we include a scheme with a complex gauge-boson mass that obeys all Ward identities. The detailed discussion of numerical results comprises integrated cross sections as well as photon-energy distributions for all different final states. 相似文献
3.
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
4.
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
5.
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
6.
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
7.
In the light of the recent muon ( gμ−2) result by the E821 experiment at the Brookhaven National Laboratory, we study the event rates of the charged lepton-flavour-violating (LFV) processes in the supersymmetric standard model (SUSY SM) with the heavy right-handed neutrinos (SUSY see-saw model). Since the left-handed sleptons get the LFV masses via the neutrino Yukawa interaction in this model, the event rate of μ→ eγ and the SUSY-SM correction to ( gμ−2)/2 (δ aμSUSY) are strongly correlated. When the left-handed sleptons have a LFV mass between the first and second generations (
) in the mass matrix, it should be suppressed by 10 −3 (10 −9/δ aμSUSY) compared with the diagonal components ( mSUSY2), from the current experimental bound on μ→ eγ. The recent ( gμ−2) result indicates δ aμSUSY10 −9. The future charged LFV experiments could cover
. These experiments will give a significant impact on the flavour models and the SUSY-breaking models. In the SUSY see-saw model
is proportional to square of the tau-neutrino Yukawa-coupling constant. In the typical models where the neutrino-oscillation results are explained and the top-quark and tau-neutrino Yukawa couplings are unified at the GUT scale, a large LFV mass of
is generated, and the large LFV event rates are predicted. We impose a so-called no-scale condition for the SUSY-breaking parameters at the GUT scale, which suppresses the FCNC processes, and derive the conservative lower bound on μ→ eγ. The predicted Br(μ→ eγ) could be covered at the future LFV experiments. 相似文献
8.
We compute non-perturbative contributions to B → Xsl+l− that are not explicitly suppressed by powers of the b-quark mass. They are proportional to
and arise from an interference between the free-quark amplitude and high order terms in the matrix element of the four-quark operator s−γ μ(1 − γ 5) cc−βγ μ(1 − γ 5) bβ. This correction is found to be small over most of the dalitz plot except near the charm threshold. Unfortunately, the perturbative computation we have performed is invalid near charm threshold and we do not except to see the structure found at lowest order reproduced in the data. We conclude that these non-perturbative contributions do not significantly modify the previous analysis of B → Xsl+l−. 相似文献
9.
The similarity between the weak interactions of electron and muon is extended to the principle that all e and μ interactions in gauge models are invariant under e ↔ μ exchange. This necessitates the existence of two Higgs doublets φ e and φ μ, and an extended e ↔ μ permutation invariance. After symmetry-breaking, a multiplicatively conserved “permutation parity” π = ± 1 for all particles naturally emerges, with π μ = −π e = 1. The model forbids μ → eγ but predicts e −e − → μ −μ − mediated by π = −1 Higgs bosons at 10 −11 times the rate of typical weak cross sections. 相似文献
10.
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
11.
We present the results of a study of the exclusive reactions π + 4He → π − 4p (double charge exchange, DCE) and π + 4He → π + π − 3pn (one-pion production). The experiment was performed with the Oxford/RHEL helium bubble chamber irradiated with a 1.7 GeV/ c π + beam. A general review of DCE models is presented and experimental results at other energies are discussed in the light of these models. None of the existing theoretical models is compatible with out data at 1.7 GeV/ c. We propose a new mechanism for DCE, involving three nucleons, which reproduces the integrated as well as the differential cross sections. Some consequences for other models are also discussed. 相似文献
12.
The recently measured angular distribution of one of the leptons produced in π −N → μ +μ -+… is compared with theoretical calculations done in the framework of QCD. 相似文献
13.
We calculate the amplitude of the decays η → π 0π +π − and η → π 0π 0π 0 to first non-leading order in the low-energy expansion. It is shown that the infrared singularities of chiral perturbation theory strongly enhance the perturbation generated by the quark mass term. The corrections of order ms increase the current algebra prediction for the rate by more than a factor of two and remove the well-known discrepancy between the low-energy theorems of current algebra and experiment. 相似文献
14.
The anomalous eμ and 2-prong μx events produced in e +e − annihilation are used to determine the properties of the proposed τ charged lepton. We find the τ mass is 1.90 ± 0.10 GeV/ c2; the mass of the associated neutrino, ν τ, is less than 0.6 GeV/ c2 with 95% confidence; V - A coupling is favored over V + A coupling for the τ − ν τ current; and the leptonic branching ratios are 0.186 ± 0.010 ± 0.028 from the eμ events and 0.175 ± 0.027 ± 0.030 from the μ x events where the first error is statistical and the second is systematic. 相似文献
15.
We present predictions for flavour-violating charged-lepton decays induced by the seesaw mechanism implemented within the constrained minimal supersymmetric standard model (CMSSM) with universal input soft supersymmetry breaking terms. We assume that one heavy singlet neutrino almost decouples from the seesaw mechanism, as suggested by the pattern of light neutrino masses and mixing angles. This is suggested independently by sneutrino inflation with a low reheating temperature, TRH10 7 GeV, so as to avoid overproducing gravitinos. This requirement further fixes the mass of the weakly-coupled sneutrino, whose decays may lead to leptogenesis. We find that BR(μ→ eγ)10 −13 but BR(τ→μγ)10 −9 in the bulk of the acceptable parameter space, apart from a few isolated points. The ratio BR(μ→ eγ)/ BR(τ→ eγ) depends on only one complex parameter, and is particularly interesting to compare with experiment. 相似文献
16.
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
17.
The reaction γ p → π °γ′ p has been measured with the TAPS BaF 2 calorimeter at the Mainz Microtron accelerator facility MAMI for energies between √2 = 1221–1331 MeV. Cross sections differential in angle and energy have been determined for the photon γ′ in three bins of the excitation energy. This reaction channel provides access to the magnetic dipole moment of the Δ +(1232) resonance and, for the first time, a value of μ Δ+ = (2.7 +1.0−1.3( stat) ± 1.5( syst) ± ( theor)) π N has been extracted. 相似文献
18.
The average polarization of 12B produced by the capture of polarized muons in 12C has been measured using recoil implantation techniques. From the result we deduce the average polarization of the 12B ground state 12B(0) corresponding to the 12C → 12B(0) Gamow-Teller reaction: Jμ(0) = 0.43 ± 0.10. The sizeable deviation of this polarization from the value of 2/3, characteristic of a “bare” 0 + → 1 + process, is a fair evidence for induced axial-vector interaction(s) in muon capture. The ratio of the induced pseudoscalar and the axial-vector coupling constants is deduced to be: gμP/ gμA = 12 ± 5. 相似文献
19.
A spherical harmonic moment analysis of the reactions K −p → K −π +n and K +p → K +π −Δ ++ at 13 GeV/ c demonstrates the existence of a broad K * state with mass in the vicinity of 1800 MeV and spin parity 3 −. 相似文献
20.
It is shown that the weak amplitude must also be radiatively corrected for a satisfactory analysis of the neutral current in e +e −→μ +μ −. Radiative asymmetry can be of possible help in isolating the weak effect provided the symmetric cross section can be measured to 1% accuracy at PEP/PETRA energies. 相似文献
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