Dynamical aspects of photonuclear reactions below pion threshold

We study the reactions (γ,p), (γ,n), (γ,pp) and (γ,nn) below pion threshold in view of a systematic treatment of electromagnetic meson exchange currents. As a numerical example we calculate the integrated and differential cross sections for (γ,p) and (γ,n) on ⁴He.     

Positive pion production at threshold on A 10B target

The angular distribution of low-energy positive pions (E_π ≈ 11 MeV) produced by 154 MeV protons on a ¹⁰B target has been measured between 16.2° and 54.8° c.m. angles. Only pions corresponding to the ground state of ¹¹B were observed. The data have been compared with theoretical angular distributions calculated in the framework of the one-nucleon mechanism using the DWBA formalism.     

Evidence for a narrow S = +1 baryon resonance in photoproduction from the neutron

The gamman-->K(+)K(-)n reaction on 12C has been studied by measuring both K+ and K- at forward angles. A sharp baryon resonance peak was observed at 1.54+/-0.01 GeV/c(2) with a width smaller than 25 MeV/c(2) and a Gaussian significance of 4.6sigma. The strangeness quantum number (S) of the baryon resonance is +1. It can be interpreted as a molecular meson-baryon resonance or alternatively as an exotic five-quark state (uuddsmacr;) that decays into a K+ and a neutron. The resonance is consistent with the lowest member of an antidecuplet of baryons predicted by the chiral soliton model.     

Spin correlation in pp elastic scattering and in pion production near threshold

An internal polarized gas target in conjunction with a beam of polarized protons stored in the IUCF Cooler storage ring has been used to measure analyzing power and spin correlation parameters between 200 MeV and 450 MeV over a wide range of angles. A consistent absolute beam and target polarization calibration was established by ramping the stored protons up and down in energy. The analyzing power A _y and the spin correlations A _xx , A _yy and A _xz were measured with transverse beam polarization, while determination of A _zz required the development of a longitudinally polarized beam. First experiments on the spin dependence on π₀-production have recently been carried out.     

Novel approach to pion and eta production in proton-proton collisions near threshold

We evaluate the threshold matrix–element for the reaction pp→ppπ⁰ in a fully relativistic Feynman diagrammatic approach. We employ a simple effective range approximation to take care of the S–wave pp final–state interaction. The experimental value for the threshold amplitude A = (2.7 −i0.3) fm⁴ can be reproduced by contributions from tree level chiral (long–range) pion exchange and short–range effects related to vector meson exchanges, with ω-exchange giving the largest individual contribution. Pion loop effects appear to be small. We stress that the commonly used heavy baryon formalism is not applicable in the NN–system above the pion production threshold due to the large external momentum, |p|≃ (Mm _π)^−1/2, with M and m _π the nucleon and the pion mass, respectively. We furthermore investigate the reaction pp→pnπ⁺ near threshold within the same approach. We extract from the data the triplet threshold amplitude as B = (2.8 −i1.5) fm⁴. Its real part can be well understood from (relativistic) tree level meson–exchange diagrams. In addition, we investigate the process pp→ppη near threshold. We use a simple factorization ansatz for the ppη final–state interaction and extract from the data the modulus of the threshold amplitude, |C|= 1.32 fm⁴. With g _ηN= 5.3, this value can be reproduced by (relativistic) tree level meson–exchange diagrams and η–rescattering, whose strength is fixed by the ηN scattering length. We also comment on the recent near threshold data for η^′–production. Received: 27 November 1998     

Inelastic response functions for pion electroproduction near threshold

Longitudinal and transverse response functions are calculated for pion electroproduction from nuclei near threshold, for an experimental situation in which only the final electron is detected. For the case of calcium, a Fermi gas model is used, and the calculated response functions and cross sections are at least an order of magnitude smaller than the measured quasielastic contributions, indicating that electroproduction is a relatively unimportant background. For the case of oxygen, it is found that pion electroproduction accompanied by excitation of collective GDR states differs in magnitude and spectrum shape from that predicted by a quasifree model.     

No evidence for precritical phenomena of pion condensation in A = 48 nuclei

The ⁴⁸Ca(e,e′)(1⁺, 10.24 MeV) reaction, the ⁴⁸Ca(p,p′)(1⁺, 10.24 MeV) reaction and the ⁴⁸Ca(p,n)⁴⁸Sc(1⁺, 16.8 MeV) reaction have been theoretically investigated with respect to the occurrence of precritical phenomena related to pion condensation. No evidence for precritical phenomena is found.     

Search for short-lived particles below charm threshold

We have searched for the production by virtual photons of short-lived particles that exhibit muonic decay. We used Brookhaven's 10.5 GeV/c muon beam and examined the two and three-muon final states. The sample of 158 trimuon events is due almost entirely to QED tridents. The sample of 673 dimuons can be accounted for in part by asymmetric tridents and in part by the decay of π and K mesons. We obtain an upper limit for the production of other short-lived particles of the order of Bδ < 150 pb/nucleon, where B is the branching ratio for muonic decay. From the trident sample we establish that muons obey Fermi-Dirac statistics at the level of 8 standard deviations.     

Implications of gauge invariance for pion electroproduction at threshold

We discuss the implications of gauge invariance in the problem of the on-shell extrapolation of the electroproduction low-energy theorems. We show that there is an invariant amplitude which can be evaluated at the Breit threshold either using gauge invariance and on-shell dispersion relations or following the Fubini and Furlan [5] extrapolation method starting from the current-algebra low-energy value of the amplitude. Comparing the two expressions, we find a relation between the electromagnetic pion form factor, F_π (k²), and the axial-vector nucleon form factors, g_A (k²) and h_A (k²).     

A single quark model for single pion photoproduction at threshold

Single pion photoproduction is studied in a single quark model in which a quark absorbs the photon and then emits the pion, propagating as a quark between the two interactions. Two different chiral bag models are used which allow for the coupling of the quarks to the pion. Surprisingly, the single quark model reproduces the results of phenomenological models and agrees with the experimental amplitudes to within 30% at threshold.     

Radiative decay of doubly excited states in helium below the He+ (N = 2) ionization threshold

The radiative decay of (1)P(o) doubly excited states in helium has been investigated using a novel apparatus in which metastable atoms and vacuum ultraviolet photons are detected. The intensity ratio of the energetically narrow (sp,2n-)(1)P(o) and (2p,nd)(1)P(o) series to the broader (sp,2n+)(1)P(o) series is strikingly enhanced in comparison with conventional photoabsorption, photoion, or photoelectron measurements using synchrotron radiation. The experimental approach is a new way forward for the study of energetically narrow doubly excited states.     

Search for narrow dibaryon resonances in neutral pion photoproduction from the deuteron

The reaction γd↦π⁰ X has been measured with TAPS at MAMI in the energy range E _γ = 140-300 MeV. Using the Glasgow tagging spectrometer a photon energy resolution of 0.8 MeV was achieved. The energy excitation functions of integral and differential total cross-sections show no structures of statistical significance > 2σ. Upper limits for the production of narrow isoscalar or isovector dibaryons with masses m? 2100 MeV/c² were deduced. They are in the range 2-5 μb averaged over the 0.8 MeV energy resolution. Received: 25 October 2000 / Accepted: 24 November 2000     

Ab initio calculation of photoionization and inelastic photon scattering spectra of He below the N=2 threshold in a dc electric field

We study the Stark effect on doubly excited states of the helium atom below N=2. We present the ab initio photoionization and total inelastic photon scattering cross sections calculated with the method of complex scaling for field strengths F 相似文献   

Discovery of a 1731-MeV narrow resonance

Results obtained by studying a previously unknown narrow meson of mass about 1731 MeV are presented. This state was found in a system of two K _S mesons. The experimental-data set subjected to the present analysis was obtained with a 6-m spectrometer of the Institute of Theoretical and Experimental Physics (ITEP, Moscow). The statistical sample of event of double K _S -meson production was accumulated in π^? p interactions by using a beam of 40-GeV pions from the U-70 accelerator of the Institute of High Energy Physics (IHEP, Protvino). There are 58 events at the respective maximum, its statistical significance being close to six standard deviations. The parameters of the observed meson are the following; the mass is M = 1731 ± 2.5 MeV, the width is σ = 3.2 ± 2 MeV, and the product of the production cross section and the respective branching ratio is not less than 20 nb. The fact that the observed meson is extremely narrow does not have a satisfactory theoretical explanation.     

Three-boson problem near a narrow Feshbach resonance

We consider a three-boson system with resonant binary interactions and show that for sufficiently narrow resonances three-body observables depend only on the resonance width and the scattering length. The effect of narrow resonances is qualitatively different from that of wide resonances revealing novel physics of three-body collisions. We calculate the rate of three-body recombination to a weakly bound level and the atom-dimer scattering length and discuss implications for experiments on Bose-Einstein condensates and atom-molecule mixtures near Feshbach resonances.     

Further evidence for formation of a narrow baryon resonance with positive strangeness in K + collisions with Xe nuclei

We have continued our investigation of the charge-exchange reaction K ⁺Xe → K ⁰ pXe’ in the bubble chamber DIANA. In agreement with our previous results based on part of the present statistics, formation of a narrow pK ⁰ resonance with mass of 1537 ± 2 MeV/c ² is observed in the elementary transition K ⁺ n → K ⁰ p on a neutron bound in the xenon nucleus. The visible width of the peak is consistent with being entirely due to instrumental resolution and allows one to place an upper limit on its intrinsic width: Γ < 9 MeV/c ². A more precise estimate of the resonance intrinsic width, Γ = 0.36 ± 0.11 MeV/c ², is obtained from the ratio between the numbers of resonant and nonresonant charge-exchange events. The signal is observed in a restricted interval of incident K ⁺ momentum that is consistent with smearing of a narrow pK ⁰ resonance by Fermi motion of the target neutron. The statistical significance of the signal is some 7.3, 5.3, and 4.3 standard deviations for the estimators and , respectively. This observation confirms and reinforces our earlier results, and offers strong evidence for formation of a pentaquark baryon with positive strangeness in the charge-exchange reaction K ⁺ n → K ⁰ p on a bound neutron. (The DIANA Collaboration) The text was submitted by the authors in English.     

The study of the negative pion production in neutron-proton collisions at beam momenta below 1.8 GeV/c

A detailed investigation of the reaction np ? \rightarrow pp p^- \pi^{{-}}_{} has been carried out using the data obtained with the continuous neutron beam produced by charge exchange scattering of protons off the deuterium target. Partial-wave event-by-event analysis based on the maximum likelihood method was applied to determine contributions of different partial waves to the pion production process. A combined analysis of the np ? \rightarrow pp p^- \pi^{{-}}_{} and pp ? \rightarrow pp p⁰ \pi^{{0}}_{} data measured in the same energy region allows us to determine the contribution of isoscalar partial waves (I = 0 in the momentum range from 1.1 up to 1.8GeV/c . The decay of isoscalar partial waves into the (¹ S ₀)_pp p \pi channel provides us with a good tool for the determination of the pp S -wave scalar scattering length in the final state which was found to be a _pp = - 7.5±0.3 fm.