Subthreshold and near threshold K+ meson photoproduction\newline on nuclei

The inclusive K⁺ meson production in photon–induced reactions in the near threshold and subthreshold energy regimes is analyzed with respect to the one–step (γN→K ⁺ Y, Y=Λ,Σ) incoherent production processes on the basis of an appropriate new folding model, which takes properly into account the struck target nucleon removal energy and internal momentum distribution (nucleon spectral function), extracted from recent quasielastic electron scattering experiments and from many–body calculations with realistic models of the NN interaction. Simple parametrizations for the total and differential cross sections of the K⁺ production in photon–nucleon collisions are presented. Comparison of the model calculations of the K⁺ differential cross sections for the reaction γ+C¹² in the threshold region with the existing experimental data is given, that displays the contributions to the K⁺ production at considered incident energies coming from the use of the single–particle part as well as high momentum and high removal energy part of the nucleon spectral function. Detailed predictions for the K⁺ total and differential cross sections from γH², γC¹² and γPb²⁰⁸ reactions at subthreshold and near threshold energies are provided. The influence of the uncertainties in the elementary K⁺ production cross sections on the K⁺ yield is explored. Received: 12 April 1999 / Revised version: 11 September 1999     

Elastic and inelastic scattering of polarized protons from even n=82 nuclei near isobaric analog resonances

Excitation curves for the elastic and the inelastic scattering of polarized protons on the target nuclei ¹³⁸Ba, ¹⁴⁰Ce, ¹⁴²Nd and ¹⁴⁴Sm have been measured in the energy region of prominent isobaric analog resonances. A detailed analysis of the data is presented. Definite J^π assignments are given for a number of parent analog states. Spectroscopic factors and amplitudes for configurations with the core in the ground state or in the 2₁⁺ excited state are derived from the elastic and inelastic proton decay amplitudes. They show consistency with sum rules including orthogonality conditions. Different methods for the-calculation of single particle decay widths have been applied. The qualitative agreement with present unified model calculations is remarkable; an overestimattion of the weak coupling structure, however, becomes apparent.     

Pion production by protons on nuclei near the threshold

We propose including in the calculation of the cross section of the process p + N → N + N + π the process of pion production and absorption that proceeds through creation of a Δ-resonance. By taking into account the energy dependence of Δ-resonance width, the contribution of deuteron channel is included in the cross section of π⁰-, π⁺-meson production.     

Measurement of η photoproduction on the proton from threshold to 1500 MeV

Beam asymmetry and differential cross-section for the reaction γp → ηp were measured from production threshold to 1500MeV photon laboratory energy. The two dominant neutral decay modes of the η -meson, η → 2γ and η → 3π⁰ , were analyzed. The full set of measurements is in good agreement with previously published results. Our data were compared with three models. They all fit satisfactorily the results but their respective resonance contributions are quite different. The possible photoexcitation of a narrow state N(1670) was investigated and no evidence was found.     

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     

Eta-meson photoproduction on protons,neutrons, and light nuclei in the nucleon-resonance region

Eta-meson photoproduction on protons, neutrons, and light nuclei is reviewed. New results obtained for photoproduction on protons and neutrons at the GRAAL facility are analyzed in detail. The experiment in question was performed in a gamma-ray beam that was obtained by means of Compton backscattering of laser photons on electrons at the European Synchrotron Radiation Facility (ESRF, Grenoble, France). A comparative analysis of data on the differential cross sections and the asymmetries for eta-meson photoproduction allowed us to reveal some special features and to study the mechanisms of eta-meson photoproduction on nucleons in detail. Data available from the literature on eta-meson photoproduction on light nuclei are discussed in detail. Results that suggest the existence of bound states (etamesic nuclei) are presented. Particular attention is given to the newmethod of “taggedmesons,” which, according to the GRAAL experiment and to simulated results, makes it possible to study thoroughly the interaction of unstable mesons with intranuclear nucleons.     

Inclusive ω photoproduction from nuclei and ω in the nuclear medium

With the aim of extracting information on the shift of the ω mass in the nuclear medium we analyze data obtained at ELSA from where claims for evidence of a mass shift of the ω have been made. We develop a Monte Carlo simulation code which takes into account the possible reactions in the experimental set-up of (γA → π⁰γX) in the vicinity of the ω production region with subsequent ω → π⁰γ decay. We compare our results with experiment for the distribution of π⁰γ invariant masses and conclude that the distribution is compatible with an enlarged ω width of about 90MeV at nuclear-matter density and no shift in the mass. This change in the width would be compatible with the preliminary results obtained from the transparency ratio in the A-dependence of ω production. The discrepancy of the present conclusions with former claims of an evidence for a shift of the ω mass stems from a different choice of background which is discussed in the paper.     

Aspects of near-threshold neutral pion photoproduction off protons

We investigate near-threshold neutral pion photoproduction off protons to fourth order in heavy-baryon chiral perturbation theory in the light of the new data from MAMI. We show that the unitarity cusp at the secondary π⁺ n threshold is in agreement with expectations from the final-state theorem. We also analyze the fourth-order corrections to the P-wave low-energy theorems and show that potentially large Δ isobar contributions are cancelled by sizeable pion loop effects. This solidifies the parameter-free third-order predictions, which are in good agreement with the data. Received: 25 May 2001 / Accepted: 21 June 2001     

pp → ppπnear threshold in pionless effective theory

The total cross-section of the pp → ppπ⁰ reaction near threshold is calculated in pionless effective field theory with a di-baryon and external pions. The amplitudes for a leading one-body and subleading contact neutral pion production vertex are obtained including the initial- and final-state interactions. After estimating a low-energy constant in the contact vertex, we compare our results for the total cross-section with the experimental data.     

Study of elastic and inelastic scattering of protons from 74Ge at isobaric analogue resonances

Excitation functions of the elastic and the inelastic proton scattering leading to the ground state and to the 2⁺ (0.596 MeV) state in ⁷⁴Ge were measured at 90°, 125°, 140° and 165° at bombarding energies from 3.34 to 5.4 MeV. Ten isobaric analogue resonances in ⁷⁵As were identified. Five of these resonances decay to the 2⁺ excited state in ⁷⁴Ge. Angular distributions of the inelastically scattered protons were measured at bombarding energies corresponding to these resonances. The partial widths of the isobaric analogue states for decay to the ground state and to the 2⁺ excited state in ⁷⁴Ge were determined as well as the l-values of the captured protons and the spins of the resonances. The Coulomb displacement energy for the pair ⁷⁵As-⁷⁵Ge was found to be 10.01 MeV. A comparison is made with the parent analogue states in ⁷⁵Ge. The results indicate that several levels of ⁷⁵Ge may be described as having a large component of a single particle state coupled to the first 2⁺ excited state in ⁷⁴Ge, and that the 2⁺ state has a configuration with a large 2p-2h component.     

Cross sections and isomeric ratios in the photoproduction of 44Sc from heavier nuclei

The yields and isomeric yield ratios in the photoproduction of⁴⁴Sc from ⁴⁵Sc, ⁵⁵Mn, Fe, ⁵⁹Co and ⁷⁵As have been measured by activation methods in the energy region 100–800 MeV. Mean cross sections and isomeric cross-section ratios in the energy region 250–800 MeV have been deduced and the number of valence neutrons available for the (γ, n) reaction estimated. The experimental cross sections are compared both to cascade-evaporation calculations based upon the free-nucleon photopion cross sections and to cross sections calculated with a semi-empirical formula. The isomeric cross-section ratios are compared to calculations based on the statistical formalism by Huizenga and Vandenbosch together with cascade-evaporation theory.     

Emission of deuterons in the interactions of 9 GeV protons with lead nuclei

The emission of deuterons with energies ranging from 17 to 104 MeV in interactions of 9 GeV protons with lead nuclei has been studied using nuclear emulsion. These deuterons constitute about $\text{1}\text{6}$ of the secondary heavily ionising particles (β < 0.7). Observed features of the emission of the deuterons with energies above 28 MeV can be interpreted in terms of the mechanism proposed by Butler and Pearson.     

Incoherent pion photoproduction on the deuteron including polarization effects

Incoherent pion photoproduction on the deuteron including polarization effects is studied in the energy region from π-threshold up to the Δ(1232)-resonance with inclusion of all leading πNN effects. For the elementary pion photoproduction operator, a realistic effective Lagrangian approach is used which displays chiral symmetry, gauge invariance, and crossing symmetry, as well as a consistent treatment of the spin-3/2 interaction. The interactions in the final two-body subsystems are taken in separable form. Effects of final state interaction are investigated and their role in unpolarized and polarization observables are found to be significant. The extracted cross sections and spin asymmetries are compared with available experimental data and predictions of other works, and a satisfactory agreement is obtained. In addition, the sensitivity of results to the elementary N(γ, π)N operator is investigated. Considerable dependence of the d(γ, π)NN results on the elementary amplitude is found. This indicates that it can serve as a filter for different elementary operators.     

Baryon resonances and polarization transfer in hyperon photoproduction

A partial-wave analysis of data on photoproduction of hyperons including single and double polarization observables is presented. The large spin transfer probability reported by the CLAS Collaboration can be successfully described within an isobar partial-wave analysis.     

E1 isobaric analogue resonances of strongly deformed nuclei in (e,e′p) reactions

The cross sections of the (e, e′p) reaction on the deformed nuclei ¹⁵⁹Tb, ¹⁶⁵Ho, ¹⁶⁹Tm, ¹⁷⁵Lu and ¹⁸¹Ta have been measured in the vicinity of the ground isobaric analogue state. Resonances corresponding to the E1 isobaric analogue states are found and displacement energies are calculated from these resonances. These energies are smaller than those obtained from proton scattering, which shows their dependence on the reaction type. The radiative widths of the E1 transitions of the isobaric analogue resonances were obtained. The ratio of these to the single-particle width calculated with the Nilsson model is order of 0.1–1 for non-spin-flip transitions and more than 10² for spin-flip transitions. These results are similar to those of the spherical nuclei ¹³⁹La, ¹⁴¹Pr, ²⁰⁷Pb and ²⁰⁹Bi.     

Giant resonances in f-p shell nuclei studied by inelastic scattering of 80 MeV 3He ions

Spectra of ⁴⁸Ti, ⁵⁶Fe, ⁵⁹Co and ⁶⁰Ni have been investigated for excitation energies of 10–20 MeV by inelastic scattering of 80 MeV ³He particles. Broad peaks with excitation energies of about $\text{63±1 A}{}^{\mathrm{<mtext>?</mtext><mtext>1</mtext><mtext>3</mtext>}}\text{MeV}$ and FWHM of 6 MeV have been observed. Much narrower satellite peaks occur at excitation energies about $\text{51±1 A}{}^{\mathrm{<mtext>?</mtext><mtext>1</mtext><mtext>3</mtext>}}\text{MeV}$. The angular distributions of the sum of these peaks were compared to extended optical model DWBA calculations which confirm the predominant L = 2 character of the GR. A tentative assignment of L = 2 or 4 was made for the 13.5 MeV satellite peak in ⁵⁶Fe. We see no definite evidence for the presence of an L = 1 GR component.