Full Coupled-channel Research Articles

LOW ENERGY DOUBLE-PION PHOTOPRODUCTION IN NUCLEI

We have investigated π0π0 and π±π0 photoproduction off complex nuclei. Simulations have been performed by means of a semi-classical BUU transport model including a full coupled-channel treatment of the final state interactions. We show that the target mass dependence of the π0π0 invariant mass spectrum as measured by the TAPS collaboration can be explained without introducing medium effects beyond absorption and quasi-elastic scattering of the final state particles. On the other hand, we find considerable discrepancies with the data in the π±π0 channel, which are not understood.

Higher order effects in theO16(d,p)O17andO16(d,n)F17transfer reactions

Full Coupled Channels Calculations were performed for the $^{16}O(d,n)^{17}F$ and $^{16}O(d,p)^{17}O$ transfer reactions at several deuteron incident energies from $E_{lab}=2.29$ MeV up to 3.27 MeV. A strong polarization effect between the entrance channel and the transfer channels $^{16}O(d,n)^{17}F(1/2^{+},0.495)$ and $^{16}O(d,p)^{17}O(1/2^{+},0.87)$ was observed. This polarization effect had to be taken into account in order to obtain realistic spectroscopic factors from these reactions.

Low-energy ππ photoproduction off nuclei

In the present Letter we investigate π0π0 and π±π0 photoproduction off complex nuclei at incident beam energies of 400–460 MeV. Simulations of two pion photoproduction on protons and nuclei are performed by means of a semi-classical BUU transport model including a full coupled-channel treatment of the final state interactions. Elastic scattering of the final state pions with the nucleons in the surrounding nuclear medium is found to yield a downward shift of the ππ invariant mass distribution. We show that the target mass dependence of the π0π0 invariant mass spectrum as measured by the TAPS Collaboration can be explained without introducing medium effects beyond absorption and quasi-elastic scattering of the final state particles. On the other hand, we find considerable discrepancies with the data in the π±π0 channel, which are not understood.

Hadron formation and attenuation in deep inelastic lepton scattering off nuclei

We investigate hadron formation in deep inelastic lepton scattering on N, Kr and Xe nuclei in the kinematic regime of the HERMES experiment. The elementary electron–nucleon interaction is described within the event generator PYTHIA while a full coupled-channel treatment of the final state interactions is included by means of a BUU transport model. We find a good agreement with the measured charged hadron multiplicity ratio RMh for N and Kr targets by accounting for the deceleration and absorption of the primarily produced particles as well as for the creation of secondary hadrons in the final state interactions.

Impacts of the Cutoff of Coupled-Channels Effects on Heavy-Ion Fusion Reactions at Extreme Sub-Barrier Energies

Based on the one dimensional barrier penetration model with semiclassical WKB method, the dynamic polarization potentials in heavy-ion reactions are extracted from the fusion cross sections. By choosing the proper parameters, the real polarization potential can be calculated from the imaginary part according to the dispersion relation in the framework of the linear-schematic model. The imaginary potential comes from the coupled-channels effects and shows a rapid cuttoff at energies far below the Coulomb barrier. Due to the absence of the coupled-channels effects, the fusion excitation function fall off more quickly than the usual expectation. Moreover, the pure potential in fusion reactions is proposed in hopes to reproduce the fusion cross sections at all energies in the full coupled-channels calculation.

Inclusive $\omega$ photoproduction off nuclei

We investigate inclusive omega photoproduction off complex nuclei, concentrating on the feasibility to examine a possible in-medium change of the omega meson properties by observing the pi^0 gamma invariant mass spectrum. The simulations are performed by means of a BUU transport model including a full coupled-channel treatment of the final state interactions. In-medium changes of the omega spectral density are found to yield a moderate modification of the observables as compared to the situation in free space. Also the effects of a momentum dependence of the strong omega potential are discussed.

The unitary convolution approximation for heavy ions

The convolution approximation for the impact-parameter dependent energy loss is reviewed with emphasis on the determination of the stopping force for heavy projectiles. In this method, the energy loss in different impact-parameter regions is well determined and interpolated smoothly. The physical inputs of the model are the projectile-screening function (in the case of dressed ions), the electron density and oscillators strengths of the target atoms. Moreover, the convolution approximation, in the perturbative mode (called PCA), yields remarkable agreement with full semi-classical-approximation (SCA) results for bare as well as for screened ions at all impact parameters. In the unitary mode (called UCA), the method contains some higher-order effects (yielding in some cases rather good agreement with full coupled-channel calculations) and approaches the classical regime similar as the Bohr model for large perturbations ( Z/ v≫1). The results are then used to compare with experimental values of the non-equilibrium stopping force as a function of the projectile charge as well as with the equilibrium energy loss under non-aligned and channeling conditions.

Collisions of dressed ground-state atoms.

We study collisions between dressed ground-state atoms in a full coupled-channel approach. In this way we examine collisions in the microwave trap, in the cryogenic hydrogen maser, and in the context of rf-induced evaporative cooling. The results for dressed H atoms confirm earlier results obtained within the so-called degenerate internal states approximation. {copyright} {ital 1996 The American Physical Society.}

A meson-exchange model for the antihyperon-hyperon production

A meson-exchange model for strangeness production processes p p→ YY ( Y = Λ, Σ) is presented, which is based on a full coupled-channel treatment. The transition interactions are given by K- and K ∗-meson exchange. The elastic part of the p p and Y Y interactions has been derived from a one-boson-exchange version of the Bonn NN potential and a corresponding extension to the hyperon-nucleon case whereas the annihilation part is taken into account by introducing phenomenological optical potentials. Results for cross sections, polarizations, depolarization and spin correlation parameters are presented for the reactions p p → ΛΣ 0, Σ 0Λ as well as p → Σ ±Σ ± , Σ 0Σ 0 . As in p →- Λ , tensor and spin-triplet d p p → ΣΣ but is somewhat reduced in p → ΛΣ 0, Σ 0Λ . Although Σ −Σ − cannot be reached from p p by a one step transition the corresponding cross section is only slightly smaller than that for \\ ̄ gS +Σ + . For pp → ΛΣ 0, Σ 0Λ, the results are compared to those derived from different transition interactions based on (i) K-exchange only and (ii) the constituent quark model ( 3S 1 version). Predictions are made for the Y Y → Y Y cross sections.

Meson exchange K+N interaction and K+-nucleus cross sections

A meson exchange model for the K+-nucleon interaction is presented, which is based on a full coupled-channel (KN,K∗N,KΔ) treatment including the direct K∗N and KΔ interactions based likewise on meson exchange. This microscopic model, which provides a good description of K+N elastic and pion production data, is then used to calculate K+−2d and K+−12C cross sections. Characteristics discrepancies to the empirical data are found, which are similar to those obtained when starting from the empirical K+N amplitude. Rough agreement results if the masses of the vector mesons ω,ϱ in the K+N interaction are reduced by 4% in 12C, which is considerably smaller than suggested by recent estimates. Possible reasons are discussed.

Reaction p-barp--> Lambda -bar Lambda in the meson-exchange picture.

A study of the strangeness production process {ital {bar p}p}{r arrow}{bar {Lambda}}{Lambda} at threshold and intermediate energies has been performed doing a full coupled-channel ({ital {bar p}p},{bar {Lambda}}{Lambda}) calculation. The elastic part of the {ital {bar p}p} and {bar {Lambda}}{Lambda} interactions has been derived from a one-boson-exchange version of the Bonn {ital NN} potential and a corresponding extension to the hyperon-nucleon case whereas the annihilation part is taken into account by introducing suitable optical potentials. The transition interaction between {ital {bar p}p} and {bar {Lambda}}{Lambda} is based on {ital K}- and {ital K}{sup *}-meson exchange. A reasonably good description of empirical cross sections as well as polarization data is achieved. It is demonstrated that the results are quite sensitive even to short-range modifications of the channel interactions. We also compare our model with other approaches and suggest a new experiment which would be able to check the very different predictions of the meson-exchange and constituent quark model, respectively.

The ground state of nuclear matter with virtual Δ-isobar excitations

The equation of state of nuclear matter is studied in the framework of the Bethe-Brueckner-Goldstone theory using the local Argonne v 28 potential which explicitly includes Δ-isobar degrees of freedom. The self-consistent two-body reaction matrix is obtained with all possible allowed couplings to the NΔ and ΔΔ states. Saturation occurs around k F =1.2 fm −1 with a binding energy per particle B A =−10.31 MeV , confirming the effect of the isobar in pushing the saturation point to the upper side of the Coester band. The present results are very close to the ones obtained in full coupled-channel relativistic calculations.

Theory of transfer reactions in peripheral heavy-ion collisions.

The total absorption from the elastic channel due to transfer and inelastic processes in peripheral heavy-ion collisions at low bombarding energies is calculated in a microscopic coupled-channel approach. It is demonstrated for the first time that considering the depopulation of the entrance channel as an incoherent depopulation due to transfer processes is a good approximation. Using the corresponding absorptive potential within the framework of the Born approximation to calculate the transfer to individual channels, the results of full coupled-channels calculations are accurately reproduced.

Coupled-channels study of antiproton-proton reactions.

Low energy antiproton-proton elastic, charge exchange, charged pion, and charged kaon production reactions are studied in a coupled-channels model. The need for a coupled-channels approach to antiproton-proton reactions is first demonstrated. Meson exchange potentials describe the long-range N\ifmmode\bar\else\textasciimacron\fi{}N interaction; smooth phenomenological forms are used for the N\ifmmode\bar\else\textasciimacron\fi{}N interaction at short distances. Effective channels are introduced to simulate strong absorptive effects, such as multimeson production processes. Transition potentials for kaon and pion production are derived in coordinate space and shown to include an important spin-times-derivative coupling form. These transition potentials, along with a one-boson-exchange model with effective absorptive channels, are used to determine the role of multistep processes of the type \ensuremath{\pi}\ifmmode\bar\else\textasciimacron\fi{}\ensuremath{\pi}\ensuremath{\leftrightarrows}p\ifmmode\bar\else\textasciimacron\fi{}p\ensuremath{\leftrightarrows}n\ifmmode\bar\else\textasciimacron\fi{}n\ensuremath{\leftrightarrows}K\ifmmode\bar\else\textasciimacron\fi{}K. Control over such coupled-channels effects is sought as a step toward unfolding short-distance quark effects in a reliable way. The p\ifmmode\bar\else\textasciimacron\fi{}p elastic and charge exchange differential cross sections at three p\ifmmode\bar\else\textasciimacron\fi{}p laboratory momenta (780, 690, and 590 MeV/c) are examined first, since ${\ensuremath{\pi}}^{\mathrm{\ensuremath{-}}}$${\ensuremath{\pi}}^{+}$ and ${K}^{\mathrm{\ensuremath{-}}}$${K}^{+}$ data also exist at these energies. Results are then presented that manifest strong coupling effects, especially between the pion and kaon production channels. Based on that result, a full coupled-channels fit to the p\ifmmode\bar\else\textasciimacron\fi{}p, n\ifmmode\bar\else\textasciimacron\fi{}n, ${K}^{\mathrm{\ensuremath{-}}}$${K}^{+}$, and ${\ensuremath{\pi}}^{\mathrm{\ensuremath{-}}}$${\ensuremath{\pi}}^{+}$ data is presented and analyzed.

Direct versus sequential four-particle transfer in heavy ion collisions with superfluid nuclei: Sn+Sn reaction.

The direct transfer of four nucleons in reactions with superfluid systems is compared with the feeding of the same final channel by a successive transfer of correlated nucleon pairs. A quantitative analysis is carried out for the reaction $^{120}$Sn${+}^{112}$Sn which shows the overwhelming dominance of the direct multiparticle transfer at bombarding energies below the Coulomb barrier. Results of simple estimates---supplemented by full coupled-channel calculations---provide orientation for selecting the experimental conditions which best exhibit the specific characteristics of the superconductive phase. The possibility of detecting an interference pattern between the two processes is also discussed.

Cross sections for the Ti(p,n) reaction to analogs of ground and 2+ excited states.

Cross sections for the (p,n) reaction to analog states have been measured for the five stable isotopes of titanium at energies between 11.5 and 26 MeV. In addition to the ground-state analogs, cross sections to the 2/sub 1//sup +/ analog states for the even A isotopes have been determined. A two-channel analysis of the ground-state analog data, searching on the charge-exchange parameters, did not yield a satisfactory globally consistent description of the 0/sub 1//sup +/ analog cross sections. However, in a full coupled-channel description of the 0/sub 1//sup +/ analog angular distributions the N-Z dependence and the strength of the 2/sub 1//sup +/ analog cross sections for /sup 46,48,50/Ti are explained by strong inelastic couplings and by rather different charge-exchange couplings among the three isotopes. The isovector deformation parameters used in the charge-exchange couplings were determined from electromagnetic and (p,p') values, which were obtained from experiment. The experimental energy dependence of the cross section shows a resonance behavior which is not explained by the channel coupling but which can be adequately accounted for by an energy dependent V/sub 1/.

Peripheral inelasticities in T=1 nucleon-nucleon collisions

Improvements of the earlier coupled-channels Delta -isobar model for T=1 NN scattering inelasticities are discussed. These include the comparison for a full coupled-channels calculation with the Born approximate treatment at high partial waves, the inclusion of one-pion-exchange tensor coupling between 3F4 and 3H4 states and, most important of all, the taking into account of the spectator nucleon recoil when calculating the Delta width. After these modifications, much better agreement with other calculations is obtained.

Properties of the (α,α*) Reaction at Very Forward Angles: Coupled-Channels Effects in Single and Mutual Excitation

Use was made of a special feature in the detection of unbound ejectiles, to extend previous measurements of the ($\ensuremath{\alpha}$,${\ensuremath{\alpha}}^{*}$) reaction on $^{24}\mathrm{Mg}$ and $^{28}\mathrm{Si}$ to very forward angles. The characteristic differential cross sections obtained for mutual as well as for single excitation are well reproduced in a full coupled-channels calculation in which the strong couplings to the first excited ${2}^{+}$ states of the target nuclei are taken into account.