Showing papers in "European Physical Journal A in 2004"

The Binary Cascade

Abstract: The Binary Cascade introduces a novel approach towards intra-nuclear cascade calculations. Like many QMD codes, it uses a detailed 3-dimensional model of the nucleus, and is based exclusively on binary scattering between reaction participants and nucleons within this nuclear model. Like a classical cascade, it uses optical potentials to describe the time evolution of particles passing through the nuclear medium. In the present paper we introduce the model, and investigate its predictive power for hadron spectra in nucleon nuclear reactions final states.

Measurement of gamma p ---> K+ Lambda and gamma p ---> K+ Sigma0 at photon energies up to 2.6-GeV

Abstract: The reactions $\gamma p \rightarrow K^ + \Lambda$ and $\gamma p \rightarrow K^ + \Sigma^0$ were measured in the energy range from threshold up to a photon energy of 26 GeV The data were taken with the SAPHIR detector at the electron stretcher facility, ELSA Results on cross-sections and hyperon polarizations are presented as a function of the kaon production angle and the photon energy The total cross-section for $\Lambda$ production rises steeply with energy close to threshold, whereas the $\Sigma^0$ cross-section rises slowly to a maximum at about $E_\gamma = 145$ GeV Cross-sections together with their angular decompositions into Legendre polynomials suggest contributions from resonance production for both reactions In general, the induced polarization of $\Lambda$ has negative values in the kaon forward direction and positive values in the backward direction The magnitude varies with energy The polarization of $\Sigma^0$ follows a similar angular and energy dependence as that of $\Lambda$ , but with opposite sign

Enhanced electron screening in d(d,p)t for deuterated metals

Abstract: The electron screening effect in the d(d,p)t reaction has been studied for deuterated metals, insulators, and semiconductors, i.e. 58 samples in total. As compared to measurements performed with a gaseous D2 target, a large effect has been observed in most metals, while a small (gaseous) effect is found, e.g., for the insulators, semiconductors, and lanthanides. The periodic table provides the ordering of the observed small and large effects in the samples. An explanation of the large effects in metals is possibly provided by the classical plasma screening of Debye applied to the quasi-free metallic electrons. The data also provide information on the solubility of hydrogen in the samples.

Improving the convergence of the chiral expansion for nuclear forces - I: Peripheral phases

Abstract: We study the two-pion exchange potential at next-to-next-to-leading order in chiral effective field theory. We propose a new cut-off scheme for the pion loop integrals based on spectral-function regularization. We show that this method allows for a consistent implementation of constraints from pion-nucleon scattering. It leads to a much improved description of the partial waves with angular momentum $l \geq 2$ as compared to the calculation utilizing dimensional regularization.

Improving the convergence of the chiral expansion for nuclear forces - II: Low phases and the deuteron

Abstract: Recently, we have proposed a new cut-off scheme for pion loop integrals in the two-pion exchange potential. This method allows for a consistent implementation of constraints from pion-nucleon scattering and has been successfully applied to peripheral nucleon-nucleon partial waves. We now consider low partial waves in the non-perturbative regime, where the regularized Lippmann-Schwinger equation has to be solved in order to generate the bound and scattering states. We observe an improved description of most of the phase shifts when going from next-to- to next-to-next-to-leading order in the chiral expansion. We also find a good description of the deuteron properties. In addition, the new cut-off scheme allows to avoid the presence of unphysical deeply bound states. We discuss the cut-off dependence of the four-nucleon low-energy constants and show that their numerical values can be understood in terms of resonance saturation. This connects the effective field theory approach to boson exchange phenomenology.

Production and decay of the isotope 271 Ds (Z = 110)

Abstract: Production and decay of the isotope 271Ds were studied. The isotope was produced by 208Pb + 64Ni $\rightarrow$ 271Ds + n reaction. Fourteen $\alpha$ -decay chains have been assigned to decays originating from the isotope 271Ds. An excitation function of the production of this isotope was measured. The results have provided a good confirmation of production and decay of 271Ds reported by Hofmann et al. The presence of an isomeric state in 271Ds has been confirmed. The existence of a possible isomeric state in 267Hs is presented.

Electroproduction of nucleon resonances

Abstract: The unitary isobar model MAID has been extended and used for a partial wave analysis of pion photo- and electroproduction in the resonance region W < 2 GeV. Older data from the world data base and more recent experimental results from Mainz, Bates, Bonn and JLab for Q 2 up to 4.0 (GeV/c)2 have been analyzed and the Q 2 dependence of the helicity amplitudes have been extracted for a series of four star resonances. We compare single-Q 2 analyses with a superglobal fit in a new parametrization of Maid2003 together with predictions of the hypercentral constituent quark model. As a result we find that the helicity amplitudes and transition form factors of constituent quark models should be compared with the analysis of bare resonances, where the pion cloud contributions have been subtracted.

Tetraquarks in a chiral constituent-quark model

Abstract: We analyze the possibility of heavy-light tetraquark bound states by means of a chiral constituent-quark model. The study is done in a variational approach. Special attention is paid to the contribution given by the different terms of the interacting potential and also to the role played by the different color channels. We find a stable state for both $qq\bar{c}\bar{c}$ and $qq\bar{b}\bar{b}$ configurations. Possible decay modes of these structures are analyzed.

Generalized parton distributions of the pion in a Bethe-Salpeter approach

Abstract: We calculate generalized parton distribution functions in a field theoretic formalism using a covariant Bethe-Salpeter approach for the determination of the bound-state wave function. We describe the procedure in an exact calculation in scalar Electrodynamics proving that the relevant corrections outside our scheme vanish. We extend the formalism to the Nambu-Jona-Lasinio model, a realistic theory of the pion. We go in both cases beyond all previous calculations and discover that all important features required by general physical considerations, like symmetry properties, sum rules and the polynomiality condition, are explicitly verified. We perform a numerical study of their behavior in the weak- and strong-coupling limits.

Signatures of chiral dynamics in low-energy Compton scattering off the nucleon

Abstract: We present a projection formalism which allows us to define dynamical polarizabilities of the nucleon from a multipole expansion of the nucleon Compton amplitudes. We give predictions for the energy dependence of these dynamical polarizabilities both from Dispersion Theory and from leading-one-loop Chiral Effective Field Theory. Based on the good agreement between the two theoretical frameworks, we conclude that the energy dependence of the dynamical polarizabilities is dominated by chiral dynamics, except in those multipole channels where the first nucleon resonance $\Delta$ (1232) can be excited. Both the Dispersion Theory framework and a Chiral Effective Field Theory with explicit $\Delta$ (1232) degrees of freedom lead to a very good description of the available low-energy proton Compton data. We discuss the sensitivity of the proton Compton cross-section to dynamical polarizabilities of different multipole content and present a fit of the static electric and magnetic dipole polarizabilities from low-energy Compton data up to $\omega \sim 170\;\mathrm{MeV}$ , finding $\bar{\alpha}_{E} = (11.04\pm1.36)\cdot 10^{-4}\;\mathrm{fm}^3,\; \bar{\beta}_{M} = (2.76\mp1.36)\cdot 10^{-4}\;\mathrm{fm}^3$ .

Fission barriers of heavy nuclei within a microscopic approach

Abstract: The fission barriers of twenty-six isotopes of Thorium, Uranium, Plutonium, Californium, Fermium and Nobelium have been microscopically calculated up to and beyond the second saddle point within a constrained Hartree-Fock plus pairing approach. The Skyrme density-dependent effective force in its SkM* parametrization --rather well suited to the description of fission barriers-- has been used in the particle-hole channel, whereas the usual HF plus BCS formalism with either a seniority force or a delta force has been implemented to treat pairing correlations. The energy correction due to the rotational zero-point motion has been approximately taken into account and the effects of triaxial and reflection asymmetric deformations have been investigated. When known, the experimental fission barrier heights are reproduced within about 1-2MeV.

New isotope 265 Bh

Abstract: A new isotope (265)Bh was produced and identified at the Sector Focus Cyclotron of the Heavy Ion Research Facility in Lanzhou. This experiment was performed via the reaction of an Am-243 target with 168 MeV Mg-26 ions. Identification was made by observation of correlated alpha-particle decays between the new isotope (265)Bh and its (261)Db and Lr-257 daughter nuclei rising a set of rotating-wheels system. A total of 8 correlated decay events of (265)Bh and 4 decay events Of (264)Bh were observed. (265)Bh decays with a 0.94(-0.31)(+0.70) s half-life by emission of a-particles with an average energy of 9.24 +/- 0.05 MeV. The half-life and a decay energy of (265)Bh from this experiment are in agreement with theoretical predictions.

Many-body effects in nuclear structure

Abstract: We calculate, for the first time, the state-dependent pairing gap of a finite nucleus (120Sn) diagonalizing the bare nucleon-nucleon potential (Argonne v 14) in a Hartree-Fock basis (with effective k-mass \(m_k \approx 0.7 m\)), within the framework of the generalized Bogoliubov-Valatin approximation including scattering states up to 800 MeV above the Fermi energy to achieve convergence. The resulting gap accounts for about half of the experimental gap. The combined effect of the bare nucleon-nucleon potential and of the induced pairing interaction arising from the exchange of low-lying surface vibrations between nucleons moving in time-reversal states close to the Fermi energy accounts for the experimental gap.

Search for cluster structure of excited states in 14 C

Abstract: We have studied three different 2n-transfer reactions on a 12C target, the 2p pick-up reaction on 16O and the 5He transfer in the reaction 9Be(7Li,d)14C. Combined with a systematic search through experimental results for transfer reactions, inelastic excitations and other data, we have established an almost complete spectroscopy for 14C up to 18 MeV excitation. We identify states with single-particle structure that have oblate shapes and states corresponding to proton excitations that are connected to oblate (triangular) cluster states. Further we list states of prolate shape which have no simple structure related to the low-lying oblate states of 12C. These are proposed to have strong \(\alpha\)-clustering and to form rotational bands as a parity inversion doublet, with high moment of inertia. With these results it is possible for the first time to identify chain states expected in the isotope 14C.

Deep electroproduction of photons and mesons on the deuteron

Abstract: We study deeply virtual Compton scattering and deep exclusive meson electroproduction on a deuteron target. We model the generalized quark distributions in the deuteron by using the impulse approximation for the lowest Fock-space state on the light cone. We study the properties of the resulting GPDs, and verify that sum rules violations are quite small in the impulse approximation. Numerical predictions are given for the unpolarized cross-sections and polarization asymmetries for the kinematical regimes relevant for JLab experiments and for HERMES at HERA. We conclude that the signal of coherent scattering on the deuteron is comparable to the one on the proton at least for low momentum transfer, providing support to the feasibility of the experiments. The short-distance structure of the deuteron may thus be scrutinized in the near future.

Non-extensive resonant reaction rates in astrophysical plasmas

Abstract: We study two different physical scenarios of thermonuclear reactions in stellar plasmas proceeding through a narrow resonance at low energy or through the low-energy wing of a wide resonance at high energy. Correspondingly, we derive two approximate analytical formulae in order to calculate thermonuclear resonant reaction rates inside very coupled and non-ideal astrophysical plasmas in which non-extensive effects are likely to arise. Our results are presented as simple first-order corrective factors that generalize the well-known classical rates obtained in the framework of Maxwell-Boltzmann statistical mechanics. As a possible application of our results, we calculate the dependence of the total corrective factor with respect to the energy at which the resonance is located, in an extremely dense and non-ideal carbon plasma.

A precision measurement of pp elastic scattering cross-sections at intermediate energies

Abstract: We have measured differential cross sections for pp elastic scattering with internal fiber targets in the recirculating beam of the proton synchrotron COSY. Measurements were made continuously during acceleration for projectile kinetic energies between 0.23 and 2.59 GeV in the angular range 30 ◦ ≤ �c.m. ≤ 90 ◦ . Details of the apparatus and the data analysis are given and the resulting excitation functions and angular distributions presented. The precision of each data point is typically better than 4%, and a relative normalization uncertainty of only 2.5% within an excitation function has been reached. The impact on phase shift analysis as well as upper bounds on possible resonant contributions in lower partial waves are discussed. PACS. 25.40.Cm Elastic proton scattering - 13.75.Cs Nucleon-nucleon interactions - 13.85.Dz Elastic scattering - 21.30.-x Nuclear forces

Photoproduction of πmesons from nuclei

Abstract: Photoproduction of neutral pions from nuclei (carbon, calcium, niobium, lead) has been studied for incident-photon energies from 200 MeV to 800 MeV with the TAPS detector using the Glasgow photon tagging spectrometer at the Mainz MAMI accelerator. Data were obtained for the inclusive photoproduction of neutral pions and the, partial channels of quasifree single-pi(0), double-pi(0), and pi(0)pi(+/-) photoproduction. They have been analyzed in terms of the in-medium behavior of nucleon resonances and the pion-nucleus interaction. They are compared to earlier measurements from the deuteron and to the predictions of a Boltzmann-Uehling-Uhlenbeck (BUU) transport model for photon-induced pion production from nuclei.

Updated dispersion-theoretical analysis of the nucleon electromagnetic form factors

Abstract: In the light of the new data on the various neutron and proton electromagnetic form factors taken in recent years, we update the dispersion-theoretical analysis of the nucleon electromagnetic form factors from the mid-nineties. The parametrization of the spectral functions includes constraints from unitarity, perturbative QCD, and recent measurements of the neutron charge radius. We obtain a good description of most modern form factor data, with the exception of the Jefferson Lab data on G E p /G M p in the four-momentum transfer range $Q^2 = 3 \ldots 6$ GeV2. For the magnetic radii of the proton and the neutron we find r M p = 0.857 fm and r M n = 0.879 fm, which is consistent with the recent determinations using continued-fraction expansions.

Model-independent properties of two-photon exchange in elastic electron-proton scattering

Abstract: We derive from general symmetry properties of the hadron electromagnetic interaction, such as C-invariance and crossing symmetry, the general characteristics of two-photon exchange in electron-proton elastic scattering. We show that the presence of this mechanism destroys the linearity of the Rosenbluth separation.

Alpha-gamma decay studies of 251, 253No and their daughter products 247, 249Fm

Abstract: The isotopes 251No and 253No were produced in nuclear reactions 206Pb( 48Ca,3n) 251No and 207Pb( 48Ca,2n) 253No. Radioactive decay of these isotopes and their daughter products has been investigated by means of α- and α-γ- spectroscopy. An isomeric state 251mNo, having a half-life of T1/2 = (0.93±0.06) s and decaying by emission of α particles of Eα = (8665±8) keV was identified. The measured decay data allowed for the construction of partial level schemes for 251No, 247, 249Fm and 243, 245Cf and an extrapolation of energy systematics of single particle levels of N = 145, 147 and 149 isotones with even Z towards higher atomic numbers up to Z = 102.

Formation of heavy and superheavy elements by reactions with massive nuclei

Abstract: The effects of the entrance channel and shell structure on the experimental evaporation residues have been studied by analyzing the 32S + 182W, 48Ti + 166Er and 60Ni + 154Sm reactions leading to 214Th*; the 40Ar + 181Ta reaction leading to 221Pa*; the 48Ca + 243Am, 248Cm, 249Cf reactions leading to the 291115, 296116 and 297118 superheavy compound nuclei, respectively. The fusion mechanism and the formation of evaporation residues of heavy and superheavy nuclei have been studied. In calculations of the excitation functions for capture, fusion and evaporation residues we used such characteristics as mass asymmetry of nuclei in the entrance channel, binding energies and shape of colliding nuclei, potential energy surface, driving potential, partial-fusion cross-sections and survival probability of the compound nucleus, $\Gamma_{\rm n} / \Gamma_{\rm f}$ ratio at each step along the de-excitation cascade of the compound nucleus. The calculations have allowed us to make useful conclusions about the mechanism of the fusion-fission process, which is in competition with the quasifission process, and the production of the evaporation residues.

Helicity dependence of the gamma p -> N pi channels and multipole analysis in the Delta region

Abstract: A high-quality double-polarization data set for the helicity dependence of the total and differential cross-sections for both gammap --> Npi channels in the Delta region has been obtained in the framework of the GDH experiment. The experiment, performed at the Mainz microtron MAMI, used a 4pi detection system, a circularly polarized photon beam, and a longitudinally polarized frozen-spin target. These data are included in the database to perform a multipole analysis to determine the properties of the Delta(1232)-resonance. For the resonant Delta(1232) multipoles we find a very good agreement with previous analyses, while the nonresonant ones show significant deviations.

Energy dependence of the Λ/Σproduction cross-section ratio in p-p interactions

Abstract: The production of the Λ- and Σ0-hyperons has been measured via the pp → pK+Λ/Σ0 reaction at the internal COSY-11 facility in the excess energy range between 14 and 60 MeV The transition of the Λ/Σ0 cross-section ratio from about 28 at Q≤13 MeV to the high-energy level of about 25 is covered by the data showing a strong decrease of the ratio between 10 and 20 MeV excess energy Effects from the final-state interactions in the p-Σ0 channel seem to be much smaller than in the p-Λ channel Estimates of the effective range parameters are given for the NΛ and the NΣ systems

Status of the BLAST experiment

Abstract: The BLAST experiment is beginning operation at the MIT-Bates Linear Accelerator Laboratory. The experiment will study the spin dependent electro-magnetic interaction in few nucleon systems at momentum transfers between 0.1 and 1.0 GeV2. This will provide improved measurements of the nucleon form factors, particularly G n E, as well as study the structure of D and 3He. Other reaction channels such as pion production and inclusive scattering will also be studied. The experiment, physics goals, and current status are described briefly.

Effects of phonon-phonon coupling on low-lying states in neutron-rich Sn isotopes

Abstract: Starting from an effective Skyrme interaction we present a method to take into account the coupling between one- and two-phonon terms in the wave functions of excited states. The approach is a development of a finite rank separable approximation for the quasiparticle RPA calculations proposed in our previous work. The influence of the phonon-phonon coupling on energies and transition probabilities for the low-lying quadrupole and octupole states in the neutron-rich Sn isotopes is studied.

Possible explanation of fine structures in mass-energy distribution of fission fragments

Abstract: Within an improved scission point model, experimental data on the relative yield, mean value and variance of the total kinetic-energy distribution of fission fragments are described. It is shown that for a fixed mass and charge fragmentation, the potential energy of the scission configuration has several minima as a function of the deformation parameters of the fragments. The scission at these minima leads to a relatively enhanced yield of the fragments with a certain TKE and creates fine structures in the TKE-mass distribution which are different from those produced by the odd-even effect.

Beta-decay properties of 25Si and 26P

Abstract: The $\beta$ -decay properties of the neutron-deficient nuclei 25Si and 26P have been investigated at the GANIL/LISE3 facility by means of charged-particle and $\gamma$ -ray spectroscopy. The decay schemes obtained and the Gamow-Teller strength distributions are compared to shell-model calculations based on the USD interaction. B(GT) values derived from the absolute measurement of the $\beta$ -decay branching ratios give rise to a quenching factor of the Gamow-Teller strength of 0.6. A precise half-life of $43.7\: (6)$ ms was determined for 26P, the $\beta$ -(2)p decay mode of which is described.

Electromagnetic form factors of the baryon octet in the perturbative chiral quark model

Abstract: We apply the perturbative chiral quark model at one loop to analyze the electromagnetic form factors of the baryon octet. The analytic expressions for baryon form factors, which are given in terms of fundamental parameters of low-energy pion-nucleon physics (weak pion decay constant, axial nucleon coupling, strong pion-nucleon form factor), and the numerical results for baryon magnetic moments, charge and magnetic radii are presented. Our results are in good agreement with experimental data.

Inclusive $\omega$ photoproduction off nuclei

Abstract: We investigate inclusive $\omega$ photoproduction off complex nuclei, concentrating on the feasibility to examine a possible in-medium change of the $\omega$ 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 situa tion in free space. Also the effects of a momentum dependence of the strong $\omega$ potential are discussed.