Showing papers in "European Physical Journal A in 2007"

Unitary Isobar Model - MAID2007

Abstract: The unitary isobar model MAID2007 has been developed to analyze the world data of pion photo- and electroproduction. The model contains both a common background and several resonance terms. The background is unitarized according to the K -matrix prescription, and the 13 four-star resonances with masses below 2GeV are described by appropriately unitarized Breit-Wigner forms. The data have been analyzed by both single-energy and global fits, and the transverse and longitudinal helicity amplitudes have been extracted for the four-star resonances below 2GeV. Because of its inherent simplicity, MAID2007 is well adopted for predictions and analysis of the observables in pion photo- and electroproduction.

The reaction 48Ca + 238U → 286112* studied at the GSI-SHIP

Abstract: The synthesis of element 116 in fusion-evaporation reactions of a 48Ca beam with radioactive 248Cm targets was studied at the velocity filter SHIP of GSI in Darmstadt. At excitation energies of the compound nuclei of 40.9MeV, four decay chains were measured, which were assigned to the isotope 292116, and one chain, which was assigned to 293116. Measured cross-sections of (3.4−1.6+2.7) pb and (0.9−0.7+2.1), respectively, and decay data of the chains agree with data measured previously at the Flerov Laboratory of Nuclear Reactions in Dubna. As a new result, one α-decay chain was measured, which terminates after four α decays by spontaneous fission. The α energies of the second-to-fourth decay are considerably higher than those measured for the α decays of 289114, 285Cn, and 281Ds and the spontaneous fission half-life is significantly longer than that of 277Hs measured in previous experiments. A possible assignment is discussed in the frame of excited quasiparticle states of nuclei populated in the decay chain from 293116. Also other possible assignments were considered and are discussed. At an excitation energy of 45.0MeV no events were observed resulting in a one-event cross-section limit of 1.6 pb. The technical aspects related with the use of radioactive target material at SHIP are described in detail. The experience gained in this experiment will serve as a basis for future experiments aiming to study still heavier elements at the velocity filter SHIP.

Universal properties of the four-body system with large scattering length

Abstract: Few-body systems with large scattering length have universal properties that do not depend on the details of their interactions at short distances. We study the universal bound-state properties of the four-boson system with large scattering length in an effective quantum mechanics approach. We compute the four-body binding energies using the Yakubovsky equations for positive and negative scattering length. Moreover, we study the correlation between three- and four-body energies and present a generalized Efimov plot for the four-body system. These results are useful for understanding the cluster structure of nuclei and for the creation of weakly bound tetramers with cold atoms close to a Feshbach resonance.

Large-scale mean-field calculations from proton to neutron drip lines using the D1S Gogny force

Abstract: Large-scale axial mean-field calculations from proton to neutron drip lines have been performed within the Hartree-Fock-Bogoliubov method based on the D1S Gogny force. Nearly 7000 nuclides have been studied under the axial symmetric hypothesis and various properties are displayed on an Internet web site for every individual nucleus. Some global properties are presented such as the positions of the drip lines, the nuclide ground-state deformations and binding energies as well as regions where possible super- or hyper-deformation might be encountered.

Evidence for a Kπ = 1/2+ isomer in neutron-rich 185Ta

Abstract: Excited states in neutron-rich 185Ta have been populated following the one-proton pickup reaction of 186W(18O,19F). In-beam γ -rays were measured in coincidence with scattered particles detected by a high-resolution ΔE -E Si telescope for reaction channel selection. Several low-lying levels including a T1/2 = 0.9(3) μs isomer at 406 keV have been identified. A spin assignment of I = 3/2 and the 1/2+[411] Nilsson configuration are given to the isomer in comparison with the level energy systematics and the isomer decay rates in the region.

Lattice simulations for light nuclei: Chiral effective field theory at leading order

Abstract: We discuss lattice simulations of light nuclei at leading order in the chiral effective field theory. Using lattice pion fields and auxiliary fields, we include the physics of instantaneous one-pion exchange and the leading-order S-wave contact interactions. We also consider higher-derivative contact interactions which adjust the S-wave scattering amplitude at higher momenta. By construction our lattice path integral is positive definite in the limit of exact Wigner SU(4) symmetry for any even number of nucleons. This SU(4) positivity and the approximate SU(4) symmetry of the low-energy interactions play an important role in suppressing sign and phase oscillations in Monte Carlo simulations. We assess the computational scaling of the lattice algorithm for light nuclei with up to eight nucleons and analyze in detail calculations of the deuteron, triton, and helium-4.

Nuclear forces with Δ excitations up to next-to-next-to-leading order, part I: Peripheral nucleon-nucleon waves

Abstract: We study the two-nucleon force at next-to-next-to-leading order in a chiral effective field theory with explicit Δ degrees of freedom. Fixing the appearing low-energy constants from a next-to-leading-order calculation of pion-nucleon threshold parameters, we find an improved convergence of most peripheral nucleon-nucleon phases compared to the theory with pions and nucleons only. In the delta-full theory, the next-to-leading-order corrections are dominant in most partial waves considered.

Axial Resonances in the Open and Hidden Charm Sectors

Abstract: A SU(4) flavor symmetrical Lagrangian is constructed for the interaction of the pseudo-scalar mesons with the vector mesons. SU(4) symmetry is broken to SU(3) by suppression of terms in the Lagrangian where the interaction should be driven by charmed mesons. Chiral symmetry can be restored by setting this new SU(4) symmetry-breaking parameters to zero. Unitarization in coupled channels leads to the dynamical generation of resonances. Many known axial resonances can be identified including the new controversial X(3872) and the structure found recently by Belle around 3875MeV in the hidden charm sector. Also new resonances are predicted, some of them with exotic quantum numbers.

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

Abstract: 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.

Polarization observable measurements for γp → K + Λ and γp → K + Σfor energies up to 1.5 GeV

Abstract: Beam asymmetries and hyperon recoil polarizations for the reactions γp → K +Λ and γp → K +Σ0 have been measured from the threshold production to 1500MeV with the GRAAL facility located at the ESRF in Grenoble. These results complement the database for the beam asymmetry, covering for the first time the production threshold region. Recent theoretical analyses are presented for which the beam asymmetry data bring interesting new information and allow to better determine some resonance parameters. Most importantly, these results strengthen the need of a new D13 state around 1900MeV.

Measurement of the beam asymmetry in η photoproduction off the proton

Abstract: The beam asymmetry, Σ , was measured at ELSA in the reaction $ \boldsymbol\gamma$ p → ηp using linearly polarised tagged photon beams, produced by coherent bremsstrahlung off a diamond. The crystal was oriented to provide polarised photons in the energy range E γ = 800 to 1400MeV with the maximum polarisation of P γ = 49 % obtained at 1305MeV. Both dominant decay modes of the η into two photons and 3π0 were used to extract the beam asymmetry from the azimuthal modulation of the cross-section. The measurements cover the angular range Θcm ≃ 50 -150 degrees. Large asymmetries up to 80% are observed, in agreement with a previous measurement. The eta-MAID model and the Bonn-Gatchina partial wave analysis describe the measurements, but the required partial waves differ significantly.

Four-nucleon force using the method of unitary transformation

Abstract: We discuss in detail the derivation of the leading four-nucleon force in chiral effective field theory using the method of unitary transformation. The resulting four-nucleon force is given in both momentum and configuration space. It does not contain any unknown parameters and can be used in few- and many-nucleon studies.

Solutions of Dirac equations with the Pöschl-Teller potential

Abstract: By using the basic concepts of the supersymmetric quantum mechanics formalism and the function analysis method, we solve the Dirac equation with vector and scalar potentials and obtain the bound-state solutions for the nuclei in the relativistic Poschl-Teller potential. All of the analyses are prepared under the conditions of the exact spin symmetry and pseudospin symmetry. The exact energy equation and corresponding two-component spinor wave functions for s -wave bound states are obtained analytically.

Excitation functions of fusion reactions and neutron transfer in the interaction of 6He with 197Au and 206Pb

Abstract: Excitation functions for evaporation residues in the reactions 197Au(6He, xn)203-xnTl, x = 2-7, and 206Pb(6He, 2n)210Po, as well as for neutron transfer reactions for the production of 196Au and 198Au in the interaction of 6He with 197Au were measured. The 6He beam was obtained from the accelerator complex for radioactive beams DRIBs (JINR). The maximum energy of the beam was about 10AMeV and the intensity reached 2×107pps. The stacked-foil activation technique was used directly in the beam extracted from the cyclotron or in the focal plane of the magnetic spectrometer MSP-144. The identification of the reaction products was done by their radioactive γ- or α-decay. The fusion reaction with the evaporation of two neutrons was characterized by an increase in the cross-section compared to statistical model calculations. The analysis of the data in the framework of the statistical model for the decay of excited nuclei, which took into account the sequential fusion of 6He has shown good agreement between the experimental and the calculated values of the cross-sections in the case of sub-Coulomb-barrier fusion in the 206Pb + 6He reaction. An unusually large cross-section was observed below the Coulomb barrier for the production of 198Au in the interaction of 6He with 197Au. Possible mechanisms of formation and decay of transfer reaction products are discussed.

Semi-Inclusive Deep Inelastic Scattering processes from small to large P(T)

Abstract: We consider the azimuthal and PT-dependence of hadrons produced in unpolarized Semi-Inclusive Deep Inelastic Scattering (SIDIS) processes, within the factorized QCD parton model. It is shown that at small PT values, P T ≲ 1GeV/c, lowest-order contributions, coupled to unintegrated (transverse-momentum-dependent) quark distribution and fragmentation functions, describe all data. At larger PT values, P T ≳ 1GeV/c, the usual pQCD higher-order collinear contributions dominate. Having explained the full PT range of available data, we give new detailed predictions concerning the azimuthal and PT-dependence of hadrons which could be measured in ongoing or planned experiments by HERMES, COMPASS and JLab Collaborations.

Static properties and semileptonic decays of doubly heavy baryons in a nonrelativistic quark model

Abstract: We evaluate static properties and semileptonic decays for the ground state of doubly heavy Ξ, Ξ', Ξ * and Ω, Ω', Ω * baryons. Working in the framework of a nonrelativistic quark model, we solve the three-body problem by means of a variational ansatz made possible by heavy-quark spin symmetry constraints. To check the dependence of our results on the inter-quark interaction we use five different quark-quark potentials that include a confining term plus Coulomb and hyperfine terms coming from one-gluon exchange. Our results for static properties (masses, charge radii and magnetic moments) are, with a few exceptions for the magnetic moments, in good agreement with a previous Faddeev calculation. Our much simpler wave functions are used to evaluate semileptonic decays of doubly heavy Ξ, Ξ'(J = 1/2) and Ω, Ω'(J = 1/2) baryons. Our results for the decay widths are in good agreement with calculations done within a relativistic quark model in the quark-diquark approximation.

Baryon resonances and polarization transfer in hyperon photoproduction

Abstract: 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.

A gauge-invariant chiral unitary framework for kaon photo-and electroproduction on the proton

Abstract: We present a gauge-invariant approach to photoproduction of mesons on nucleons within a chiral unitary framework. The interaction kernel for meson-baryon scattering is derived from the chiral effective Lagrangian and iterated in a Bethe-Salpeter equation. Within the leading-order approximation to the interaction kernel, data on kaon photoproduction from SAPHIR, CLAS and CBELSA/TAPS are analyzed in the threshold region. The importance of gauge invariance and the precision of various approximations in the interaction kernel utilized in earlier works are discussed.

Dirac-Brueckner-Hartree-Fock calculations for isospin asymmetric nuclear matter based on improved approximation schemes

Abstract: We present Dirac-Brueckner-Hartree-Fock calculations for isospin asymmetric nuclear matter which are based on improved approximations schemes. The potential matrix elements have been adapted for isospin asymmetric nuclear matter in order to account for the proton-neutron mass splitting in a more consistent way. The proton properties are particularly sensitive to this adaption and its consequences, whereas the neutron properties remains almost unaffected in neutron-rich matter. Although at present full Brueckner calculations are still too complex to apply to finite nuclei, these relativistic Brueckner results can be used as a guidance to construct a density-dependent relativistic mean-field theory, which can be applied to finite nuclei. It is found that an accurate reproduction of the Dirac-Brueckner-Hartree-Fock equation of state requires a renormalization of these coupling functions.

Masses of neutron-rich Ni and Cu isotopes and the shell closure at Z = 28 , N = 40

Abstract: The Penning trap mass spectrometer JYFLTRAP, coupled to the Ion Guide Isotope Separator On-Line (IGISOL) facility at Jyvaskyla, was employed to measure the atomic masses of neutron-rich 70-73Ni and 73, 75Cu isotopes with a typical accuracy less than 5keV. The mass of 73Ni was measured for the first time. Comparisons with the previous data are discussed. Two-neutron separation energies show a weak subshell closure at 68 28Ni40 . A well established proton shell gap is observed at Z = 28 .

Photoproduction of η-mesons off protons

Abstract: Photoproduction of η -mesons was studied with the Crystal-Barrel detector at ELSA for photon energies in the range from 0.75 to 3GeV. Total and differential cross-sections are presented. The η mesons are detected in two decay modes, in η → 2γ and in η → 3π0 → 6γ . The cross-sections vary slowly as functions of energy and η production angle suggesting that only few resonances make significant contributions to the cross-section. For photon energies above 1.5GeV, the differential cross-sections show a strong forward peak due to meson exchanges in the t channel. A comparison of the η -photoproduction cross-sections with SAID and MAID as well as with a partial-wave analysis performed on our data in combination with other data sets shows good agreement. The Bonn-Gatchina partial-wave analysis suggests that the largest contribution to η photoproduction for energies below 3GeV proceeds via three resonances, the well-known N(1535)S 11 , via N(1720)P 13 , and a newly suggested N(2070)D 15 .

Trigonometric quark confinement potential of QCD traits

Abstract: We make the case that the Coulomb- plus linear quark confinement potential predicted by lattice QCD is an approximation to the exactly solvable trigonometric Rosen-Morse potential that has the property to interpolate between the Coulomb- and the infinite wells. We test the predictive power of this potential in the description of the nucleon (considered as a quark-diquark system) and provide analytic expressions for its mass spectrum and the proton electric form factor. We compare the results obtained in this fashion to data and find quite good agreement. We obtain an effective gluon propagator in closed form as the Fourier transform of the potential under investigation.

¯N interaction from meson-exchange and quark-gluon dynamics

Abstract: We investigate the ¯N interaction at low energies using a meson exchange model supplemented with a short-distance contribution from one-gluon exchange The model is developed in close analogy to the meson-exchange KN interaction of the Julich group utilizing SU(4) symmetry constraints The main ingredients of the interaction are provided by vector meson (ρ , ω exchange and higher-order box diagrams involving ¯* N , ¯Δ , and ¯*Δ intermediate states The short-range part is assumed to receive additional contributions from genuine quark-gluon processes The predicted cross-sections for ¯N for excess energies up to 150MeV are of the same order of magnitude as those for KN but with average values of around 20mb, roughly a factor two larger than for the latter system It is found that the ω -exchange plays a very important role Its interference pattern with the ρ -exchange, which is basically fixed by the assumed SU(4) symmetry, clearly determines the qualitative features of the ¯N interaction --very similiar to what happens also for the KN system

Inclusive ω photoproduction from nuclei and ω in the nuclear medium

Abstract: 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 (γ A → π 0 γ X) in the vicinity of the ω production region with subsequent ω → π 0 γ decay. We compare our results with experiment for the distribution of π 0 γ invariant masses and conclude that the distribution is compatible with an enlarged ω width of about 90 MeV 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 stem from a different choice of background which is discussed in the paper.

Photoproduction of πmesons off protons from the Δ(1232) region to Eγ = 3 GeV

Abstract: Photoproduction of π0-mesons was studied with the Crystal-Barrel detector at ELSA for incident energies from 300MeV to 3GeV. Differential cross-sections dσ/dΩ, dσ/dt, and the total cross-section are presented. For E γ < 3GeV, the angular distributions agree well with the SAID parametrization. At photon energies above 1.5GeV, a strong forward peaking indicates t-channel exchange to be the dominant process. The rapid variations of the cross-section with energy and angle indicate production of resonances. An interpretation of the data within the Bonn-Gatchina partial-wave analysis is briefly discussed.

Identification of a K isomer in 252No

Abstract: A K isomer with a half-life of (110±10) ms at an excitation energy of 1254keV has been identified in the nucleus 252No . The isomer is interpreted as a two-quasineutron state with K π = 8- . The isotope 252No was produced in the fusion-evaporation reaction 206Pb(48Ca, 2n)252No . The experiment has been performed at the velocity filter SHIP of GSI Darmstadt.

Two-particle scattering on the lattice: Phase shifts, spin-orbit coupling, and mixing angles

Abstract: We determine two-particle scattering phase shifts and mixing angles for quantum theories defined with lattice regularization. The method is suitable for any non-relativistic effective theory of point particles on the lattice. In the center-of-mass frame of the two-particle system we impose a hard spherical wall at some fixed large radius. For channels without partial-wave mixing the partial-wave phase shifts are determined from the energies of the nearly spherical standing waves. For channels with partial-wave mixing further information is extracted by decomposing the standing wave at the wall boundary into spherical harmonics, and we solve coupled-channels equations to extract the phase shifts and mixing angles. The method is illustrated and tested by computing phase shifts and mixing angles on the lattice for spin-1/2 particles with an attractive Gaussian potential containing both central and tensor force parts.

Photoproduction of η -mesons on the deuteron above S11(1535) in the presence of a narrow P11(1670) -resonance

Abstract: Incoherent photoproduction of η -mesons on the deuteron is considered. The main attention is paid to the region above the S 11(1535) -resonance where a rather narrow resonance-like structure in the total cross-section extracted for γn → ηn has been reported. The corresponding experimental results are analyzed from the phenomenological standpoint within the model containing a baryon P11 with a mass of about 1670MeV and a width less than 30MeV. This resonance was suggested in some recent works as a nonstrange member of the pentaquark antidecuplet with J P = 1/2+ . The calculation is also performed for the polarized and unpolarized angular distributions of η -mesons. In addition, we present our predictions for the cross-sections of the neutral kaons and double pion photoproduction, where the same narrow P 11(1670) -resonance is assumed to contribute through the decay into K 0Λ and πΔ configuration.

The symmetry energy in nuclei and in nuclear matter

Abstract: We discuss to what extent information on ground-state properties of finite nuclei (energies and radii) can be used to obtain constraints on the symmetry energy in nuclear matter and its dependence on the density. The starting point is a generalized Weizsacker formula for ground-state energies. In particular, effects from the Wigner energy and shell structure on the symmetry energy are investigated. Strong correlations in the parameter space prevent a clear isolation of the surface contribution. Use of neutron skin information improves the situation. The result of the analysis appears consistent with a rather soft density dependence of the symmetry energy in nuclear matter.

Angular anisotropy of the fusion-fission and quasifission fragments

Abstract: The anisotropy in the angular distribution of the fusion-fission and quasifission fragments for the 16O + 238U , 19F + 208Pb and 32S + 208Pb reactions is studied by analyzing the angular-momentum distributions of the dinuclear system and compound nucleus which are formed after capture and complete fusion, respectively. The orientation angles of the axial symmetry axes of the colliding nuclei relative to the beam direction are taken into account for the calculation of the variance of the projection of the total spin onto the fission axis. It is shown that there is a large contribution of the quasifission fragments in the 32S + 208Pb reaction to the deviation of the experimental angular anisotropy from the statistical model results. Enhancement of anisotropy at low energies in the 16O + 238U reaction is connected with the quasifission of the dinuclear system having low temperature and relatively small effective moment of inertia.