Naturalness of nonlinear \sigma, \omega self-couplings in relativistic mean-field models for neutron stars

We investigate dense hadronic matter in a relativistic mean-field approach. For a generalized baryon-meson Lagrangian effective field theory, we confront expansions from naive dimensional analysis for the nonlinear self-couplings of the , fields with estimates from microscopic pair creation models from Quantum Chromodynamics. Upon adjusting the model parameters to ordinary nuclear matter, we discuss implications of the approach to dense hadronic matter and in particular to neutron stars.Received: 30 September 2002, Published online: 22 October 2003PACS: 11.10.-z Field theory - 21.65. + f Nuclear matter - 26.60. + c Nuclear matter aspects of neutron stars     

\pi\pi correlations in \gamma + A reactions

Preliminary differential cross-sections of the reactions and with A = ¹ H, ¹² C, and ^nat Pb are presented. A significant nuclear-mass dependence of the invariant-mass distribution is found in the channel. The dependence is not observed in the channel. The in-medium observation in the channel is consistent with an in-medium modification of the interaction in the I = J = 0 channel, changing width and pole position of a resonant state.Received: 30 September 2002, Published online: 22 October 2003PACS: 13.60.Le Meson production - 21.65. + f Nuclear matterJ.G. Messchendorp: For the TAPS and A2 Collaborations     

\alpha-decay spectroscopy of light odd-odd Bi isotopes - I: 188,190Bi nuclei

Detailed fine-structure -decay studies of ^188,190Bi were performed using the complete-fusion reactions of ^50,52Cr ions with a ¹⁴²Nd target at the velocity filter SHIP. The evaporation residues were separated in-flight and subsequently identified on the basis of recoil- , recoil- - /X-ray and excitation function measurements. Improved data on the -decay of ^188,190Bi were obtained and a number of new low-lying excited states in the daughters ^184,186Tl were observed. An intruder, presumably 10^- state was identified in ¹⁸⁴Tl, extending the systematics of such states in the odd-odd Tl nuclei beyond the neutron mid-shell at N = 104. Some technical aspects of experiment at recoil separators are also discussed.Received: 14 March 2003, Revised: 1 May 2003, Published online: 2 September 2003PACS: 23.60.+e decay - 27.70.+q - 27.80.+w C.D. OLeary: Present address: Department of Physics, University of York, York, UK.M.J. Taylor: Present address: Department of Physics and Astronomy, Rutgers Univesity, NJ, USA.L. Weissman: Present address: NSCL, Michigan State University, East Lansing, MI 48824-1321 USA.K. Heyde: Present address: EP-ISOLDE, CERN, CH-1211, Geneva 23, Switzerland.     

Helicity dependence of the \gamma p\rightarrow N \pi channels and multipole analysis in the \Delta region

A high-quality double-polarization data set for the helicity dependence of the total and differential cross-sections for both channels in the region has been obtained in the framework of the GDH experiment. The experiment, performed at the Mainz microtron MAMI, used a 4 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 -resonance. For the resonant (1232) multipoles we find a very good agreement with previous analyses, while the nonresonant ones show significant deviations.Received: 11 July 2003, Revised: 16 January 2004, Published online: 17 August 2004PACS: 13.60.Le Meson production - 14.20.Gk Baryon resonances with S = 0 - 25.20.Lj Photoproduction reactions     

Different effect of the hyperons $\Lambda$ and $\Xi$ on the nuclear core

We demonstrate the different effect of strange impurities ( and ) on the static properties of nuclei within the framework of the relativistic mean-field model. Systematic calculations show that the glue-like role of the -hyperon is universal for all -hypernuclei considered. However, the -hyperon has a glue-like role only for the protons distribution in nuclei, while for the neutrons distribution it plays a repulsive role. On the other hand, the -hyperon attracts the surrounding neutrons and reveals a repulsive force to the protons. Possible explanations of the above observation are discussed.Received: 10 March 2003, Revised: 6 November 2003, Published online: 5 May 2004PACS: 21.80. + a Hypernuclei - 24.10.Cn Many-body theory     

First structure information on the exotic 149 La from the $\beta^-$-decay of 149 Ba

The -decay of ¹⁴⁹Ba to ¹⁴⁹La has been investigated by means of - and X-ray spectroscopy, and a partial level scheme of ¹⁴⁹La has been constructed for the first time. It includes thirteen -rays and ten excited states. The exotic ¹⁴⁹La is the heaviest lanthanum for which spectroscopic information is now available. We have applied the shell correction approach with the axially deformed Woods-Saxon potential in order to calculate the deformation energy for the ¹⁴⁹La ground state. The deformed ground state was found at 0.23, 0 and 0.12.Received: 18 July 2003, Revised: 12 January 2004, Published online: 26 May 2004PACS: 27.60. + j - 29.30.Kv X- and -ray spectroscopy     

Quasi-free \pi^0 photoproduction from the bound nucleon

Differential cross-sections for quasi-free photoproduction from the proton and neutron bound in the deuteron have been measured for MeV at using the Glasgow photon tagger at MAMI, the Mainz 48 cm cm NaI(Tl) photon detector and the Göttingen SENECA recoil detector. For the proton measurements made with both liquid-deuterium and liquid-hydrogen targets allow direct comparison of free photoproduction cross-sections as extracted from the bound-proton data with experimental free cross-sections which are found to be in reasonable agreement below 320 MeV. At higher energies the free cross-sections extracted from quasi-free data are significantly smaller than the experimental free cross-sections and theoretical predictions based on multipole analysis. For the first time, free neutron cross-section have been extracted in the -region. They are also in agreement with the predictions from multipole analysis up to 320 MeV and significantly smaller at higher photon energies.Received: 28 May 2003, Published online: 27 January 2004PACS: 13.60.Le Meson production - 14.20.Dh Protons and neutrons - 25.20.Lj Photoproduction reactionsK. Kossert: Part of the Doctoral Thesis.,Present address: Physikalisch-Technische Bundesanstalt, Bundesallee 100, D-38116 Braunschweig, Germany.Present address F. Wissmann: Physikalisch-Technische Bundesanstalt, Bundesallee 100, D-38116 Braunschweig, Germany. Present address B. Seitz: II. Physikalisches Institut der Universität Gießen, Germany.     

Absolute resonance strengths in the $^{6,7}Li(\alpha, \gamma)^{10,11}B$ reactions

The absolute strengths of the keV resonance in the reaction and of the keV resonance in the reaction have been measured to meV and meV, respectively, in good agreement with previous values. These resonances can be used to measure the absolute acceptance of the recoil separator ERNA to a precision of about 10%.Received: 12 December 2003, Revised: 10 February 2004, Published online: 3 August 2004PACS: 24.30.-v Resonance reactions - 25.40.Lw Radiative capture - 26.20. + f Hydrostatic stellar nucleosynthesis     

Experimental signals of \sigma-mesons in nuclei and chiral symmetry

The interaction in the scalar isoscalar channel is studied as a function of the baryonic density in the framework of a chiral unitary approach which successfully reproduces vacuum - phase shifts. We present some results on the -meson pole, which is generated dynamically in our model, in nuclear matter. Finally, we analyze the ( ) reaction on nucleons and nuclei in the kinematical region where the scalar isoscalar scattering amplitude is influenced by the low mass of the in nuclei, and thus, presents a large enhancement close to the position of its pole. Indeed, we find, that the final-state interaction of the pions modified by the nuclear medium produces a large shift of strength of the two-pion invariant-mass distribution in the ( ) channel consistent with recent experimental data.Received: 30 September 2002, Published online: 22 October 2003PACS: 14.40.-n Properties of specific particles: Mesons - 25.20.-x Photonuclear reactions - 21.65. + f Nuclear matter     

Near-threshold production of a0(980)-mesons in the reaction \rm {pp \rightarrow d K^ + \bar{K^0}}

Using an effective Lagrangian approach as well as the Quark-Gluon Strings Model, we analyze near-threshold production of a ₀(980)-mesons in the reaction as well as the background of non-resonant -pair production. We argue that the reaction at an energy release MeV is dominated by the intermediate production of the a ₀(980)-resonance. At larger energies the non-resonant -pair production --where the kaons are produced in a relative P-wave-- becomes important. The effects of final-state interactions are evaluated in a unitarized scattering length approach and found to be in the order of a 20% suppression close to threshold. Thus, in present experiments at the Cooler Synchrotron COSY-Jülich for MeV the a ₀ ⁺ signal can reliably be separated from the non-resonant background.Received: 26 February 2004, Published online: 21 September 2004PACS: 25.10. + s Nuclear reactions involving few-nucleon systems - 13.75.-n Hadron-induced low- and intermediate-energy reactions and scattering (energy GeV)     

Total and differential cross-sections for the pp\rightarrow pp\eta^\prime reaction near threshold

The -meson production in the reaction has been studied at excess energies of Q = 26.5, 32.5 and 46.6 MeV using the internal beam facility COSY-11 at the cooler synchrotron COSY. The total cross-sections as well as one angular distribution for the highest Q-value are presented. The excitation function of the near-threshold data can be described by a pure s-wave phase space distribution with the inclusion of the proton-proton final-state interaction and Coulomb effects. The obtained angular distribution of the -mesons is also consistent with pure s-wave production.Received: 11 August 2003, Revised: 14 November 2003, Published online: 6 April 2004PACS: 13.60.Le Meson production - 13.75.-n Hadron-induced low- and intermediate-energy reactions and scattering (energy GeV) - 13.85.Lg Total cross-sections - 25.40.-h Nucleon-induced reactions     

Delayed crossing in the \pi h_{9/2} 1/2-[541] band of 173 Lu

High-spin states in the odd-proton nucleus ¹⁷³Lu have been populated in a ¹⁷⁰Er(⁷Li,4n) reaction and the emitted -radiation was detected with the GASP array. The favoured and unfavoured sequences of the band have been considerably extended. The favoured sequence does not show any indication of a band crossing up to a rotational frequency of 0.45 MeV. This is the largest crossing frequency ever observed for 1/2^- [541] bands of odd-proton nuclei with and . Previous Cranked Shell Model (CSM) calculations underestimate the crossing frequency by 100 keV.Received: 19 May 2003, Published online: 11 November 2003PACS: 21.10.-k Properties of nuclei; nuclear energy levels - 21.10.Re Collective levels - 23.20.Lv transitions and level energies - 27.70. + q      

Observation of bound excited states in 15B

The structure of the A/Z = 3 nucleus ¹⁵B has been investigated using the in-beam -spectroscopy technique with a fragmentation reaction of a ³⁶S beam on a ⁹Be target at 77.5 . The fragments were identified and selected by their energy loss and time of flight using the SPEG spectrograph. -ray energies and intensities have been measured in coincidence with the projectile-like fragments. From this information as well as from the -coincidence relationships a level scheme is proposed for ¹⁵B up to the neutron separation energy. The experimental results have been interpreted using shell model calculations in the psd valence space. Effects of the weakly bound nature of the valence neutrons have been observed.Received: 4 May 2004, Revised: 2 September 2004, Published online: 19 October 2004PACS: 25.70.Mn Projectile and target fragmentation - 23.20.Lv transitions and level energies - 27.20. + n - 21.60.Cs Shell model     

\alpha-decay spectroscopy of light odd-odd Bi isotopes - II: 186Bi and the new nuclide 184Bi

Alpha-decay of the new nuclide ¹⁸⁴Bi has been studied in the complete-fusion reaction ⁹³Nb(⁹⁴Mo, 3n)¹⁸⁴Bi at the velocity filter SHIP. The evaporation residues were separated in-flight and subsequently identified on the basis of recoil- , recoil- - analysis and excitation functions measurements. Two -decaying isomeric states in ¹⁸⁴Bi with half-life values of 13(2) ms and 6.6(1.5) ms were identified. The -branching ratio of ¹⁸⁰Tl was deduced for the first time as . Improved data on the fine-structure -decay of ¹⁸⁶Bi were obtained in the ⁹³Nb(⁹⁵Mo, 2n)¹⁸⁶Bi reaction. A similarity of the decay energies and half-life values of ^184,186Bi is pointed out and a possible explanation for this effect is suggested.Received: 14 March 2003, Revised: 29 April 2003, Published online: 2 September 2003PACS: 23.60.+e decay - 27.70.+q - 27.80.+w K. Heyde: Present address: EP-ISOLDE, CERN, CH-1211, Geneva 23, Switzerland.     

Exclusive and semi-inclusive strangeness and charm production in \pi N and NN reactions

Using the Quark-Gluon Strings Model (QGSM) combined with Regge phenomenology we consider the reactions and which are dominated by the contributions of the K ^* and D ^* Regge trajectories, respectively. The spin structure of the amplitudes is described by introducing Reggeized Born terms. It is found that the existing data for the reaction are in reasonable agreement with the model predictions. To describe the absolute values of the cross-sections it is necessary to introduce also suppression factors which can be related to absorption corrections. Furthermore, assuming the SU(4) symmetry to hold for Regge residues and the universality of absorption corrections we calculate the cross-section of the reaction . Employing the latter results from reactions we then estimate the contributions of the pion exchange mechanism to the cross-sections of the reactions and and compare them with the contributions of the K and D exchanges. We find that the NN reactions are dominated not by pion exchange but by K and D exchanges, respectively. Moreover, assuming the SU(4) symmetry to hold approximately for the coupling constants = we analyze also the production of leading -hyperons in the reaction . It is shown that the non-perturbative mechanism should give an essential contribution to the yield for .Received: 30 September 2002, Published online: 22 October 2003PACS: 13.85.Fb Inelastic scattering: two-particle final states - 13.85.Hd Inelastic scattering: many-particle final states - 14.20.Lq Charmed baryons - 14.40.Lb Charmed mesons     

Inclusive $\omega$ photoproduction off nuclei

We investigate inclusive photoproduction off complex nuclei, concentrating on the feasibility to examine a possible in-medium change of the properties by observing the 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 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 potential are discussed.Received: 28 October 2003, Revised: 26 November 2003, Published online: 8 June 2004PACS: 25.20.-x Photonuclear reactions - 13.60.-r Photon and charged-lepton interactions with hadrons - 13.60.Le Meson production - 14.40.-n Mesons     

Prospects for measurements of parity-violatingphotoproduction of \pi^\pm

I address the experimental feasibility of two measurements related to the photoproduction of pions, in the context of the G⁰ experiment. The first involves an extraction of the parity-violating pion-nucleon coupling constant via a measurement of parity-violating threshold pion photoproduction. The second involves an extraction of , which parameterizes parity-violating photoproduction of pions on the -resonance. The second process might have an anomalously large asymmetry, from a model based on hyperon decays.Received: 30 September 2002, Published online: 22 October 2003PACS: 11.30.Er Charge conjugation, parity, time reversal, and other discrete symmetries - 13.60.Le Meson production - 14.20.Gk Baryon resonances with S = 0J.W. Martin: For the G⁰ Collaboration     

Cluster structure of <Superscript>16</Superscript>O

A folding potential describing the -scattering on ¹⁶O over a broad energy range 25.8-146.0 MeV is constructed on the basis of -like cluster and unclustered-nucleon configurations of ¹⁶O. The resulting potential does not need any renormalization to fit the angular distribution of elastic cross-sections. The effects of the repulsive part of - and -nucleon interactions are investigated. The analysis suggests that both the - repulsive potential and the unclustered nucleonic configuration in the target are important to describe the scattering data over a broad range of incident energies. The root-mean-square radius for the ¹⁶O nucleus is deduced.Received: 5 December 2002, Published online: 9 October 2003PACS: 25.55.Ci Elastic and inelastic scattering - 24.10.Ht Optical and diffraction models - 21.60.Gx Cluster models     

Prompt fission neutron spectra of <Superscript>238 </Superscript>U(n,f) above emissive fission threshold

Model calculations were performed to interpret prompt fission neutron spectra (PFNS) of the ²³⁸ U(n, f) reaction for incident neutron energies -18 MeV. Pre-fission (pre-saddle) reaction neutron spectra were calculated with Hauser-Feshbach statistical model, ²³⁸ U fission and reaction cross-section data being described consistently. The increase of the cut-off energy of (n, nf) reaction neutron spectra with excitation energy of fissioning nucleus is described. For -9 MeV the low-energy PFNS component, which is due to the contribution of pre-fission (n, nf) neutrons, is compatible with measured data. Average energy of prefission (n, nf) neutrons is shown to be rather dependent on . For -18 MeV, a decrease of measured PFNS average neutron energies is interpreted. Spectra of neutrons, evaporated from fission fragments, were approximated as a sum of two Watt distributions. The reduced fission fragment velocity is assumed for the neutron emission during fragment acceleration. Several interpretations of observed soft neutron excess are investigated, i.e., possible uncertainties of emissive fission contributions and additional neutron source. We claim the soft neutron excess cannot be attributed to the pre-saddle neutrons contribution.Received: 3 February 2003, Revised: 24 April 2003, Published online: 9 October 2003PACS: 25.85.Ec Neutron-induced fission