The short-range properties of the kaon-nucleon $\mathrm{}\left(\mathrm{KN}\right)\mathrm{}$ interaction are studied within the meson-exchange model of the Jülich group. Specifically, dynamical explanations for the phenomenological short-range repulsion, required in this model for achieving agreement with the empirical $\mathrm{KN}$ data, are explored. Evidence is found that contributions from the exchange of a heavy scalar-isovector meson $\left[a_0\mathrm{}\right(980\left)\mathrm{}\right]$ as well as from genuine quark-gluon exchange processes are needed. Taking both mechanisms into account, a satisfactory description of the $\mathrm{KN}$ phase shifts can be obtained without resorting to phenomenological pieces.

Weak-interaction currents are studied in a recently proposed effective field theory of the nuclear many-body problem. The Lorentz-invariant effective field theory contains nucleons, pions, as well as isoscalar, scalar $\left(\sigma \right)$ and vector $\left(\omega \right)$ fields, and isovector, vector $\left(\rho \right)$ fields. The theory exhibits a nonlinear realization of $\mathrm{SU}{\mathrm{}\left(2\right)\mathrm{}}_L\times \mathrm{SU}{\mathrm{}\left(2\right)\mathrm{}}_R$ chiral symmetry and has three desirable features: it uses the same degrees of freedom to describe the axial-vector current and the strong-interaction dynamics, it satisfies the symmetries of the underlying theory of quantum chromodynamics, and its parameters can be calibrated using strong-interaction phenomena, like hadron scattering or the empirical properties of finite nuclei. Moreover, it has recently been verified that for normal nuclear systems, it is possible to systematically expand the effective Lagrangian in powers of the meson fields (and their derivatives) and to reliably truncate the expansion after the first few orders. Here it is shown that the expressions for the axial-vector current, evaluated through the first few orders in the field expansion, satisfy both PCAC and the Goldberger-Treiman relation, and it is verified that the corresponding vector and axial-vector charges satisfy the familiar chiral charge algebra. Explicit results are derived for the Lorentz-covariant, axial-vector, two-nucleon amplitudes, from which axial-vector meson-exchange currents can be deduced.

We have extended the experiment reported in Bradfield-Smith et al. [Phys. Rev. C $59,$ 3402 (1999)] concerning the ${}^{18}\mathrm{Ne}(\alpha {,p)}^{21}\mathrm{Na}$ reaction, which is of importance to nuclear astrophysics. This study covered an energy region $\sim 1.7–2.9\mathrm{MeV}$ in the center of mass frame of the ${}^{18}\mathrm{Ne}+\alpha$ system. A change in the detector geometry resulted in an increase in the detection efficiency and significantly reduced the proton background that hindered the previous measurement. A direct measurement of the energy loss of the ${}^{18}\mathrm{Ne}$ beam, as it passed through He gas, was undertaken to reduce a major source of uncertainty in the determination of the stellar reaction rate. Eight states have been identified in the compound nucleus, ${}^{22}\mathrm{Mg}.$ These resonances were used to calculate an enhanced stellar reaction rate which shows good agreement with theoretical predictions [Görres, Wiescher, and Thielemann, Phys. Rev. C $51,$ 392 (1995)] at and above a temperature of 1.5 GK. The impact of this enhanced stellar reaction rate upon the nucleosynthesis that occurs during the peak phase of an x-ray burst has been investigated.

Recent observations by the Chandra observatory suggest that some neutron stars may cool rapidly, perhaps by the direct URCA process which requires a high proton fraction. The proton fraction is determined by the nuclear symmetry energy whose density dependence may be constrained by measuring the neutron radius of a heavy nucleus, such as ${}^{208}\mathrm{Pb}.$ Such a measurement is necessary for a reliable extrapolation of the proton fraction to the higher densities present in a neutron star. A large neutron radius in ${}^{208}\mathrm{Pb}$ implies a stiff symmetry energy that grows rapidly with density, thereby favoring a high proton fraction and allowing direct URCA cooling. Predictions for the neutron radius in ${}^{208}\mathrm{Pb}$ are correlated to the proton fraction in dense matter by using a variety of relativistic effective field-theory models. Models that predict a neutron ${(R}_n)$ minus proton ${(R}_p)$ root-mean-square radius in ${}^{208}\mathrm{Pb}$ to be $R_n-R_p\lesssim 0.20\mathrm{fm}$ have proton fractions too small to allow the direct URCA cooling of ${1.4M}_{\odot }$ neutron stars. Conversely, if $R_n-R_p\gtrsim 0.25\mathrm{fm},$ the direct URCA process is allowed (by all models) to cool down a ${1.4M}_{\odot }$ neutron star. The Parity Radius Experiment at Jefferson Laboratory aims to measure the neutron radius in ${}^{208}\mathrm{Pb}$ accurately and model independently via parity-violating electron scattering. Such a measurement would greatly enhance our ability to either confirm or dismiss the direct URCA cooling of neutron stars.

Spectroscopic factors and asymptotic normalization coefficients (ANCs) have been determined for bound states in ${}^{15}\mathrm{O}$ using the ${}^{14}\mathrm{N}{(}^3\mathrm{He}{,d)}^{15}\mathrm{O}$ reaction. These results are used to calculate the astrophysical S factor for direct capture in the ${}^{14}\mathrm{N}(p,\gamma {)}^{15}\mathrm{O}$ reaction. We also discuss how uncertainties in optical-model parameters influence both the spectroscopic factors and the ANCs, and the effect that this has on the predicted direct-capture reaction rate.

The ${}^{22}\mathrm{Ne}(\alpha {,n)}^{25}\mathrm{Mg}$ reaction is thought to be the neutron source during the s process in massive and intermediate mass stars as well as a secondary neutron source during the s process in low-mass stars. Therefore, an accurate determination of this rate is important for a better understanding of the origin of nuclides heavier than iron as well as for improving s-process models. Also, the s process produces seed nuclides for a later p process in massive stars, so an accurate value for this rate is important for a better understanding of the p process. Because the lowest observed resonance in direct ${}^{22}\mathrm{Ne}(\alpha {,n)}^{25}\mathrm{Mg}$ measurements is considerably above the most important energy range for s-process temperatures, the uncertainty in this rate is dominated by the poorly known properties of states in ${}^{26}\mathrm{Mg}$ between this resonance and threshold. Neutron measurements can observe these states with much better sensitivity and determine their parameters (except ${\Gamma }_{\alpha })$ much more accurately than direct ${}^{22}\mathrm{Ne}(\alpha {,n)}^{25}\mathrm{Mg}$ measurements. I have analyzed previously reported ${}^{\mathrm{nat}}\mathrm{Mg}+n$ total and ${}^{25}\mathrm{Mg}(n,\gamma )$ cross sections to obtain a much improved set of resonance parameters for states in ${}^{26}\mathrm{Mg}$ between threshold and the lowest observed ${}^{22}\mathrm{Ne}(\alpha {,n)}^{25}\mathrm{Mg}$ resonance, and an improved estimate of the uncertainty in the ${}^{22}\mathrm{Ne}(\alpha {,n)}^{25}\mathrm{Mg}$ reaction rate. For example, definitely two, and very likely at least four, of the states in this region have natural parity and hence can contribute to the ${}^{22}\mathrm{Ne}(\alpha {,n)}^{25}\mathrm{Mg}$ reaction, but two others definitely have non-natural parity and so can be eliminated from consideration. As a result, a recent evaluation in which it was assumed that only one of these states has natural parity has underestimated the reaction rate uncertainty by at least a factor of 10, whereas evaluations that assumed all these states could contribute probably have overestimated the uncertainty.

Recently, results of a $\mathrm{}(p,t)\mathrm{}$ experiment revealed 13 excited $0^{+}$ states below $\sim 3.2\mathrm{MeV}$ in ${}^{158}\mathrm{Gd}.$ Such data provide a challenge to theory. We show calculations with the geometric collective model and the interacting boson approximation model that may account for many of these $0^{+}$ states. The calculations suggest that many of the $0^{+}$ states may be of two-phonon octupole character.

Light-baryon resonances with u, d, and s quarks only can be classified using the nonrelativistic quark model. When we assign intrinsic orbital angular momenta L and spin S to baryon resonances with total angular momenta J, we make the following observations: plotting the squared masses of the light-baryon resonances against these intrinsic orbital angular momenta L, ${\Delta }^{*}’\mathrm{s}$ with even and odd parity can be described by the same Regge trajectory. For a given L, nucleon resonances with spin $S=3/2\mathrm{}$ are approximately degenerate in mass with $\Delta$ resonances of same total orbital momentum L. To which total angular momentum L and S couple has no significant impact on the baryon mass. Nucleons with spin 1/2 are shifted in mass; the shift is—in units of squared masses—proportional to the component in the wave function which is antisymmetric in spin and flavor. Sequential resonances in the same partial wave are separated in mass square by the same spacing as observed in orbital angular momentum excitations. Based on these observations, a new baryon mass formula is proposed which reproduces nearly all known baryon masses.

Within the transport Boltzmann-Uehling-Uhlenbeck theory we study the influence of the choice for the $\Delta$-resonance lifetime on pion-nucleus reactions and on heavy-ion collisions at 1A GeV. A quite small effect of these modifications on the pion absorption on nuclei is found provided that the absorption probability of the $\Delta$ resonance is modified consistently. Observable effects in the case of heavy-ion collisions are demonstrated.

The photoproduction and electroproduction of the $\eta ,{\eta }^{\prime }$ mesons on nucleons are investigated within a relativistic chiral unitary approach based on coupled channels. The s-wave potentials for electroproduction and meson-baryon scattering are derived from a chiral effective Lagrangian, which includes ${\eta }^{\prime }$ as an explicit degree of freedom and incorporates important features of the underlying QCD Lagrangian such as the axial U(1) anomaly. The effective potentials are iterated in a Bethe-Salpeter equation and the cross sections for $\eta ,{\eta }^{\prime }$ photoproduction and electroproduction from nucleons are obtained. The results for the ${\eta }^{\prime }$ photoproduction cross section on protons reproduce the appearance of an $S_{11}$ resonance around 1.9 GeV observed at ELSA. The inclusion of electromagnetic form factors increases the predicted $\eta$ electroproduction cross sections on the proton, providing a qualitative explanation for the hard form factor of the photocoupling amplitude observed at CLAS.

Lifetimes of 34 high-spin levels in ${}^{81}\mathrm{Y}$ were measured using the Doppler-shift attenuation method. The high-spin states were populated using the ${}^{58}\mathrm{Ni}{(}^{32}\mathrm{S},2\mathrm{}\alpha p)$ reaction at 135 MeV, and the recoils were stopped in a thick Ta backing. Prompt multi-$\gamma$ coincidences were detected using GAMMASPHERE, and evaporated charged particles were detected with the MICROBALL. Transition quadrupole moments inferred from the lifetimes in the lowest positive- and negative-parity bands show a relatively gradual decrease with spin, in agreement with Hartree-Fock-Bogolyubov cranking calculations. The calculations also predict a shape change in each band from predominantly prolate at low spin to triaxial above the first quasiparticle alignment. Thus the observed decrease in collectivity with spin is interpreted as the result of competing shapes which tend to lower the amount of deformation. Other negative-parity bands show evidence for structures with large deformation.

A terminating rotational band has been identified in ${}^{51}\mathrm{Mn}$ following the ${}^{28}\mathrm{Si}{(}^{32}\mathrm{S},2\mathrm{}\alpha {1p)}^{51}\mathrm{Mn}$ fusion-evaporation reaction at 130 MeV beam energy. Spins and tentative positive parities of the band members are assigned based on angular distribution and correlation measurements of transitions, which connect the rotational structure with previously known yrast states. Configuration-dependent cranked Nilsson-Strutinsky calculations suggest a configuration of the band, which comprises one particle in the ${1g}_{9/2}$ intruder orbit.

The pending question of the existence of three-neutron resonances near the physical energy region is reconsidered. Finite rank neutron-neutron forces are used in Faddeev equations, which are analytically continued into the unphysical energy sheet below the positive real energy axis. The trajectories of the three-neutron S-matrix poles in the states of total angular momenta and parity $J^{\pi }={\frac{1}{2}}^{\pm }$ and $J^{\pi }={\frac{3}{2}}^{\pm }$ are traced out as a function of artificial enhancement factors of the neutron-neutron forces. The final positions of the S-matrix poles removing the artificial factors are found in all cases to be far away from the positive real energy axis, which provides a strong indication for the nonexistence of nearby three-neutron resonances. The pole trajectories close to the threshold $E=0\mathrm{}$ are also predicted out of auxiliary generated three-neutron bound state energies using the Padé method and agree very well with the directly calculated ones.

The influence of three-particle breakup singularities on s-wave meson production in nucleon-nucleon collisions is studied within the distorted wave Born approximation. This study is based on a simple scalar model for the two-nucleon interaction and the production mechanism. An algorithm for the exact numerical treatment of the inherent three-body cuts, together with its straightforward implementation is presented. It is also shown that three often-used approximations to avoid the calculation of the three-body breakup are not justified. The possible impact on pion production observables is discussed.

The influence of different approximations for the single-particle energy on neutron-proton pairing in nuclear matter is studied. A strong suppression of the gap due to the energy dependence of the Brueckner-Hartree-Fock self-energy is observed. A quasiparticle approximation neglecting the imaginary part of the self-energy is found to be insufficient.

A method of model-independent spin-parity determination is proposed for the ${(d,}^2\mathrm{He})$ reaction by using the tensor analyzing power $A_{\mathrm{zz}}$ at $\theta =0\mathrm{}°,$ which shows extreme values for $0^{-}$ and natural-parity states solely by parity conservation. It is applied to the ${}^{12}\mathrm{C}{(d,}^2\mathrm{He}{)}^{12}\mathrm{B}$ reaction at $E_d=270\mathrm{}\mathrm{MeV}$ and a possible indication of $0^{-}$ state is found at $E_x=9.3\mathrm{}\mathrm{MeV}$ in ${}^{12}\mathrm{B}.$ The bump at $E_x=7.5\mathrm{}\mathrm{MeV}$ appears to have more $2^{-}$ strength than $1^{-}$ strength at $\theta \sim 0°,$ consistent with our earlier work.

The pion single-charge-exchange reactions are studied for ${}^{13}\mathrm{C}$ and ${}^{15}\mathrm{N}$ above the delta resonance. The effect of the nuclear higher configurations is investigated for the cross section and the asymmetry under the distorted-wave impulse approximation. The cross section and the asymmetry are shown to be appreciably affected with the nuclear higher configurations.

We use a microscopic three-cluster model with the $\alpha +\alpha +n$ configuration to analyze with astrophysical interests the photodisintegration cross section of ${}^9\mathrm{Be}.$ The valence neutron in ${}^9\mathrm{Be}$ is treated in the molecular orbit model, including continuum states by the box discretization method. It is shown that good agreement is achieved with the recently measured $B\left(E1\mathrm{}\right)\mathrm{}$ transition probability from the ground state to the first ${1/2}^{+}$ state when the box size is enlarged, resulting in reasonable reproduction of the experimental data for the photodisintegration cross section. The Coulomb dissociation of ${}^9\mathrm{Be}$ is also discussed.

The energy spectra of the antimony isotope ${}^{108}\mathrm{Sb}$ in the $\mathrm{sdgh}$ shell are calculated in the nuclear shell model approach by using both the CD-Bonn and Nijmegen1 nucleon-nucleon effective interaction. The modified Drexel University parallel shell model (DUPSM) code was used to perform the calculations. The energy levels are compared to recent experimental results.