Approximations made to estimate two-nucleon transfer probabilities in ground-state to ground-state transitions and the physical interpretation of these probabilities are discussed. Probabilities are often calculated by approximating both ground states of the initial nucleus $A$ and of the final nucleus $A\pm 2$ by the same quasiparticle vacuum. We analyze two improvements of this approach. First, the effect of using two different ground states with average numbers of particles $A$ and $A\pm 2$ is quantified. Second, by using projection techniques, the role of particle number restoration is analyzed. Our analysis shows that the improved treatment plays a role close to magicity, leading to an enhancement of the pair-transfer probability. In midshell regions, part of the error made by approximating the initial and final ground states by a single vacuum is compensated by projecting onto a good particle number. Surface effects are analyzed by using pairing interactions with a different volume-to-surface mixing. Finally, a simple expression of the pair-transfer probability is given in terms of occupation probabilities in the canonical basis. We show that, in the canonical basis formulation, surface effects that are visible in the transfer probability are related to the fragmentation of single-particle occupancies close to the Fermi energy. This provides a complementary interpretation with respect to the standard quasiparticle representation where surface effects are generated by the integrated radial profiles of the contributing wave functions.

We investigate both bound and resonance states in ${}^{12}$C embedded in a three-$\alpha$-cluster continuum using a three-cluster microscopic model. The model relies on the hyperspherical harmonics basis to enumerate the channels describing the three-cluster discrete and continuous spectrum states. It yields the most probable distribution of the three $\alpha$ clusters in space, and the dominant decay modes of the three-cluster resonances.

Background: Momentum distributions of fragmentation products as a function of fragment mass have been used to study the fragmentation mechanism. The parallel component of the momentum distribution has been well studied previously and modeled. The perpendicular component, however, is much less measured or understood.

Purpose: Measure both components of the linear momentum of a wide range of fragmentation products and compare the widths of the momentum distributions to previous results and descriptions.

Method: The parallel and perpendicular components of the momentum vector have been measured for projectile-like fragments produced in the reactions of ${}^{76}$Ge with ${}^9$Be and ${}^{197}$Au at 130 MeV/nucleon in a magnetic spectrometer.

Results: The measured parallel momentum distributions of all fragments follow established systematics. The perpendicular momentum distributions of fragments produced by fragmentation by the ${}^{197}$Au target with masses near that of the projectile exhibit a clear peak near the momentum corresponding to the grazing angle that diminishes with decreasing fragment mass.

Conclusions: The interplay between Coulomb and nuclear scattering can be used to describe results for the most peripheral collisions.

The production of $\phi$ mesons in proton collisions with C, Cu, Ag, and Au targets has been studied via the $\phi \to K^{+}{K}^{-}$ decay at an incident beam energy of 2.83 GeV using the ANKE detector system at COSY. For the first time, the momentum dependence of the nuclear transparency ratio, the in-medium $\phi$ width, and the differential cross section for $\phi$-meson production at forward angles have been determined for these targets over the momentum range of 0.6–1.6 GeV/$c$. There are indications of a significant momentum dependence in the value of the extracted $\phi$ width, which corresponds to an effective $\phi N$ absorption cross section in the range of 14–21 mb.

The $g$ factor of the $2_1^{+}$ state of ${}^{168}$Hf was measured using the perturbed angular correlation technique in a static external magnetic field. The result, $g\left(2_1^{+}\right)=0.17\left(3\right)$, is discussed in relation to the systematics of the previously reported $g$ factors in the Hf isotopes and compared to the predictions of several models. An interesting outcome of the analysis presented in this paper has to do with the relatively small result for the $g$ factor. This indicates that in the Hf isotopes, a minimum in the $g$($2_1^{+}$) dependence on $N$ occurs at $N\le 98$ and not at midshell, as expected from IBA-2 or large-scale shell-model calculations. The pairing plus quadrupole model of Kumar and Baranger predicts a minimum at $N=98$ and gives the best description of the experimental data. The present result clearly shows the importance of $g$-factor measurements in “fine-tuning” among different models.

Although many random-phase approximation (RPA) calculations of the Gamow-Teller (GT) response exist, this is not the case for calculations going beyond the mean-field approximation. We apply a consistent model, that includes the coupling of the GT resonance to low-lying vibrations, to nuclei of the $fp$ shell. Among other motivations, our goal is to see if the particle-vibration coupling can redistribute the low-lying GT${}^{+}$ strength that is relevant for electron-capture processes in core-collapse supernova. We conclude that the lowering and fragmentation of that strength are consistent with the experimental findings and validate our model. However, the particle-vibration coupling cannot account for the quenching of the total value of the low-lying strength.

A model-independent relation which holds for the long-distance part of the Fourier transform of the electromagnetic form factors of the nucleon in the large-$N_c$ and chiral limits is demonstrated. This relation was previously conjectured based on the fact that it emerged in all semiclassical chiral models independent of the details of the model. Here we show that the result is, in fact, model independent by deriving it directly in large-$N_c$ chiral perturbation theory, which is known to capture the long-distance behavior of the form factors. The relation is valid when the large-$N_c$ limit is formally taken before the chiral limit. A new relation is derived for the case where the chiral limit is taken prior to the large-$N_c$ limit.

The ${}^{12}$C($\alpha$,$\gamma$)${}^{16}$O reaction plays a crucial role in stellar evolution. The rate of this reaction determines directly the ${}^{12}$C-to-${}^{16}$O abundance ratio at the end of the helium burning phase of stars and consequently has a big effect on the subsequent nucleosynthesis and even on the evolution of massive stars. However, despite many experimental studies, the low-energy cross section of ${}^{12}$C($\alpha$,$\gamma$)${}^{16}$O remains uncertain. The extrapolation of the measured cross sections to stellar energies ($E\sim 300$ keV) is made particularly difficult by the presence of the 2${}^{+}$ ($E_x=6.92$ MeV) and 1${}^{-}$ ($E_x=7.12$ MeV) subthreshold states of ${}^{16}$O. To further investigate the contribution of these two subthreshold resonances to the ${}^{12}$C($\alpha$,$\gamma$)${}^{16}$O cross section, we determine their $\alpha$-reduced widths via a measurement of the transfer reaction ${}^{12}$C(${}^7$Li, $t$)${}^{16}$O at two incident energies, 28 and 34 MeV. The uncertainties on the determined $\alpha$-spectroscopic factors and the $\alpha$-reduced widths were reduced thanks to a detailed distorted-wave Born approximation analysis of the transfer angular distributions measured at the two incident energies. The $R$-matrix calculations of ${}^{12}$C($\alpha$,$\gamma$)${}^{16}$O cross section using our obtained $\alpha$-reduced widths for the 2${}^{+}$ and 1${}^{-}$ subthreshold resonances lead to an $E1$ $S$ factor at 300 keV of $100\pm 28$ keV b, which is consistent with values obtained in most of the direct and indirect measurements as well as the NACRE collaboration compilation while the result for the $E2$ component $S_{E2}\left(300\text{keV}\right)=50\pm 19$ keV b disagrees with the NACRE adopted value .

In this paper we investigate the thermal evolution of hybrid stars, objects composed of a quark matter core, enveloped by ordinary hadronic matter. Our purpose is to investigate how important the microscopic properties of the quark core are to the thermal evolution of the star. In order to do that we use a simple Massachusetts Institute of Technology (MIT) bag model for the quark core and a relativistic-mean-field model for the hadronic envelope. By choosing different values for the microscopic parameters (bag constant, strange quark mass, strong coupling constant), we obtain hybrid stars with different quark core properties. We also consider the possibility of color superconductivity in the quark core. With this simple approach, we have found a set of microscopic parameters that lead to a good agreement with those of observed cooling neutron stars. Our results can be used to obtain clues regarding the properties of the quark core in hybrid stars and to refine more sophisticated models for the equation of state of quark matter.

We recently proposed a nonlocal form for the 3-body induced interaction that is consistent with the Fock space representation of interaction operators but leads to a fractional power dependence on the density. Here we examine the implications of the nonlocality for the excitation spectrum. In the two-component weakly interacting Fermi gas, we find that it gives an effective mass that is comparable to the one in many-body perturbation theory. Applying the interaction to nuclear matter, it predicts a huge enhancement to the effective mass. Since the saturation of nuclear matter is partly due to the induced 3-body interaction, fitted functionals should treat the effective mass as a free parameter, unless the 2- and 3-body contributions are determined from basic theory.

We describe the investigation of high-spin states in ${}^{24}$Mg populated with the ${}^{12}\mathrm{C}{{(}^{16}\mathrm{O},\alpha )}^{24}\mathrm{Mg}$ reaction at 62- and 68-MeV beam energy. The excited states were established through the coincident detection of up to four $\alpha$ and $\gamma$ particles in complete decay cascades toward a final state of angular momentum zero, the ground state of either ${}^{20}$Ne or ${}^{16}$O. We describe a new analysis method for the angular correlations in such events and apply it with the goal to assign spin and parity to $\alpha$-unbound states in ${}^{24}$Mg. We establish a number of high-spin, natural-parity states between spins 6 and 12, including the lowest known ${10}^{+}$ and ${12}^{+}$ levels. The energy systematics of positive- and negative-parity high-spin states are compared to the predictions of modern shell-model calculations.

Neutron rich He nuclei have been investigated using the relativistic mean field approach in co-ordinate space. Elastic partial scattering cross sections for proton scattering in inverse kinematics have been calculated using the theoretically obtained density for ${}^{6,8}$He and compared with the experiment. The energies of the low-lying resonance states in the neutron unstable nuclei ${}^{5,7}$He have also been calculated and compared with experimental observations.

Neutron single-particle states in the neutron-rich nucleus ${}^{35}$Si, which is located beside the $N=20$ shell breaking nucleus ${}^{32}$Mg, were investigated through their isobaric analog resonances. The excitation function for ${}^{34}$Si$+p$ elastic scattering was measured around $0^{\circ }$ in the laboratory frame by the thick target inverse kinematics method with a ${}^{34}$Si beam around 5 MeV/nucleon and a thick polyethylene target. Eight resonances were successfully observed. Angular momenta and proton and total widths of the resonances were assigned using $R$-matrix analysis. With the help of information of the $\beta$ decay study, six isobaric analog resonances were identified. Spectroscopic factors and spin-parities of the corresponding parent states in ${}^{35}$Si are presented.

Measurements of angular distributions for one- and two-proton stripping reactions for ${}^{28}$Si $+$ ${}^{90,94}$Zr systems were performed at 120 MeV. The experiment was carried out with the ${}^{28}$Si beam at Inter University Accelerator Center, New Delhi. The theoretical calculations were performed using the quantum mechanical coupled reaction channels code fresco. The distorted wave Born approximation calculations reproduced the experimental angular distributions for the one-proton transfer channel for both the systems reasonably well but failed for the two-proton transfer channel. Coupled channels calculations including various intermediate states (involving target and projectile inelastic excitations before and/or after transfer) along with the sequential transfer were able to reproduce the two-proton transfer angular distributions for both the systems reasonably well. It seems that at an energy above the Coulomb barrier, there is significant contribution of the indirect multistep and sequential transfer to the two-proton stripping reaction.

Indirect neutron capture ($n$,$\gamma$) cross sections have been extracted for the key thorium cycle nuclei ${}^{232}$Th, ${}^{231}$Pa, and ${}^{230}$Th using the surrogate reaction method. Final nucleus $\gamma$-decay probabilities were measured between the neutron binding energy and around 1 MeV above it using the ${}^{232}$Th(d,p)${}^{233}$Th, ${}^{232}$Th(${}^3$He,t)${}^{232}$Pa, and ${}^{232}$Th(${}^3$He,$\alpha$)${}^{231}$Th reactions in experiments with the CACTUS $\gamma$-detector array and Silicon Ring charged-particle detectors at the Oslo Cyclotron Laboratory. Because the neutron capture cross section for ${}^{232}$Th is already well known from direct measurements a comparison with these results provides a stringent test of the applicability of the surrogate method in the actinide region for indirect ($n$,$\gamma$) cross-section measurements. In addition, a new technique for correcting measured $\gamma$-ray decay probabilities below the neutron emission energy threshold is proposed and used. We find good agreement between indirect and direct ($n$,$\gamma$) cross-section measurements in the range 500 keV–1 MeV, but large discrepancies outside this range. Explanations for the observed differences are proposed.

We present an unquenched quark model calculation of the $s\overline{s}$ sea pairs’ contribution to the magnetic moment of the proton (the strange magnetic moment) and its charge radius (the strange radius). In our approach the effects of the $s\overline{s}$ pairs are taken explicitly into account through a microscopic, QCD-inspired, quark-antiquark pair creation mechanism. Our results for the two “strangeness” observables are compatible with the latest experimental results and recent lattice calculations.

We describe systems of two and three nucleons trapped in a harmonic-oscillator potential with interactions from the pionless effective field theory up to next-to-leading order (NLO). We construct the two-nucleon interaction using two-nucleon scattering information. We calculate the trapped levels in the three-nucleon system with isospin $T=1/2$ and determine the three-nucleon force needed for stability of the triton. We extract neutron-deuteron phase shifts, and show that the quartet scattering length is in good agreement with experimental data.

We present results of an exclusive measurement of the first-excited state of the $\Sigma$ hyperon, $\Sigma {\left(1385\right)}^{+}$, produced in $p+p\to {\Sigma }^{+}+K^{+}+n$ at $3.5\mathrm{GeV}$ beam energy. The extracted data allow the detailed study of the invariant mass distribution of the $\Sigma {\left(1385\right)}^{+}$. The mass distribution is well described by a relativistic Breit-Wigner function with a maximum at $m_0=1383.2\pm 0.9\mathrm{MeV}/c^2$ and a width of $40.2\pm 2.1\mathrm{MeV}/c^2$. The exclusive production cross section comes out to be $22.27\pm 0.89\pm 1.{56}_{-2.10}^{+3.07}\mu b$. Angular distributions of the $\Sigma {\left(1385\right)}^{+}$ in different reference frames are found to be compatible with the hypothesis that $33\%$ of $\Sigma {\left(1385\right)}^{+}$ result from the decay of an intermediate broad ${\Delta }^{++}$ excitation at about $2000\mathrm{MeV}/c^2$.

The ${}^{17}$O($p,\gamma$)${}^{18}$F reaction plays an important role in hydrogen-burning nucleosynthesis. Conflicting values for the low-energy behavior of its cross section exist in the literature. We present direct measurements of the astrophysical $S$ factor of the ${}^{17}$O($p,\gamma$)${}^{18}$F reaction at center-of-mass energies between 250 and 500 keV. These measurements were conducted in inverse kinematics at the DRAGON recoil separator.