Background: The structure of weakly bound and unbound nuclei close to particle drip lines is one of the major science drivers of nuclear physics. A comprehensive understanding of these systems goes beyond the traditional configuration interaction approach formulated in the Hilbert space of localized states (nuclear shell model) and requires an open quantum system description. The complex-energy Gamow shell model (GSM) provides such a framework as it is capable of describing resonant and nonresonant many-body states on equal footing.

Purpose: To make reliable predictions, quality input is needed that allows for the full uncertainty quantification of theoretical results. In this study, we carry out the optimization of an effective GSM (one-body and two-body) interaction in the $psdf$-shell-model space. The resulting interaction is expected to describe nuclei with $5\le A\lesssim 12$ at the $p-sd$-shell interface.

Method: The one-body potential of the ${}^4\mathrm{He}$ core is modeled by a Woods-Saxon + spin-orbit + Coulomb potential, and the finite-range nucleon-nucleon interaction between the valence nucleons consists of central, spin-orbit, tensor, and Coulomb terms. The GSM is used to compute key fit observables. The ${\chi }^2$ optimization is performed using the Gauss-Newton algorithm augmented by the singular value decomposition technique. The resulting covariance matrix enables quantification of statistical errors within the linear regression approach.

Results: The optimized one-body potential reproduces nucleon-${}^4\mathrm{He}$ scattering phase shifts up to an excitation energy of 20 MeV. The two-body interaction built on top of the optimized one-body field is adjusted to the bound and unbound ground-state binding energies and selected excited states of the helium, lithium, and beryllium isotopes up to $A=9$. A very good agreement with experimental results was obtained for binding energies. First applications of the optimized interaction include predictions for two-nucleon correlation densities and excitation spectra of light nuclei with quantified uncertainties.

Conclusion: The new interaction will enable comprehensive and fully quantified studies of structure and reactions aspects of nuclei from the $psd$ region of the nuclear chart.

The question of the competition between $\mathrm{\Lambda }$ and ${\mathrm{\Xi }}^{0,-}$ in the ground state of multistrange hypernuclei is addressed within a nonrelativistic density functional approach, partially constrained by ab initio calculations and experimental data. The exploration of the nuclear chart for $10<Z<120$ as a function of the strangeness number is performed by adding hyperons to a nuclear core, imposing either conserved total charge $Q$ or conserved proton number $Z$. We find that almost all $\mathrm{\Lambda }$ hypernuclei present an instability with respect to the strong interaction decay of $\mathrm{\Lambda }$ towards ${\mathrm{\Xi }}^{0,-}$ and that most of the instabilities generates ${\mathrm{\Xi }}^{-}$ (${\mathrm{\Xi }}^0$) in the case of conserved total charge $Q$ (proton number $Z$). The strangeness number at which the first ${\mathrm{\Xi }}^{0,-}$ appear is generally lower for configurations explored in the case of conserved $Q$ compared to the case of conserved $Z$, and corresponds to the crossing between the $\mathrm{\Lambda }$ and the neutron or proton chemical potentials. About two to three hundred thousand pure $\mathrm{\Lambda }$ hypernuclei may exist before the onset of ${\mathrm{\Xi }}^{0,-}$. The largest uncertainty comes from the unknown $\mathrm{\Lambda }\mathrm{\Xi }$ interaction, since the $N\mathrm{\Lambda }$ and the $N\mathrm{\Xi }$ ones can be constrained by a few experimental data. The uncertainty on the $\mathrm{\Lambda }\mathrm{\Xi }$ interaction can still modify the previous estimation by 30–40%, while the impact of the unknown $\mathrm{\Xi }\mathrm{\Xi }$ interaction is very weak.

In a series of Coulomb excitation experiments the first excited $2^{+}$ states in semimagic ${}^{112,116,118,120,122,124}\mathrm{Sn}$ isotopes were excited using a ${}^{58}\mathrm{Ni}$ beam at safe Coulomb energy. The $B(E2$; $0^{+}\to 2^{+})$ values were determined with high precision $(\sim 3\%)$ relative to ${}^{58}\mathrm{Ni}$ projectile excitation. These results disagree with previously reported $B(E2\uparrow )$ values [A. Jungclaus et al., Phys. Lett. B 695, 110 (2011).] extracted from Doppler-shift attenuation lifetime measurements, whereas the reported mass dependence of $B(E2\uparrow )$ values is very similar to a recent Coulomb excitation study [J. M. Allmond et al., Phys. Rev. C 92, 041303(R) (2015)]. The stable Sn isotopes, key nuclei in nuclear structure, show no evidence of reduced collectivity and we, thus, reconfirm the nonsymmetric behavior of reduced transition probabilities with respect to the midshell $A=116$.

We investigate the ${}^{26}\mathrm{O}$ unbound nucleus in a ${}^{24}\mathrm{O}+n+n$ three-body model. We use the hyperspherical formalism, which relies only on ${}^{24}\mathrm{O}+n$ and $n+n$ interactions. As ${}^{26}\mathrm{O}$ is unbound, even in its ground state, the long-range behavior of the wave functions is crucial. We adapt the $R$-matrix technique to derive the wave functions and the associated phase shifts. We start from a ${}^{24}\mathrm{O}+n$ potential which nicely reproduces the known ${}^{25}\mathrm{O}$ properties. Using a weakly repulsive three-body potential, we find the ${}^{26}\mathrm{O}$ ground state at 0.08 MeV, i.e., just above the threshold, in agreement with the latest experimental value. An analysis of the wave function suggests a dominant dineutron configuration. We extend the model to $J=1^{-}$ and $J=2^{+}$. For $J=2^{+}$, we find a relatively narrow resonance near $E_R=1.3$ MeV ($\mathrm{\Gamma }\approx 0.4$ MeV), which is supported by recent experimental data. Broad structures are seen in the $0^{+}$ and $1^{-}$ phase shifts but cannot be associated with physical resonances.

A unitary description for wobbling motion in even-even and even-odd nuclei is presented. In both cases compact formulas for wobbling frequencies are derived. The accuracy of the harmonic approximation is studied for the yrast as well as for the excited bands in the even-even case. Important results for the structure of the wave function and its behavior inside the two wells of the potential energy function corresponding to the Bargmann representation are pointed out. Applications to ${}^{158}\mathrm{Er}$ and ${}^{163}\mathrm{Lu}$ reveal a very good agreement with available data. Indeed, the yrast energy levels in the even-even case and the first four triaxial superdeformed bands, TSD1, TSD2, TSD3, and TSD4, are realistically described. Also, the results agree with the data for the $E2$ and $M1$intra- as well as interband transitions. Perspectives for the formalism development and an extensive application to several nuclei from various regions of the nuclides chart are presented.

The prediction of one-nucleon-removal cross sections by the Liège intranuclear-cascade model has been improved using a refined description of the matter and energy densities in the nuclear surface. Hartree-Fock-Bogoliubov calculations with the Skyrme interaction are used to obtain a more realistic description of the radial-density distributions of protons and neutrons, as well as the excitation-energy uncorrelation at the nuclear surface due to quantum effects and short-range correlations. The results are compared with experimental data covering a large range of nuclei, from carbon to uranium, and projectile kinetic energies. We find that the new approach is in good agreement with experimental data of one-nucleon-removal cross sections covering a broad range in nuclei and energies. The new ingredients also improve the description of total reaction cross sections induced by protons at low energies, the production cross sections of heaviest residues close to the projectile, and the triple-differential cross sections for one-proton removal. However, other observables such as quadruple-differential cross sections of coincident protons do not present any sizable sensitivity to the new approach. Finally, the model is also tested for light-ion-induced reactions. It is shown that the new parameters can give a reasonable description of the nucleus-nucleus total reaction cross sections at high energies.

The free nucleon emission from intermediate-energy nucleus-nucleus collisions is proposed as a possible probe to study the high-momentum tail (HMT) in nucleon momentum distribution of nuclei. Within the framework of the isospin-dependent Boltzmann-Uehling-Uhlenbeck (IBUU) transport model, the effect of HMT on free nucleon emission from the reactions of a light system ${}^{12}\mathrm{C}+{}^{12}\mathrm{C}$ and heavy one ${}^{124}\mathrm{Sn}+{}^{124}\mathrm{Sn}$ at the beam energies of 50 and 140 MeV/nucleon was investigated. We show that the probability of free nucleon emission, especially high-energy nucleons, is sensitive to the high-momentum component in nucleon momentum distribution of nuclei. The momentum distribution with HMT can lead to an increase of both high-energy free proton and neutron emission as compared to the free Fermi gas case for both symmetric and asymmetric nuclear systems.

Relative values of the neutron-induced fission cross section $\sigma (n,f)$ of ${}^{242}\mathrm{Pu}$ have been measured with respect to the standard ${}^1\mathrm{H}(n,p)$ elastic scattering cross section, at average energies of 1.0, 1.4, and $1.9\mathrm{MeV}$. The measurements are part of an international effort to reduce uncertainties and provide independent nuclear data relevant for fast-spectrum reactors. The shape of the measured cross section is in good agreement with data from Tovesson et al. [Phys. Rev. C 79, 014613 (2009)] and with the most recent data from Matei et al. [Phys. Rev. C 95, 024606 (2017)], but disagrees with the shapes of ENDF/B-VII.1 and JEFF3.2 evaluations. Absolute values of $\sigma (n,f)$, obtained under some assumptions, indicate an overestimation of $\sigma (n,f)$ in the evaluated libraries at 1.0 and $1.4\mathrm{MeV}$, while a good agreement is found with ENDF/B-VII.1 at $1.9\mathrm{MeV}$. A careful analysis of the impact of scattered neutrons and anisotropy of the fission fragment angular distribution has been performed. The measurement of the neutron flux by means of a proton-recoil detector is discussed. A comprehensive study of corrections applied to the data, of associated uncertainties, and of correlations between the measurements at the different energies is presented.

Pre- and post-scission $\alpha$-particle multiplicities have been measured for the reaction ${}^{16}O+{}^{194}Pt$ at 98.4 MeV forming ${}^{210}Rn$ compound nucleus. $\alpha$ particles were measured at various angles in coincidence with the fission fragments. Moving source technique was used to extract the pre- and post-scission contributions to the particle multiplicity. Study of the fission mechanism using the different probes are helpful in understanding the detailed reaction dynamics. The neutron multiplicities for this reaction have been reported earlier. The multiplicities of neutrons and $\alpha$ particles were reproduced using standard statistical model code joanne2 by varying the transient (${\tau }_{tr}$) and saddle to scission (${\tau }_{ssc}$) times. This code includes deformation dependent-particle transmission coefficients, binding energies and level densities. Fission time scales of the order of 50–65 $\times {10}^{-21}$ s are required to reproduce the neutron and $\alpha$-particle multiplicities.

We propose a method for calculating the nucleon form factors (FFs) of $G$-parity-even operators by combining chiral effective field theory ($\chi \mathrm{EFT}$) and dispersion analysis. The FFs are expressed as dispersive integrals over the two-pion cut at $t>4M_{\pi }^2$. The spectral functions are obtained from the elastic unitarity condition and expressed as products of the complex $\pi \pi \to N\overline{N}$ partial-wave amplitudes and the timelike pion FF. $\chi \mathrm{EFT}$ is used to calculate the ratio of the partial-wave amplitudes and the pion FF, which is real and free of $\pi \pi$ rescattering in the $t$ channel ($N/D$ method). The rescattering effects are then incorporated by multiplying with the squared modulus of the empirical pion FF. The procedure results in a marked improvement compared to conventional $\chi \mathrm{EFT}$ calculations of the spectral functions. We apply the method to the nucleon scalar FF and compute the scalar spectral function, the scalar radius, the $t$-dependent FF, and the Cheng-Dashen discrepancy. Higher-order chiral corrections are estimated through the $\pi N$ low-energy constants. Results are in excellent agreement with dispersion-theoretical calculations. We elaborate several other interesting aspects of our method. The results show proper scaling behavior in the large-$N_c$ limit of QCD because the $\chi \mathrm{EFT}$ calculation includes $N$ and $\mathrm{\Delta }$ intermediate states. The squared modulus of the timelike pion FF required by our method can be extracted from lattice QCD calculations of vacuum correlation functions of the operator at large Euclidean distances. Our method can be applied to the nucleon FFs of other operators of interest, such as the isovector-vector current, the energy-momentum tensor, and twist-2 QCD operators (moments of generalized parton distributions).

In the dynamics of the QCD critical point, the net-baryon fluid, linked to the slow component of the order parameter, relaxes to a three-dimensional Ising system in equilibrium. An analytical study of shear and bulk viscosity, with constraints imposed by the dynamics of the critical net-baryon fluid, the universality property, and the requirements of a class of strong-coupling theories, is performed in the neighborhood of the critical point. It is found that the shear viscosity of the net-baryon fluid is restricted in the domain $1.6\le 4\pi \frac{\eta }{s}\le 3.7$ for $T_c<T\le 2T_c$, whereas the bulk viscosity is small, $4\pi \frac{\zeta }{s}<0.05$ (for $T>1.27T_c$), but rising towards the singularity at $T=T_c$.

The $\gamma$ rays following radiative neutron capture on ${}^{161,163}\mathrm{Dy}$ targets were measured with the highly segmented $\gamma$-ray calorimeter Detector for Advanced Neutron Capture Experiments at the Los Alamos Neutron Science Center. The $\gamma$-ray energy spectra for different multiplicities were gathered for tens of $s$-wave resonances of both possible spins in each nucleus. Analysis of these spectra within the statistical model enabled us to draw conclusions about dipole photon strength functions with emphasis on the scissors mode. The photon strength functions best describing Dy data are very similar to previously published results on even-even Gd isotopes. It was shown that the scissors mode plays a significant role in the decay of highly excited states up to the neutron separation energy. Measurement of multiple resonances allowed us to assess the fluctuations of experimental spectra and to compare the fluctuations with simulated ones. The size of measured fluctuations is on average smaller than predicted from simulations. However, the results are puzzling as the difference between simulations within the statistical model and experiment is not consistent for both nuclei.

The hyperfine coupling constants of neutron deficient ${}^{53}\mathrm{Fe}$ were deduced from the atomic hyperfine spectrum of the $3d^{6}4s^2{}^5{D}_4\leftrightarrow 3d^{6}4s4p{}^5{F}_5$ transition, measured using the bunched-beam collinear laser spectroscopy technique. The low-energy ${}^{53}\mathrm{Fe}$ beam was produced by projectile-fragmentation reactions followed by gas stopping, and used for the first time for laser spectroscopy. Ground state magnetic-dipole and electric-quadrupole moments were determined as $\mu =-0.65\left(1\right){\mu }_N$ and $Q=+35\left(15\right)e^2{\mathrm{fm}}^2$, respectively. The multiconfiguration Dirac-Fock method was used to calculate the electric field gradient to deduce $Q$ from the quadrupole hyperfine coupling constant, since the quadrupole coupling constant has not been determined for any Fe isotopes. Both experimental values agree well with nuclear shell model calculations using the GXPF1A effective interaction performed in a full $fp$ shell model space, which support the soft nature of the ${}^{56}\mathrm{Ni}$ nucleus.

The Jet Experiments in Nuclear Structure and Astrophysics (JENSA) gas-jet target was used to perform spectroscopic studies of ${}^{20}\mathrm{Ne}+p$ reactions. Levels in ${}^{19}\mathrm{Ne}$ were probed via the ${}^{20}\mathrm{Ne}(p,d){}^{19}\mathrm{Ne}$ reaction to constrain the astrophysical rate of the ${}^{18}\mathrm{F}(p,\alpha ){}^{15}\mathrm{O}$ reaction. Additionally, the first spectroscopic study of the ${}^{20}\mathrm{Ne}(p,{}^3\mathrm{He}){}^{18}\mathrm{F}$ reaction was performed. Angular distribution data were used to determine or confirm the spins of several previously observed levels, and the existence of a strong subthreshold ${}^{18}\mathrm{F}(p,\alpha ){}^{15}\mathrm{O}$ resonance was verified.

In the earlier unitary-model-operator approach (UMOA), one-body correlations have been taken into account approximately by the diagonalization of unitary-transformed Hamiltonians in the $0\text{p}0\text{h}$ and $1\text{p}1\text{h}$ space. With this prescription, the dependence of the harmonic-oscillator energy ($\hslash \omega$) on calculated observables is not negligible even at larger model spaces. In the present work, we explicitly introduce the one-body correlation operator so that it optimizes the single-particle basis states and then reduces the $\hslash \omega$ dependence. For an actual demonstration, we calculate the energy and radius for the ${}^4\mathrm{He}$ ground state with the softened nucleon-nucleon ($NN$) interactions from Argonne v18 (AV18) and chiral effective field theory ($\chi \mathrm{EFT}$) up to the next-to-next-to-next leading order ($\mathrm{N}{}^3\mathrm{LO}$). As a result, we obtain practically $\hslash \omega$-free results at sufficiently large model spaces. The present results are reasonably close to those by the other ab initio calculations with the same $NN$ interactions. This methodological development enables more systematic analysis of calculation results in the UMOA. We also discuss qualitatively the origin of the $\hslash \omega$ dependence on calculated observables in a somewhat simplified way.

The cross-shell excitations in the mass-130 region are determined by the behavior of the single-particle (or single-hole) states around the doubly magic nucleus ${}^{132}\mathrm{Sn}$ and the size of the energy gaps at $N=82$ and/or $Z=50$. The present work reports on the results from large-scale shell-model calculations with the extended paring plus quadrupole-quadrupole force, usually with additions of monopole corrections (the EPQQM model). This paper applies the EPQQM model to the northwestern quadrant of ${}^{132}\mathrm{Sn}$. The model space includes the orbits of $(0g_{7/2},1d_{5/2},1d_{3/2},2s_{1/2},0h_{11/2})$ for both protons and neutrons, with two additional neutron orbits $(1f_{7/2},2p_{3/2})$ above the $N=82$ shell gap, allowing cross-shell excitations. It is found that the experimentally known low-lying levels for ${}^{133}\mathrm{Sb}, {}^{132}\mathrm{Sb}$, and ${}^{131}\mathrm{Sb}$ can be well described. The highly excited states above 4.0 MeV are clearly explained as excitations across the neutron $N=82$ shell gap. The monopole effects in these nuclei are carefully examined. In contrast to the already studied northeastern and southwestern quadrants around ${}^{132}\mathrm{Sn}$ by the EPQQM model, the description of the current Sb data does not request particular monopole corrections. Experiments to further explore the cross-shell excitations in the Sb isotopes are called for.

We investigate systematics of the freezeout surface in heavy ion collisions due to the hadron spectrum. The role of suspected resonance states that are yet to be confirmed experimentally in identifying the freezeout surface has been investigated. We have studied two different freezeout schemes: a unified freezeout scheme where all hadrons are assumed to freeze out at the same thermal state and a flavor-dependent sequential freezeout scheme with different freezeout thermal states for hadrons with or without valence strange quarks. The data of mean hadron yields as well as scaled variance of net proton and net charge distributions have been analyzed. We find the freezeout temperature $T$ to drop by $\sim 5\%$ while the dimensionless freezeout parameters ${\mu }_B/T$ and $V{T}^3$ (${\mu }_B$ and $V$ are the baryon chemical potential and the volume at freezeout respectively) are insensitive to the systematics of the input hadron spectrum. The observed hint of flavor hierarchy in $T$ and $V{T}^3$ with only confirmed resonances survives the systematics of the hadron spectrum. It is more prominent in the range $\sqrt{s_{NN}}\sim 10–100$ GeV, where the maximum hierarchy in $T$ is $\sim 10\%$ and in $V{T}^3$ is $\sim 40\%$. However, the uncertainties in the thermal parameters due to the systematics of the hadron spectrum and their decay properties do not allow us to make a quantitative estimate of the flavor hierarchy yet.

In this work we investigate the finite-size effects on the phase structure of the Walecka model within the framework of a generalized $\zeta$-function, focusing on the influence of temperature as well as the number and length of compactified spatial dimensions. Here we concentrate on the situation of larger values of the coupling between the scalar and fermion fields, in which a phase transition of first order takes place. The phase transitions are analyzed and compared with the system in the situations of one, two, and three compactified spatial dimensions. Our findings suggest that the thermodynamic behavior of the system depends on the length and number of spatial dimensions, with the symmetric phase being favored as the size of the system diminishes.

We extend our previous study of elastic pion-nucleon scattering in the framework of chiral perturbation theory by performing a combined analysis of the reactions $\pi N\to \pi N$ and $\pi N\to \pi \pi N$. The calculation is carried out to fourth order in the chiral expansion using the heavy-baryon approach and the covariant formulation supplemented with a modified version of the extended on-mass-shell renormalization scheme. We demonstrate that a combined fit to experimental data in both channels leads to a reduced amount of correlations between the low-energy constants. A satisfactory description of the experimental data in both channels is obtained, which is further improved upon including tree-level contributions of the $\mathrm{\Delta }$(1232) resonance. We also explore a possibility of using the empirical information about $\pi N$ subthreshold parameters obtained recently by means of the Roy-Steiner equations to stabilize the fits.

Bismuth capture of neutrons is the termination point of the $s$-process cycle of nucleosynthesis in stellar environments. A new measurement is reported here for neutron activation of bismuth with an intense quasi-Maxwellian neutron source at $kT\sim 30\mathrm{keV}$. The measurement was performed at the SARAF phase I accelerator facility by bombarding a 1.5-mA proton beam on the liquid-lithium larget. The cross section of the ${}^{209}\mathrm{Bi}(n,\gamma )$ capture reaction leading to the ${}^{210}\mathrm{Bi}$ ground state was determined by combining $\beta$ measurements from the ${}^{210\mathrm{g}}\mathrm{Bi}$ decay and $\alpha$ and $\gamma$ from the subsequent ${}^{210}\mathrm{Po}$ decay, along with detailed Monte Carlo simulations of the ${}^7\mathrm{Li}(p,n)$ reaction kinematics and the activation experimental setup. Deduced Maxwellian averaged cross sections (MACS) for ${}^{209}\mathrm{Bi}(n,\gamma ){}^{210\mathrm{g}}\mathrm{Bi}$ at $kT=30\mathrm{keV}$ using the ENDF, JEFF, and JENDL databases for the corrections and extrapolations yielded a value of $1.84\pm 0.09\mathrm{mb}$. A comparison is made with previous measurements, including time-of-flight (TOF) measurements of the total bismuth capture cross section. Plans for obtaining the MACS for capture to the bismuth-210 metastable state in the reaction ${}^{209}\mathrm{Bi}(n,\gamma ){}^{210\mathrm{m}}\mathrm{Bi}$ are discussed, along with estimates based on our results in comparison with TOF measurements. The bismuth neutron activation cross section is also of importance for design of GenIV reactor coolant and subcritical accelerator driven systems, especially in light of the 3 million year half-life of the ${}^{210\mathrm{m}}\mathrm{Bi}$ isomer.