Evaporated neutron energy spectra have been measured in coincidence with low-energy discrete $\gamma$ rays in the reaction ${}^4\mathrm{He}+{}^{93}\mathrm{Nb}$ at $E\left({}^4\mathrm{He}\right)=28$ MeV. The low-energy light-ion beam provides the scope of extracting the experimental nuclear level density (NLD) in the compound nuclear reaction. Angular-momentum gated NLDs have been extracted in the excitation energy range of $E^{*}\sim 5–15$ MeV from the measured neutron energy spectra. The extracted NLDs have been compared with different theoretical calculations such as the exact pairing plus independent particle model at finite temperature (EP+IPM), Hartree-Fock plus BCS (HFBCS), and Hartree-Fock-Bogoliubov plus combinatorial method (HFBC). Interestingly, the experimental NLDs are in good agreement with the results of the EP+IPM, whereas the HFBCS and HFBC fail to describe these data. Consequently, the thermodynamic properties of ${}^{96}\mathrm{Tc}$ at finite angular momentum have been extracted using the EP+IPM NLDs. Through the analysis of the calculated thermodynamic quantities, it is shown that no pronounced bump is seen in the heat capacity of ${}^{96}\mathrm{Tc}$, in opposition with the earlier results of ${}^{96}\mathrm{Mo}$, which showed a prominent bump at $T\sim 0.7–1$ MeV. This difference is understandable since pairing in the even-even system (${}^{96}\mathrm{Mo}$) is always stronger than that in the odd-odd one (${}^{96}\mathrm{Tc}$).

Full angular distributions of the ${}^{10}\mathrm{B}$ elastically and inelastically scattered by ${}^{58}\mathrm{Ni}$ have been measured at different energies around the Coulomb barrier. The elastic and inelastic scattering of ${}^{10}\mathrm{B}$ on a medium mass target has been measured for the first time. The obtained angular distributions have been analyzed in terms of large-scale coupled reaction channel calculations, where several inelastic transitions of the projectile and the target, as well as the most relevant one- and two-step transfer reactions have been included in the coupling matrix. The roles of the spin reorientation, the spin-orbit interaction, and the large ground-state deformation of the ${}^{10}\mathrm{B}$, in the reaction mechanism, were also investigated. The real part of the interaction potential between projectile and target was represented by a parameter-free double-folding potential, whereas no imaginary potential at the surface was considered. In this sense, the theoretical calculations were parameter free and their results were compared to experimental data to investigate the relative importance of the different reaction channels. A striking influence of the ground-state spin reorientation of the ${}^{10}\mathrm{B}$ nucleus was found, while all transfer reactions investigated had a minimum contribution to the dynamics of the system. Finally, the large static deformation of the ${}^{10}\mathrm{B}$ and the spin-orbit coupling can also play an important role in the system studied.

Detection of the neutrinoless $\beta \beta$ ($0\nu \beta \beta$) decay is of high priority in the particle- and neutrino-physics communities. The detectability of this decay mode is strongly influenced by the value of the weak axial-vector coupling constant $g_A$. The recent nuclear-model analyses of $\beta$ and $\beta \beta$ decays suggest that the value of $g_A$ could be dramatically quenched, reaching ratios of $g_A^{\mathrm{free}}/g_A\approx 4$, where $g_A^{\mathrm{free}}=1.27$ is the free, neutron-decay, value of $g_A$. The effects of this quenching appear devastating for the sensitivity of the present and future $0\nu \beta \beta$ experiments since the fourth power of this ratio scales the $0\nu \beta \beta$ half-lives. This, in turn, could lead to some two orders of magnitude less sensitivity for the $0\nu \beta \beta$ experiments. In the present article it is shown that by using a consistent approach to both the two-neutrino $\beta \beta$ and $0\nu \beta \beta$ decays by the proton-neutron quasiparticle random-phase approximation, the feared two-orders-of-magnitude reduction in the sensitivity of the $0\nu \beta \beta$ experiments actually shrinks to a reduction by factors in the range $2\text{–}6$. This certainly has dramatic consequences for the potential to detect the $0\nu \beta \beta$ decay.

Background: Theoretical calculations suggest the presence of low-lying excited states in ${}^{25}\mathrm{O}$. Previous experimental searches by means of proton knockout on ${}^{26}\mathrm{F}$ produced no evidence for such excitations.

Purpose: We search for excited states in ${}^{25}\mathrm{O}$ using the ${}^{24}\mathrm{O}(d,p){}^{25}\mathrm{O}$ reaction. The theoretical analysis of excited states in unbound ${}^{25,27}\mathrm{O}$ is based on the configuration interaction approach that accounts for couplings to the scattering continuum.

Method: We use invariant-mass spectroscopy to measure neutron-unbound states in ${}^{25}\mathrm{O}$. For the theoretical approach, we use the complex-energy Gamow Shell Model and Density Matrix Renormalization Group method with a finite-range two-body interaction optimized to the bound states and resonances of ${}^{23–26}\mathrm{O}$, assuming a core of ${}^{22}\mathrm{O}$. We predict energies, decay widths, and asymptotic normalization coefficients.

Results: Our calculations in a large $spdf$ space predict several low-lying excited states in ${}^{25}\mathrm{O}$ of positive and negative parity, and we obtain an experimental limit on the relative cross section of a possible $J^{\pi }={1/2}^{+}$ state with respect to the ground state of ${}^{25}\mathrm{O}$ at ${\sigma }_{1/2+}/{\sigma }_{g.s.}=0.{25}_{-0.25}^{+1.0}$. We also discuss how the observation of negative parity states in ${}^{25}\mathrm{O}$ could guide the search for the low-lying negative parity states in ${}^{27}\mathrm{O}$.

Conclusion: Previous experiments based on the proton knockout of ${}^{26}\mathrm{F}$ suffered from the low cross sections for the population of excited states in ${}^{25}\mathrm{O}$ because of low spectroscopic factors. In this respect, neutron transfer reactions carry more promise.

The nuclear temperature is calculated from the derivative of the logarithm of the level densities in ${}^{\text{60-62}}\mathrm{Ni}$ and ${}^{\text{170-172}}\mathrm{Yb}$. The latter are obtained within a method, which includes exact pairing for the levels around the Fermi surface in combination with the independent particle model for the rest of the single-particle spectrum. It is found that the increase in this temperature is relatively slow up to the excitation energy $E^{*}=E_f^{*}$ so that, at 0 $<E^{*}\le E_f^{*}$, the level density can be described well by the constant-temperature model. The values of $E_f^{*}$ are found to be 10 MeV for ${}^{\text{170-172}}\mathrm{Yb}$ and 20 MeV for ${}^{\text{60-62}}\mathrm{Ni}$, that is much higher than the particle separation threshold. Within this energy interval, the constant temperature is found to be around 0.5 MeV for ${}^{\text{170-172}}\mathrm{Yb}$, whereas for ${}^{\text{60-62}}\mathrm{Ni}$ it can be any value between 1.3 and 1.5 MeV, in excellent agreement with the recent experimental finding. It is also shown that pairing plays an important role in maintaining this constant temperature at low excitation energy.

Nuclear reactions induced by muon neutrinos with energies from 0.2 to 1.5 GeV in the Monte Carlo calculation framework in the intranuclear cascade model are studied. This study was done by comparing the available experimental data and theoretical values of total cross section, and the energy distribution of emitted lepton energy in the reaction muon neutrino nucleus, using the targets ${}^{12}\mathrm{C}, {}^{16}\mathrm{O}, {}^{27}\mathrm{Al}, {}^{40}\mathrm{Ar}, {}^{56}\mathrm{Fe}$, and ${}^{208}\mathrm{Pb}$. A phenomenological model of primary neutrino-nucleon interaction gives good agreement between our theoretical inclusive neutrino nucleus cross section and the available experimental data. Some interesting results on the behavior of the cross section as function of $1p-1n$ and higher contributions are also sketched. The previous results on the fraction of fake events in available experiments in ${}^{12}\mathrm{C}$ were expanded for the set of studied nuclei. With the increase of mass targets, the nuclear effects in the cross sections were observed and the importance of taking into account fake events in the reactions was noted.

Various corrections to the eikonal approximations are studied for two- and three-body nuclear collisions with the goal to extend the range of validity of this approximation to beam energies of 10 MeV/nucleon. Wallace's correction does not improve much the elastic-scattering cross sections obtained at the usual eikonal approximation. On the contrary, a semiclassical approximation that substitutes the impact parameter by a complex distance of closest approach computed with the projectile-target optical potential efficiently corrects the eikonal approximation. This opens the possibility to analyze data measured down to 10 MeV/nucleon within eikonal-like reaction models.

Background: Coherent one-particle one-hole (1p1h) excitations have given us effective insights into general nuclear excitations. However, the two-particle two-hole (2p2h) excitation beyond 1p1h is now recognized as critical for the proper description of experimental data of various nuclear responses.

Purpose: The spin-flip charge-exchange reactions ${}^{48}\mathrm{Ca}(p,n){}^{48}\mathrm{Sc}$ are investigated to clarify the role of the 2p2h effect on their cross sections. The Fermi transition of ${}^{48}\mathrm{Ca}$ via the $(p,n)$ reaction is also investigated in order to demonstrate our framework.

Methods: The transition density is calculated microscopically with the second Tamm-Dancoff approximation, and the distorted-wave Born approximation is employed to describe the reaction process. A phenomenological one-range Gaussian interaction is used to prepare the form factor.

Results: For the Fermi transition, our approach describes the experimental behavior of the cross section better than the Lane model, which is the conventional method. For spin-flip excitations including the GT transition, the 2p2h effect decreases the magnitude of the cross section and does not change the shape of the angular distribution. The $\mathrm{\Delta }l=2$ transition of the present reaction is found to play a negligible role.

Conclusions: The 2p2h effect will not change the angular-distributed cross section of spin-flip responses. This is because the transition density of the Gamow-Teller response, the leading contribution to the cross section, is not significantly varied by the 2p2h effect.

The dynamics of fermionic many-body systems is investigated in the framework of Boltzmann–Langevin (BL) stochastic one-body approaches. Within the recently introduced Boltzmann–Langevin one-body (BLOB) model, we examine the interplay between mean-field effects and two-body correlations, of stochastic nature, for nuclear matter at moderate temperature and in several density conditions, corresponding to stable or mechanically unstable situations. Numerical results are compared with analytic expectations for the fluctuation amplitude of isoscalar and isovector densities, probing the link to the properties of the employed effective interaction; namely, symmetry energy (for isovector modes) and incompressibility (for isoscalar modes). For unstable systems, clusterization is observed. The associated features are compared with analytical results for the typical length and timescales characterizing the growth of unstable modes in nuclear matter and for the isotopic variance of the emerging fragments. We show that the BLOB model is generally better suited than simplified approaches previously introduced to solve the BL equation, and it is therefore more advantageous in applications to open systems, such as heavy-ion collisions.

Density dependences of the mass and self-energies of ${\mathrm{\Lambda }}_c$ in nuclear matter are studied in the parity projected QCD sum rule. Effects of nuclear matter are taken into account through the quark and gluon condensates. It is found that the four-quark condensates give dominant contributions. As the density dependences of the four-quark condensates are not known well, we examine two hypotheses. One is based on the factorization hypothesis (F-type) and the other is derived from the perturbative chiral quark model (QM-type). The F-type strongly depends on density, while the QM-type gives a weaker dependence. It is found that, for the F-type dependence, the energy of ${\mathrm{\Lambda }}_c$ increases as the density of nuclear matter grows, that is, ${\mathrm{\Lambda }}_c$ feels repulsion. On the other hand, the QM-type predicts a weak attraction ($\sim 20$ MeV at the normal nuclear density) for ${\mathrm{\Lambda }}_c$ in nuclear matter. We carry out a similar analysis of the $\mathrm{\Lambda }$ hyperon and find that the F-type density dependence is too strong to explain the observed binding energy of $\mathrm{\Lambda }$ in nuclei. Thus we conclude that the weak density dependence of the four-quark condensate is more realistic. The scalar and vector self-energies of ${\mathrm{\Lambda }}_c$ for the QM-type dependence are found to be much smaller than those of the light baryons.

We discuss collective excitations (both fundamental and solitonic excitations) of quantized superfluid vortices in neutron ${}^{3}P_2$ superfluids, which likely exist in high density neutron matter such as neutron stars. Besides the well-known Kelvin modes (translational zero modes), we find a gapful mode whose low-energy description takes the simple form of a double sine-Gordon model. The associated kink solution and its effects on spontaneous magnetization inside the vortex core are analyzed in detail.

The pairing-plus-quadrupole Hamiltonian is diagonalized in a symmetry-restored basis, i.e., the triaxial quasiparticle states with angular momentum and particle number projections, and applied for chiral doublet bands in ${}^{128}\mathrm{Cs}$. The observed energy spectra and electromagnetic transition probabilities are reproduced well without introducing any parameters. The orientation of the angular momentum in the intrinsic frame is investigated by the distributions of its components on the three principle axes ($K$ plot) and those of its tilted angles (azimuthal plot). The evolution of the chirality with spin is illustrated, and the chiral geometry is demonstrated in the angular momentum projected model for the first time.

Chiral effective field theory is a framework to derive systematic nuclear interactions. It is based on the symmetries of quantum chromodynamics and includes long-range pion physics explicitly, while shorter-range physics is expanded in a general operator basis. The number of low-energy couplings at a particular order in the expansion can be reduced by exploiting the fact that nucleons are fermions and therefore obey the Pauli exclusion principle. The antisymmetry permits the selection of a subset of the allowed contact operators at a given order. When local regulators are used for these short-range interactions, however, this “Fierz rearrangement freedom” is violated. In this paper, we investigate the impact of this violation at leading order (LO) in the chiral expansion. We construct LO and next-to-leading order (NLO) potentials for all possible LO-operator pairs and study their reproduction of phase shifts, the ${}^4\mathrm{He}$ ground-state energy, and the neutron-matter energy at different densities. We demonstrate that the Fierz rearrangement freedom is partially restored at NLO where subleading contact interactions enter. We also discuss implications for local chiral three-nucleon interactions.

Background: Isolating nuclear structure properties from knock-out reactions in a process-independent manner requires a controlled factorization, which is always to some degree scale and scheme dependent. Understanding this dependence is important for robust extractions from experiment, to correctly use the structure information in other processes, and to understand the impact of approximations for both.

Purpose: We seek insight into scale dependence by exploring a model calculation of deuteron electrodisintegration, which provides a simple and clean theoretical laboratory.

Methods: By considering various kinematic regions of the longitudinal structure function, we can examine how the components—the initial deuteron wave function, the current operator, and the final-state interactions (FSIs)—combine at different scales. We use the similarity renormalization group to evolve each component.

Results: When evolved to different resolutions, the ingredients are all modified, but how they combine depends strongly on the kinematic region. In some regions, for example, the FSIs are largely unaffected by evolution, while elsewhere FSIs are greatly reduced. For certain kinematics, the impulse approximation at a high renormalization group resolution gives an intuitive picture in terms of a one-body current breaking up a short-range correlated neutron-proton pair, although FSIs distort this simple picture. With evolution to low resolution, however, the cross section is unchanged but a very different and arguably simpler intuitive picture emerges, with the evolved current efficiently represented at low momentum through derivative expansions or low-rank singular value decompositions.

Conclusions: The underlying physics of deuteron electrodisintegration is scale dependent and not just kinematics dependent. As a result, intuition about physics such as the role of short-range correlations or $D$-state mixing in particular kinematic regimes can be strongly scale dependent. Understanding this dependence is crucial in making use of extracted properties.

Modern softened nucleon-nucleon interactions are well suited for perturbative many-body calculations, but a many-body power counting scheme is lacking. Estimates of diagrammatic contributions at finite density are important ingredients in such a scheme. Here we show how to make quantitative estimates of the particle-particle and hole-hole channel in uniform nuclear matter for soft interactions. We also use estimates to assess the role of normal-ordered three-body forces for a pure contact interaction.