The precision measurement of $\beta$-spectrum shape for ${}^{210}\mathrm{Bi}$ (historically RaE) has been performed with a spectrometer based on semiconductor Si(Li) detector. This first forbidden nonunique transition has the transition form factor strongly deviated from unity, and knowledge of its spectrum would play an important role in low-background physics in the presence of ${}^{210}\mathrm{Pb}$ background. The measured transition formfactor could be approximated as $C\left(W\right)=1+(-0.4470\pm 0.0013)W+(0.0552\pm 0.0004)W^2$, that is in good agreement with previous studies and has significantly increased parameter precision.

We use the microscopic Giessen Boltzmann-Uehling-Uhlenbeck (GiBUU) transport model to calculate dilepton ($e^{+}{e}^{-}$) production in heavy-ion collisions at SIS18 energies focusing on the effect of collisional broadening of the $\rho$ meson. The collisional width of the $\rho$ meson at finite temperature and baryon density in nuclear matter is calculated on the basis of the collision integral of the GiBUU model. A systematic comparison with HADES data on dilepton production in heavy-ion collisions is performed. The collisional broadening of the $\rho$ improves the agreement between theory and experiment for the dilepton invariant-mass distributions near the $\rho$ pole mass and for the excess radiation in $\mathrm{Au}+\mathrm{Au}$ at $1.23A\mathrm{GeV}$. We furthermore show that some remaining underprediction of the experimental dilepton spectra in $\mathrm{C}+\mathrm{C}$ at $1A\mathrm{GeV}$ and $\mathrm{Au}+\mathrm{Au}$ at $1.23A\mathrm{GeV}$ at intermediate invariant masses 0.2–0.4 GeV can be accounted for by adjusting the $pn$ bremsstrahlung cross section in a way to agree with the inclusive dilepton spectrum from $dp$ collisions at $1.25A\mathrm{GeV}$.

The double-differential cross section of the deuteron breakup reaction ${}^2\mathrm{H}(n,2n$) has been studied experimentally with a neutron beam energy of 15 MeV. Special attention has been devoted to estimation of background condition and multiple scattering effect in the scattering sample. Experimental data have been compared with models based on phase-space approximation used in the ENDF/B-VIII.0 data library and in the MCNP neutron transport code, as well as with rigorous model based on Faddeev equations used for cross section evaluations in JENDL data library. It was found that experimental data are better reproduced by Faddeev model, however, the model overestimates data in the low-energy region of the neutron spectrum ($<4$ MeV).

The particle-number-conserving (PNC) method in the framework of cranked shell model (CSM) is developed to deal with the reflection-asymmetric nuclear system by applying the $S_x$ symmetry. Based on an octupole-deformed Nilsson potential, the alternating-parity bands in ${}^{236,238}\mathrm{U}$ and ${}^{238,240}\mathrm{Pu}$ are investigated. The experimental kinematic moments of inertia (MoI) and the angular momentum alignments of all studied bands are reproduced well in the PNC-CSM calculations. The striking difference of rotational behaviors between U and Pu isotopes can be linked to the strength of octupole correlations. The upbendings of the alternating-parity bands in ${}^{236,238}\mathrm{U}$ are due to the alignments of pairs of nucleons occupying $\nu g_{9/2}, \pi f_{7/2}$ orbitals and $\nu j_{15/2}, \pi i_{13/2}$ high-$j$ intruder orbitals. In particular, the interference terms of nucleon occupying the octupole-correlation pairs of ${\nu }^2{j}_{15/2}{g}_{9/2}$ and of ${\pi }^2{i}_{13/2}{f}_{7/2}$ give a very important contribution to the suddenly gained alignments.

We present detailed calculations about the expected shape of two-pion Bose-Einstein [or Hanbury-Brown–Twiss (HBT)] correlations in high energy heavy ion collisions that include a realistic treatment of final state Coulomb interaction as well as strong interactions (dominated by $s$-wave scattering). We assume Lévy type source functions, a generalization that goes beyond the Gaussian approximation. Various recent experimental results found the use of such source types necessary to properly describe the shape of the measured correlation functions. We find that strong interaction effects for like-sign pions are small, but their consideration may become important in future precision measurements, especially if one considers source parameters beyond the Gaussian HBT radii. Precise experimental determination of these source parameters (such as Lévy stability exponent, correlation strength, etc.) may then benefit from the inclusion of the treatment of strong interaction not just for heavier particles (e.g., protons, $\mathrm{\Lambda }\mathrm{s}$) but also in case of two-pion measurements.

The theoretical uncertainty in the nuclear Schiff moment is an obstacle to set constraints on charge-parity violation beyond the standard model from experimental upper bounds on atomic electric dipole moments. We perform large-scale shell-model calculations of the ${}^{129}\mathrm{Xe}$ and ${}^{199}\mathrm{Hg}$ nuclei with realistic effective interactions. To estimate the Schiff moments caused by the $P, T$-odd $\pi NN$ interaction perturbatively, we employ the one-particle–one-hole approximation to the intermediate states. The Schiff moments of ${}^{129}\mathrm{Xe}$ and ${}^{199}\mathrm{Hg}$ are reduced due to the configuration mixing by $\sim 10\%$ from the evaluation of the independent particle model. On the other hand, the reduction is more significant in mean-field based calculations and shell-model calculations with a drastic truncation. In order to resolve the discrepancy in the Schiff moment of ${}^{199}\mathrm{Hg}$ among several nuclear models, we survey low-energy nuclear structure. The large-scale shell-model calculations reveal that the Schiff moment of ${}^{199}\mathrm{Hg}$ is considerably quenched in the second ${\frac{1}{2}}^{-}$ state.

The enhancement of radiative strength function (RSF) in the region of low $\gamma$-rays energy ($E_{\gamma }\le 12\mathrm{MeV}$), which is caused by the pygmy dipole resonance (PDR), is treated within the phonon damping model (PDM) plus exact thermal pairing (EP) without adding any extra PDR strength function. The numerical calculations performed for ${}^{161–163}\mathrm{Dy}$ show that, because of the effect of EP, the $\mathrm{EP}+\mathrm{PDM}$ can describe reasonably well the total RSF data in both low- and high-energy regions of $\gamma$ rays. Consequently, as compared to the conventional description within the phenomenological generalized Lorentzian and standard Lorentzian models, the $\mathrm{EP}+\mathrm{PDM}$ calculations can eliminate, at least, eight free parameters. This indicates the important role of microscopic approaches towards the precise description of the RSF. In particular, temperature is found to have significant contributions to the RSF below the neutron separation energy, questioning again the validity of the Brink-Axel hypothesis in this energy region.

Background: Inelastic proton scattering at energies of a few hundred MeV and very-forward scattering angles including $0^{\circ }$ has been established as a tool for the study of electric-dipole strength distributions in nuclei. The present work reports a systematic investigation of the chain of stable even-mass Nd isotopes representing a transition from spherical to quadrupole-deformed nuclei.

Purpose: Extraction of the equivalent photo-absorption cross sections and analysis of their fine structure in the energy region of the isovector giant dipole resonance (IVGDR).

Method: Proton inelastic scattering reactions of 200 MeV protons were measured at the iThemba Laboratory for Accelerator Based Sciences in Cape Town, South Africa. The scattering products were momentum-analyzed by the K600 magnetic spectrometer positioned at ${\theta }_{\mathrm{Lab}}=0^{\circ }$. Using dispersion-matching techniques, energy resolutions of $\mathrm{\Delta }E\approx 40\text{–}50$ keV (full width at half maximum) were obtained. After subtraction of background and contributions from other multipoles, the spectra were converted to photoabsorption cross sections using the equivalent virtual-photon method. Wavelet-analysis techniques are used to extract characteristic energy scales of the fine structure of the IVGDR from the experimental data.

Results: Fine structure of the IVGDR is observed even for the most deformed nuclei studied. Comparisons between the extracted experimental energy scales and those energy scales obtained from the quasiparticle-phonon model (QPM) and Skyrme separable random phase approximation (SSRPA) predictions provide insight into the role of different giant-resonance damping mechanisms. It can be seen that the scales in the spherical and most likely also in the deformed nuclei mainly result from the fragmentation of the one-particle-one-hole (1p1h) strength into several dominant transitions serving as doorway states. In cases where calculations beyond the 1p1h level are available, some impact of the spreading due to coupling of the two-particle-two-hole (2p2h) states to the 1p1h doorway states is observed.

Conclusions: New virtual-photon absorption data for the chain of stable Nd isotopes and ${}^{152}\mathrm{Sm}$ are presented, with a focus on the phenomenon of nonstatistical cross-section fluctuations, referred to as fine structure, in the energy region of the IVGDR. The wavelet-analysis techniques used allowed for the features of the fine structure to be quantified in the form of characteristic scales. Comparisons between experimental results and model predictions indicate that Landau damping seems to be the main source of the fine structure in both the spherical and deformed nuclei, but calculations including 2p2h degrees of freedom would be beneficial to confirm this for the deformed cases.

Neutrino-nucleus quasielastic scattering is studied in the plane-wave impulse approximation for three nuclear models: the relativistic Fermi gas (RFG), the independent-particle shell model (IPSM), and the natural orbitals (NO) model with Lorentzian dependence of the excitation energy. A complete study of the kinematics of the semi-inclusive process and the associated cross sections are presented and discussed for ${}^{40}\mathrm{Ar}$ and ${}^{12}\mathrm{C}$. Inclusive cross sections are also obtained by integrating the semi-inclusive expressions over the outgoing hadron. Results are consistent with previous studies restricted to the inclusive channel. In particular, a comparison with the analytical results for the RFG model is performed. Explicit expressions for the hadronic tensor and the 10 semi-inclusive nuclear responses are given. Theoretical predictions are compared with semi-inclusive experimental data from T2K experiment.

Background: The neutron-rich ${}^{28,29}\mathrm{F}$ isotopes have been recently studied via knockout and interaction cross-section measurements. The two-neutron halo in ${}^{29}\mathrm{F}$ has been linked to the occupancy of $pf$ intruder configurations.

Purpose: We investigate the bound spectrum and continuum states in ${}^{29}\mathrm{F}$, focusing on the electric dipole $\left(E1\right)$ response of low-lying excitations and the effect of dipole couplings on nuclear reactions.

Method: ${}^{29}\mathrm{F}$ $({}^{27}\mathrm{F}+n+n)$ wave functions are built within the hyperspherical harmonics expansion formalism, and total reaction cross sections are calculated using the Glauber theory. Continuum states and $B\left(E1\right)$ transition probabilities are described in a pseudostate approach using the analytical transformed harmonic oscillator basis. The corresponding structure form factors are used in continuum-discretized coupled-channels (CDCC) calculations to describe low-energy scattering.

Results: Parity inversion in ${}^{28}\mathrm{F}$ leads to a ${}^{29}\mathrm{F}$ ground state characterized by 57.5% of ${\left(p_{3/2}\right)}^2$ intruder components, a strong dineutron configuration, and an increase of the matter radius with respect to the core radius of $\mathrm{\Delta }R=0.20$ fm. Glauber-model calculations for a carbon target at 240 MeV/nucleon provide a total reaction cross section of 1370 mb, in agreement with recent data. The model produces also a barely bound excited state corresponding to a quadrupole excitation. $B\left(E1\right)$ calculations into the continuum yield a total strength of 1.59 $e^2$ ${\mathrm{fm}}^2$ up to 6 MeV, and the $E1$ distribution exhibits a resonance at $\approx 0.85$ MeV. Results using a standard shell-model order for ${}^{28}\mathrm{F}$ lead to a considerable reduction of the $B\left(E1\right)$ distribution. The four-body CDCC calculations for ${}^{29}\mathrm{F}+{}^{120}\mathrm{Sn}$ around the Coulomb barrier are dominated by dipole couplings, which totally cancel the Fresnel peak in the elastic-scattering cross section.

Conclusions: Our three-body calculations for ${}^{29}\mathrm{F}$, using the most recent experimental information on ${}^{28}\mathrm{F}$, are consistent with a two-neutron halo. Our predictions show the low-lying enhancement of the $E1$ response expected for halo nuclei and the relevance of dipole couplings for low-energy reactions on heavy targets. These findings may guide future experimental campaigns.

We present a detailed discussion of a recently proposed method to evaluate complete and incomplete fusion cross sections for weakly bound systems. The method is applied to collisions of ${}^7\mathrm{Li}$ projectiles on different heavy targets, and the results are compared with the available data. The overall agreement between experiment and theory is fairly good.

Rapid-advancing intense laser technologies enable the possibility of a direct laser-nucleus coupling. In this paper the effect of intense laser fields on a series of nuclear fission processes, including proton emission, $\alpha$ decay, and cluster decay, is theoretically studied with the help of nuclear double folding potentials. The results show that the half-lives of these decay processes can be modified by non-negligible amounts, for example on the order of 0.01 or 0.1% in intense laser fields available in the forthcoming years. In addition to numerical results, an approximate analytical formula is derived to connect the laser-induced modification to the decay half-life and the decay energy.

We study the $\eta N$ interactions in the hot and dense isospin asymmetric nuclear matter using two different approaches. In the first approach, the in-medium mass and optical potential of $\eta$ meson have been calculated in the chiral SU(3) model, considering the effect of explicit symmetry-breaking term and range terms in the $\eta N$ interaction Lagrangian density. In the second scenario, the conjunction of chiral perturbation theory and chiral SU(3) model is employed. In this case, the next to leading order $\eta N$ interactions are evaluated from the chiral perturbation theory (ChPT), and the in-medium contribution of scalar densities are taken as input from chiral SU(3) model. We observe a larger negative mass shift in the ChPT $+$ chiral model approach compared to the chiral SU(3) model alone as a function of nuclear density. Moreover, the increase in the asymmetry and temperature cause a decrease in the magnitude of mass shift. We have also studied the impact of $\eta N$ scattering length $a^{\eta N}$ on the $\eta$-meson mass $m_{\eta }^{*}$ and observed that the $m_{\eta }^{*}$ decrease mores for increasing values of scattering length.

We study the ${\mathrm{\Lambda }}_{c}N$ interaction in the covariant chiral effective field theory (ChEFT) at leading order. All the relevant low-energy constants are determined by fitting to the lattice QCD simulations from the HAL QCD Collaboration. Extrapolating the results to the physical point, we show that the ${\mathrm{\Lambda }}_{c}N$ interaction is weakly attractive in the ${}^{1}S_0$ channel, but in the ${}^{3}S_1$ channel it is only attractive at extremely low energies and soon turns repulsive for larger laboratory energy. Furthermore, we show that the neglect of the ${}^{3}S_1\text{−}{}^{3}D_1$ coupling provided by the leading-order covariant ChEFT would result in an attractive interaction in the ${}^{3}S_1$ channel at the physical point, which coincides with the previous nonrelativistic ChEFT study. As a byproduct, we predict the ${}^{3}D_1$ phase shifts and the mixing angel ${\varepsilon }_1$, which can be checked by future lattice QCD simulations. In addition, we compare the ${\mathrm{\Lambda }}_{c}N$ interaction with the $\mathrm{\Lambda }N$ and $NN$ interactions to study how the baryon-nucleon ($BN$) interactions evolve as a function of the baryon mass with the replacement of a light quark by a strange or charm quark in the baryon ($B$).

A class of phenomenological relativistic models of hadronic systems motivated by quantum chromodynamics that have dual representations as models of mesons and nucleons or quarks and gluons is investigated. These models are designed to provided qualitative insight into the role of sea quarks in hadronic structure and reactions. The model assumption is that the Hamiltonian can be divided into two parts: one that involves degrees of freedom in the same connected local and global color singlet and the remaining interactions that allow the connected local and global color singlets to interact. The first class of interactions results in infinite towers of bare “particles” with hadronic quantum numbers. All but a finite number of these remain stable when the second class of interactions is included. The model interactions are expressed in terms of subhadronic degrees of freedom, which determine the bare hadronic spectrum and the interactions involving the bare hadrons in terms of a small number of subhadronic model parameters. As a first test, this paper considers the simplest case of mesons that interact via a string-breaking interaction. One virtue of this model is that all of the bare meson masses and eigenfunctions can be computed analytically. In addition, the string-breaking interaction leads to production vertices that can also be computed analytically. The relativistic wave functions have a light-front kinematic symmetry. The goal is to find a simple relativistic quantum mechanical model based on subhadronic degrees of freedom that can provide an efficient, qualitatively consistent description of hadronic masses, lifetimes, cross sections, sea quark effects, and electromagnetic properties. The simplicity of the model makes it a potentially useful tool to study the impact of sea quarks on hadronic structure and reactions.

The electron-capture process plays an important role in the evolution of the core collapse of a massive star that precedes the supernova explosion. In this study, the electron capture on nuclei in stellar environment is described in the relativistic energy density functional framework, including both the finite-temperature and nuclear pairing effects. Relevant nuclear transitions $J^{\pi }=0^{\pm },1^{\pm },2^{\pm }$ are calculated using the finite-temperature proton-neutron quasiparticle random-phase approximation with the density-dependent meson-exchange effective interaction DD-ME2. The pairing and temperature effects are investigated in the Gamow-Teller transition strength as well as the electron-capture cross sections and rates for ${}^{44}\mathrm{Ti}$ and ${}^{56}\mathrm{Fe}$ in the stellar environment. It is found that the pairing correlations establish an additional unblocking mechanism similar to the finite-temperature effects, that can allow otherwise blocked single-particle transitions. Inclusion of pairing correlations at finite temperature can significantly alter the electron-capture cross sections, even up to a factor of 2 for ${}^{44}\mathrm{Ti}$, while for the same nucleus electron-capture rates can increase by more than one order of magnitude. We conclude that for the complete description of electron capture on nuclei both pairing and temperature effects must be taken into account.

Is the secondary component of GW190814 the lightest black hole or the heaviest neutron star ever discovered in a double compact-object system [Abbott et al. Astrophys. J. 896, L44 (2020)]? This is the central question animating this paper. Covariant density functional theory provides a unique framework to investigate both the properties of finite nuclei and neutron stars, while enforcing causality at all densities. By tuning existing energy density functionals we were able to: (i) account for a $2.6M_{\odot }$ neutron star, (ii) satisfy the original constraint on the tidal deformability of a $1.4M_{\odot }$ neutron star, and (iii) reproduce ground-state properties of finite nuclei. Yet, for the class of models explored in this work, we find that the stiffening of the equation of state required to support supermassive neutron stars is inconsistent with either constraints obtained from energetic heavy-ion collisions or from the low deformability of medium-mass stars. Thus, we speculate that the maximum neutron star mass can not be significantly higher than the existing observational limit and that the $2.6M_{\odot }$ compact object is likely to be the lightest black hole ever discovered.

The properties of neutron matter are integral to the correct description of neutron stars as well as the description of neutron-rich nuclei. One key property of neutron matter is its superfluid behavior in a range of densities relevant to the inner crust of neutron stars. This work investigates the finite-size effects in the pairing gap of a pure neutron matter superfluid system at densities found in the inner crust of cold neutron stars. The BCS (Bardeen-Cooper-Schrieffer) treatment of superfluidity gives rise to the mean-field pairing gap, while a projection after variation (PBCS theory) can provide a beyond-mean-field pairing gap through an odd-even staggering formula. While these two pairing gap results should agree in the thermodynamic limit, in this work we demonstrate that this is the case for systems far from the thermodynamic limit as well. These results can prove valuable to the study of neutron matter since they can connect ab initio approaches to other approaches found in the literature. This is a first step towards a model-independent extraction of the pairing gap in neutron matter.

The nuclear continuum may potentially be investigated by means of fusion-evaporation reactions in combination with the $Hk$ technique. The $\gamma$ radiation in such experiments can be analyzed using energy ordering which produces complex spectra. A method for computing the shape of the energy-ordered spectra is presented and benchmarked by reproducing simulated data. The spectrum shape computation is applied to experimentally relevant cases and is used for extracting the level density parameter from simulations. Furthermore, the calculation shows to be useful for the characterization of nuclear phase transitions.

The quadrupole-octupole coupling and the related spectroscopic properties have been studied for the even-even light actinides ${}^{218–238}\mathrm{Ra}$ and ${}^{220–240}\mathrm{Th}$. The Hartree-Fock-Bogoliubov approximation, based on the Gogny-D1M energy density functional, has been employed as a microscopic input, i.e., to obtain (axially symmetric) mean-field potential energy surfaces as functions of the quadrupole and octupole deformation parameters. The mean-field potential energy surfaces have been mapped onto the corresponding bosonic potential energy surfaces using the expectation value of the $sdf$ interacting boson model (IBM) Hamiltonian in the boson condensate state. The strength parameters of the $sdf$ IBM Hamiltonian have been determined via this mapping procedure. The diagonalization of the mapped IBM Hamiltonian provides energies for positive- and negative-parity states as well as wave functions which are employed to obtain transitional strengths. The results of the calculations compare well with available data from Coulomb excitation experiments and point towards a pronounced octupole collectivity around ${}^{224}\mathrm{Ra}$ and ${}^{226}\mathrm{Th}$.