The present work deals with $e^{+}\text{−}{e}^{-}$ pair production in the four-nucleon system. We first analyze the process as a purely electromagnetic one in the context of a state-of-the-art approach to nuclear strong-interaction dynamics and nuclear electromagnetic currents, derived from chiral effective field theory $\left(\chi \mathrm{EFT}\right)$. Next, we examine how the exchange of a hypothetical low-mass boson would impact the cross section for such a process. We consider several possibilities, that this boson is either a scalar, pseudoscalar, vector, or axial particle. The ab initio calculations use exact hyperspherical-harmonics methods to describe the bound state and low-energy spectrum of the $A=4$ continuum, and they fully account for initial state interaction effects in the $3+1$ clusters. While electromagnetic interactions are treated to high orders in the chiral expansion, the interactions of the hypothetical boson with nucleons are modeled in leading-order $\chi \mathrm{EFT}$ (albeit, in some instances, selected subleading contributions are also accounted for). We also provide an overview of possible future experiments probing pair production in the $A=4$ system at a number of candidate facilities.

Maximal ground-state deformation should occur when both proton and neutron Fermi surfaces are located at midshell. However, subshell gaps that stabilize large deformation can exist at proton or neutron values other than midshell. One such gap may occur at $Z=60$ in the rare-earth region, as the energy of the first $2^{+}$ states in even-even nuclei are often lowest in an isotonic chain for neodymium ($Z=60$) rather than the midshell isotopes of dysprosium ($Z=66$). Further evidence of this deformed gap has now been observed by investigating the signature splitting systematics of the $\nu i_{13/2}$ bands found in the odd-$N$, rare-earth nuclei. These were aided by the present observation of the $\nu i_{13/2}$ band in ${}^{159}\mathrm{Gd}$ and the confirmation of the same structure in ${}^{155}\mathrm{Sm}$ via the transfer of a neutron from a ${}^{160}\mathrm{Gd}$ beam to a ${}^{154}\mathrm{Sm}$ target.

We propose a novel storage scheme for three-nucleon (3N) interaction matrix elements relevant for the normal-ordered two-body approximation used extensively in ab initio calculations of atomic nuclei. This scheme reduces the required memory by approximately two orders of magnitude, which allows the generation of 3N interaction matrix elements with the standard truncation of $E_{3\max }=28$, well beyond the previous limit of 18. We demonstrate that this is sufficient to obtain the ground-state energy of ${}^{132}\mathrm{Sn}$ converged to within a few MeV with respect to the $E_{3\max }$ truncation. In addition, we study the asymptotic convergence behavior and perform extrapolations to the un-truncated limit. Finally, we investigate the impact of truncations made when evolving free-space 3N interactions with the similarity renormalization group. We find that the contribution of blocks with angular momentum $J_{\mathrm{rel}}>9/2$ to the ground-state energy is dominated by a basis-truncation artifact, which vanishes in the large-space limit, so these computationally expensive components can be neglected. For the two sets of nuclear interactions employed in this work, the resulting binding energy of ${}^{132}\mathrm{Sn}$ agrees with the experimental value within theoretical uncertainties. This work enables converged ab initio calculations of heavy nuclei.

Magnetic-dipole strength functions have been deduced from averages of large numbers of $M1$ transition strengths calculated within the shell model for the nuclides ${}^{105}\mathrm{Cd}$, ${}^{106}\mathrm{Cd}$, ${}^{111}\mathrm{Cd}$, and ${}^{112}\mathrm{Cd}$. Enhancements of the $M1$ strengths toward low transition energy have been found for all nuclides considered. These properties are compared with those of experimental photon strength functions obtained from ${}^3\mathrm{He}$-induced reactions, which seem to indicate a disappearance of the low-energy enhancement in the heavier isotopes.

We investigate the elastic scattering, inelastic scattering, breakup reaction, and total fusion reactions of the ${}^{17}\mathrm{F}+{}^{208}\mathrm{Pb}$ system using an optical model approach by including the dynamical polarization potential. In particular, we focus on the breakup reaction channels, whose effects on other reactions have been argued to be small. By exploiting two different potentials composed of a surface-type Woods-Saxon potential, which were obtained by a fitting analysis, and a Love-type optical potential, the breakup reaction cross sections are explained with other elastic and quasielastic scattering cross sections. For the breakup reaction at $E_{\text{lab}}=170$ MeV in the forward angle region, where only exclusive reaction data are known, inclusive breakup reaction data are deduced from available theoretical and experimental $E1$ strength values. We discuss the breakup reaction effects on the other nuclear reactions related to the ${}^{17}\mathrm{F}$ projectile in detail.

The present paper proposes an improved version of the $\alpha$-nucleus optical model potential ($\alpha$-OMP), in which the real part of the potential is determined by the double-folding calculation using the density-dependent CDM3Y1 interaction plus a repulsive potential. The imaginary part of the $\alpha$-OMP is treated by a Woods-Saxon form with energy-dependent depth, while its real part is corrected by including the contribution of the dispersion relation derived from the strong variation of the potential at energies in the vicinity of the Coulomb barrier. The proposed potential is validated by calculating the $(\alpha ,n)$ and $(\alpha ,\gamma )$ cross sections on different target nuclei and the obtained results are compared with those predicted by the recent $\alpha$-OMP suggested by Avrigeanu, as well as the available experimental data. The impacts of the energy-dependent imaginary part and the real dispersive term on the $\alpha$-induced cross sections are also investigated. The results obtained show that the proposed $\alpha$-OMP together with that of Avrigeanu describe reasonably well the experimental cross sections of all $(\alpha ,n)$ reactions being considered. In addition, the imaginary part of the proposed potential has a strong energy-dependent effect on the $\alpha$-induced cross section in the energy region below the Coulomb barrier, in particular for radiative $\alpha$-capture reactions. Two potentials also describe well the cross sections of $(\alpha ,\gamma )$ reactions after rescaling the radiative $\gamma$ width. This leads to our conclusion that the present $\alpha$-OMP can provide precise $\alpha$ widths for the study of reactions relevant to the production of $p$ nuclei, and therefore more intensive theoretical research on the radiative strength functions is required.

The chiral magnetic effect (CME) is predicted to occur as a consequence of a local violation of $\mathcal{P}$ and $\mathcal{C}P$ symmetries of the strong interaction amidst a strong electromagnetic field generated in relativistic heavy-ion collisions. Experimental manifestation of the CME involves a separation of positively and negatively charged hadrons along the direction of the magnetic field. Previous measurements of the CME-sensitive charge-separation observables remain inconclusive because of large background contributions. To better control the influence of signal and backgrounds, the STAR Collaboration performed a blind analysis of a large data sample of approximately 3.8 billion isobar collisions of ${}_{44}{}^{96}\mathrm{Ru}+{}_{44}{}^{96}\mathrm{Ru}$ and ${}_{40}{}^{96}\mathrm{Zr}+{}_{40}{}^{96}\mathrm{Zr}$ at $\sqrt{s_{{}_{NN}}}=200$ GeV. Prior to the blind analysis, the CME signatures are predefined as a significant excess of the CME-sensitive observables in $\mathrm{Ru}+\mathrm{Ru}$ collisions over those in $\mathrm{Zr}+\mathrm{Zr}$ collisions, owing to a larger magnetic field in the former. A precision down to 0.4% is achieved, as anticipated, in the relative magnitudes of the pertinent observables between the two isobar systems. Observed differences in the multiplicity and flow harmonics at the matching centrality indicate that the magnitude of the CME background is different between the two species. No CME signature that satisfies the predefined criteria has been observed in isobar collisions in this blind analysis.

Parity violating (PV) as well as parity and time-reversal invariance violating (PTRIV) effects are enhanced a million times in neutron reactions near $p$-wave compound resonances. We present the calculation of such effects using a statistical theory based on the properties of chaotic eigenstates and discuss a possibility to extract the strength constants of PTRIV interactions from the experimental data, including nucleon-nucleon and pion-nucleon CP-violating interactions, the QCD $\theta$-term, and the quark chromo-EDM. PV effects have random signs for all target nuclei except for ${}^{232}\mathrm{Th}$, where PV effects of a positive sign have been observed for ten statistically significant $p$-wave resonances, with energy smaller than 250 eV. This may be an indication of a possible regular (nonchaotic) contribution to PV effects. We link this regular effect to the doublets of opposite parity states in the rotation spectra of nuclei with an octupole deformation and suggest other target nuclei where this hypothesis may be tested. We also discuss a permanent sign contribution produced by doorway states. An estimate of the ratio of PTRIV effects to PV effects is presented. Although a polarized target is not needed for the measurement of PV effects, for the interpretation of the results, it may be convenient to do both PV and PTRIV experiments with a polarized target.

Weak interaction charged current transition strengths from highly excited nuclear states are fundamental ingredients for accurate modeling of compact object composition and dynamics, but are difficult to obtain either from experiment or theory. For lack of alternatives, calculations have often fallen back upon a generalized Brink-Axel hypothesis, that is, assuming the strength function (transition probability) is independent of the initial nuclear state but depends only upon the transition energy and the weak interaction properties of the parent nucleus ground state. Here we present numerical evidence for a modified “local” Brink-Axel hypothesis for Gamow-Teller transitions for $pf$-shell nuclei relevant to astrophysical applications. Specifically, while the original Brink-Axel hypothesis does not hold globally, strength functions from initial states nearby in energy are similar within statistical fluctuations. This agrees with previous work on strength function moments. Using this modified hypothesis, we can tackle strength functions at previously intractable initial energies, using semiconverged initial states at arbitrary excitation energy. Our work provides a well-founded method for computing accurate thermal weak transition rates for medium-mass nuclei at temperatures occurring in stellar cores near collapse. We finish by comparing results to previous calculations of astrophysical rates.

Neutron skin thickness ($\mathrm{\Delta }{r}_{\mathrm{np}}$) of nuclei and the inferred nuclear symmetry energy are of critical importance to nuclear physics and astrophysics. It is traditionally measured by nuclear processes with significant theoretical uncertainties. We recently proposed an indirect measurement of the $\mathrm{\Delta }{r}_{\mathrm{np}}$ by charged hadron multiplicities in central isobaric collisions at relativistic energies, which are sensitive to nuclear densities. In this Letter, we propose a direct measurement of the $\mathrm{\Delta }{r}_{\mathrm{np}}$ by using net-charge multiplicities in ultraperipheral (grazing) collisions of those isobars, under the assumption that they are simple superimposition of nucleon-nucleon interactions. We illustrate this novel approach by the trento and urqmd models.

Realistic nuclear structure calculations are presented for the event rates due to coherent elastic neutrino-nucleus scattering ($\mathrm{CE}\nu \mathrm{NS}$), assuming neutrinos from pion decay at rest, from nuclear reactors, and from Earth's interior. We focus on the currently interesting germanium isotopes, ${}^{70,73,76}\mathrm{Ge}$, which constitute detector materials of the recently planned $\mathrm{CE}\nu \mathrm{NS}$ experiments. We study in addition the potential use of ${}^{64,70}\mathrm{Zn}$ and ${}^{28}\mathrm{Si}$ isotopes as promising $\mathrm{CE}\nu \mathrm{NS}$ detectors. From nuclear physics perspectives, recently, calculations have been carried out within the framework of the deformed shell model (DSM), based on realistic nuclear forces, and assessed on the reproducibility of spectroscopic nuclear properties. The high confidence level acquired by their agreement with experimental results and by their comparison with other mostly phenomenological calculations encouraged the use of DSM to extract predictions for the $\mathrm{CE}\nu \mathrm{NS}$ event rates of the above isotopes. Our detailed estimation of the nuclear physics aspects of the recently observed neutral current coherent neutrino-nucleus scattering may shed light on unravelling the still remaining uncertainties for the $\mathrm{CE}\nu \mathrm{NS}$ process within and beyond the standard model.

Machine learning as a very powerful tool has recently been applied in many nuclear aspects. In this paper, a straightforward $k$-nearest-neighbor algorithm (KNN) combined with the general description of the fission observable (gef) method is proposed as an easy machine learning algorithm to learn existing fission product yields (FPYs) and predict unknowns. The results show that the KNN $+$ gef method can properly take account of the similarities of fission processes and the systematic properties of FPYs. Due to KNN $+$ gef, to use the experimental FPY data from the several nearest-neighboring isotopes, this method can provide an effective tool to predict the FPY data of the nuclei for which the less experimental data are available.

We propose closed-form expressions of the distributions of magnetic quantum number $M$ and total angular momentum $J$ for three and four fermions in single-$j$ orbits. The latter formulas consist of polynomials with coefficients satisfying congruence properties. Such results, derived using doubly recursive relations over $j$ and the number of fermions, enable us to deduce explicit expressions for the total number of levels in the case of three-, four-, and five-fermion systems. We present applications of these formulas, such as sum rules for six-$j$ and nine-$j$ symbols, obtained from the connection with fractional-parentage coefficients, an alternative proof of the Ginocchio-Haxton relation, or cancellation properties of the number of levels with a given angular momentum.

The nuclear multiple wobbling modes in $A\approx 60$ mass region are explored by the adiabatic and configuration-fixed constrained covariant density functional theory for nickel isotopes for the first time. The potential-energy curves and deformation parameters $\beta$ and $\gamma$ of the ground and excited states of ${}^{57\text{–}62}\mathrm{Ni}$ are obtained to search for the possible wobbling candidates. It is shown that there are suitable triaxial deformations and high-$j$ particle(s) or hole(s) configurations in ${}^{57\text{–}62}\mathrm{Ni}$, which are beneficial to the appearance of wobbling motion, including the simple, longitudinal and transverse modes. Moreover, possible multiple wobbling motions based on different high-$j$ configurations might coexist in one single nucleus, which provides a promising basis for future experiments to find wobbling bands in the $A\approx 60$ mass region. Finally, taking the nucleus ${}^{62}\mathrm{Ni}$ as an example, the experimental energy spectra, the angular momentum components, and the azimuthal plots are investigated by the combination of the covariant density functional theory and quantum particle rotor model. The transverse wobbling mode is suggested in this nucleus.

A radioactive beam of ${}^{24}\mathrm{Na}$ with $90\%$ of its content in its $1^{+}$ isomeric state (${\mathrm{E}}_{ex}$ = 0.472 MeV, $t_{1/2}$ = 20.18 ms) has been developed and used to perform a measurement of the ${}^{24}\mathrm{Na}^m(d,p){}^{25}\mathrm{Na}$ reaction at the John D. Fox Accelerator Laboratory at Florida State University. This reaction selectively populated $\ell$ = 0 transfers, allowing the study of low-spin states in ${}^{25}\mathrm{Na}$. Mirror symmetry arguments were then used to investigate the effects of the isomeric state of ${}^{24}\mathrm{Al}$ (${\mathrm{E}}_{ex}$ = 0.426 MeV, $t_{1/2}$ = 130 ms) on the astrophysical rate of the ${}^{24}\mathrm{Al}^m(p,\gamma ){}^{25}\mathrm{Si}$ reaction. Experimental parameters were extracted to provide, for the first time, an experimental reaction rate for the destruction of ${}^{24}\mathrm{Al}$ via proton captures in its isomeric state relevant to $rp$-process nucleosynthesis.

Precision tests of the standard model and searches for beyond the standard model physics often require nuclear structure input. There has been a tremendous progress in the development of nuclear ab initio techniques capable of providing accurate nuclear wave functions. For the calculation of observables, matrix elements of complicated operators need to be evaluated. Typically, these matrix elements would contain spurious contributions from the center-of-mass (c.m.) motion. This could be problematic when precision results are sought. Here, I derive a transformation relying on properties of harmonic oscillator wave functions that allows an exact removal of the c.m. motion contamination applicable to any one-body operator depending on nucleon coordinates and momenta. Resulting many-nucleon matrix elements are translationally invariant provided that the nuclear eigenfunctions factorize as products of the intrinsic and c.m. components as is the case, e.g., in the no-core shell model approach. An application of the transformation has been recently demonstrated in calculations of the nuclear structure recoil corrections for the $\beta$ decay of ${}^6\mathrm{He}$.

Capture of electrons by nuclei is an important process in stellar environments where excited nuclear states are thermally populated. However, accurate treatment for excited configurations in electron capture (EC) rates has been an unsolved problem for medium-heavy and heavy nuclei. In this work, we take the ${}^{93}\mathrm{Nb}\to {}^{93}\mathrm{Zr}$ EC rates as the example to introduce the projected-shell model (PSM) in which excited configurations are explicitly included as multi-quasiparticle states. Applying the prevalent assumption that the parent nucleus always stays in its ground state in stellar conditions, we critically compare the obtained PSM results with the recently measured Gamow-Teller transition data, and with the previous calculations by the conventional shell model and the quasiparticle random-phase approximation. We discuss important ingredients that are required in theoretical models used for stellar EC calculations, and demonstrate effects of the explicit inclusion of excited nuclear states in EC rate calculations, especially when both electron density and environment temperature are high.

${}^{72}\mathrm{As}$ is a promising positron emitter for diagnostic imaging that can be employed locally using a ${}^{72}\mathrm{Se}$ generator. However, current reaction pathways to ${}^{72}\mathrm{Se}$ have insufficient nuclear data for efficient production using regional 100–200 MeV high-intensity proton accelerators. In order to address this deficiency, stacked-target irradiations were performed at LBNL, LANL, and BNL to measure the production of the ${}^{72}\mathrm{Se}/{}^{72}\mathrm{As}$ positron emission tomography (PET) generator system via ${}^{75}\mathrm{As}(p,x)$ between 35 and 200 MeV. This work provides the most well-characterized excitation function for ${}^{75}\mathrm{As}(p,4n){}^{72}\mathrm{Se}$ starting from threshold. Additional focus was given to report the first measurements of ${}^{75}\mathrm{As}(p,x){}^{68}\mathrm{Ge}$ and bolster an already robust production capability for the highly valuable ${}^{68}\mathrm{Ge}/{}^{68}\mathrm{Ga}$ PET generator. Thick target yield comparisons with prior established formation routes to both generators are made. In total, high-energy proton-induced cross sections are reported for 55 measured residual products from ${}^{75}\mathrm{As}, {}^{\mathrm{nat}}\mathrm{Cu}$, and ${}^{\mathrm{nat}}\mathrm{Ti}$ targets, where the latter two materials were present as monitor foils. These results were compared with literature data as well as the default theoretical calculations of the nuclear model codes talys, coh, empire, and alice. Reaction modeling at these energies is typically unsatisfactory due to few prior published data and many interacting physics models. Therefore, a detailed assessment of the talys code was performed with simultaneous parameter adjustments applied according to a standardized procedure. Particular attention was paid to the formulation of the two-component exciton model in the transition between the compound and preequilibrium regions, with a linked investigation of level density models for nuclei off of stability and their impact on modeling predictive power. This paper merges experimental work and evaluation techniques for high-energy charged-particle isotope production in an extension to an earlier study of this kind.

The so-called super-short fission mode, in which a nucleus divides nearly symmetrically into two unusually energetic fragments, competes favorably with the standard asymmetric fission mode for spontaneous fission of a limited number of nuclei near ${}^{264}\mathrm{Fm}$ but it quickly fades away at finite excitations. We investigate the energy-dependent competition between those two fission modes for even fermium isotopes from ${}^{254}\mathrm{Fm}$ to ${}^{268}\mathrm{Fm}$, using the Metropolis method to simulate the strongly damped fission dynamics being driven by shape- and energy-dependent level densities. The origin of the super-short mode is discussed and its effects on the fragment mass distribution, the total fragment kinetic energy, and the neutron multiplicity are calculated. Generally good agreement with the available data is obtained.

The semi-inclusive correlator for a $\mathcal{J}$=1/2 bound system, composed by $A$ spin-1/2 fermions, is linearly expressed in terms of the light-front Poincaré covariant spin-dependent spectral function, in valence approximation. The light-front spin-dependent spectral function is fully determined by six scalar functions that allow for a complete description of the six T-even transverse momentum distributions, suitable for a detailed investigation of the dynamics inside the bound system. The application of the developed formalism to a case with a sophisticated dynamical content, like ${}^3\mathrm{He}$, reaches two goals: (i) to illustrate a prototype of an investigation path for gathering a rich wealth of information on the dynamics and also finding valuable constraints to be exploited from the phenomenological standpoint and (ii) to support for the three-nucleon system a dedicated experimental effort for obtaining a detailed three-dimensional picture in momentum space. In particular, the orbital angular momentum decomposition of the bound state can be studied through the assessment of relations among the transverse momentum distributions, as well as the relevance of the relativistic effect generated by the implementation of macroscopic locality. A fresh evaluation of the longitudinal and transverse polarizations of the neutron and proton is also provided, confirming essentially the values used in the standard procedure for extracting the neutron structure functions from both deep-inelastic scattering and semi-inclusive reactions, in the same kinematical regime.