Nuclear structure effects are essential for describing hyperfine splittings from high-precision atomic spectroscopy measurements. These effects are often parametrized by the effective or elastic Zemach radii, with their difference being poorly understood. We show that the long-standing discrepancy between the effective and elastic Zemach radii in ${}^6\mathrm{Li}$ and ${}^7\mathrm{Li}$ could be resolved by taking into account nuclear polarizability effects in ab initio nuclear structure calculations. Our results demonstrate that nuclear polarizability effects, negligible in ${}^7\mathrm{Li}$, dominate in ${}^6\mathrm{Li}$ and explain the observed significant deviation between the effective and elastic Zemach radii. Furthermore, we show that the ratios between the nuclear polarizability contributions in different nuclei are universal in the limit of closure and SU(4) symmetry of nuclear forces. In particular, the nuclear polarizability contribution in an odd-odd nucleus is enhanced by a factor of ${\mu }_p/({\mu }_p+{\mu }_n)\simeq 3$, with ${\mu }_{n,p}$ denoting the nucleon magnetic moments, compared to its odd-$A$ isotopes. The same mechanism also explains the Zemach radius deviations observed in ${}^2\mathrm{H}$ and ${}^3\mathrm{He}$. These findings establish nuclear polarizability as the dominant source of isotope-dependent nuclear corrections to hyperfine splitting in light atoms.

Forbidden $\beta$ transitions are important for understanding the reactor antineutrino anomaly. Using many-body perturbation theory with two- plus three-nucleon interactions, we have derived the valence-space effective Hamiltonian and effective operators for first-forbidden transitions. 20 dominant first-forbidden transitions have been investigated, which provide important contributions to the reactor antineutrino spectrum anomaly. Calculated $logft$ values are in reasonable agreement with experimental data. Obtained shape factors exhibit significant deviations from the values approximated with forbidden transitions treated as allowed transitions. The “5 MeV bump” observed in the experimental ${}^{235}\mathrm{U}$-fission antineutrino spectrum was discussed with self-consistent shape factors obtained in the present work. No empirical quenching factors are required in the present calculations of the first-forbidden transitions.

We consider scenarios in which a shallow bound state undergoes breakup by a probe whose energy is high compared to the binding energy. The first two scenarios, which serve as warm-up exercises, involve a single heavy particle bound to a light particle, analogous to a core nucleus bound to a neutron. We show that in quasifree kinematics, the leading effect comes from the heavy particle being knocked out by the probe, with corrections suppressed by inverse powers of the probe momentum. This formally justifies extracting neutron form factors from high-energy deuteron breakup in quasifree kinematics. In scenario 1, the probe is a local current; in scenario 2, it is hadron scattering. In scenarios 3 and 4 we consider, respectively, a local current and hadron scattering, but now on a three-body bound state of a heavy particle and two light particles. Hard knockout of the heavy particle leaves two low-energy particles behind, which can interact with one another. In all four scenarios, we prove that the amplitude is dominated by the nearby on-shell pole of the heavy-particle propagator and derive a closed-form expression for this contribution. When two bodies are left behind, the leading amplitude is the product of the scattering of the two light particles, a dynamical function depending on the probe, and a real function related to the bound-state wave function. Thus, quasifree removal of a core nucleus from a system with halo neutrons provides access to on-shell data on multineutron interactions. The resulting amplitudes are relativistic and satisfy unitarity for the remnant subsystem exactly. We also provide complementary nonrelativistic derivations. While the derivations are for spinless particles, the generalization to spin is straightforward, since the results depend only on quasifree knockout kinematics; we make no assumptions about the interparticle dynamics.

Inclusive breakup measurements detect only part of a projectile while the remaining fragments interact with the target. For a projectile with three constituents, these measurements can probe correlations that are lost when the projectile is treated as two preformed clusters. I derive cross-section sum rules within the distorted-wave Born approximation for detecting either a pair or a single constituent, retaining the full three-body projectile wave function. For pair detection, the detected state is projected from this wave function. Comparing the resulting reaction source with its factorized limit tests, at the amplitude level, what the preformed-cluster approximation omits. For single-particle detection, both unobserved constituents can interact with the target, and their absorption includes a contribution mediated by target excitation that is absent from an independent-fragment description. The optical reduction recovers the Carlson-Frederico-Hussein absorption kernel [Phys. Lett. B 767, 53 (2017)]; explicit coupling of the detected particle to target excitation adds further contributions. The ${}^6\mathrm{Li}=\alpha +n+p$ example shows how neutron-proton states beyond the bound deuteron enter the reaction source. I give a route for calculating the single-particle spectrum using inelastic target transition form factors and three-body propagation.

It has been conventionally assumed that triaxial deformation breaks the degeneracy of pseudospin doublets, leading to less discussion of pseudospin partner bands in triaxial nuclei. However, by considering the pairing correlations, the quasiparticle states of pseudospin doublets are found to remain nearly degenerate at appropriate Fermi energies in triaxial systems. In this work, we systematically investigate the pseudospin partner bands based on the $\pi (g_{7/2},d_{5/2})$ configuration in triaxial nuclei using the particle rotor model. For the first time, we propose the possible coexistence of pseudospin partner bands and wobbling excitation. Further analysis of angular momentum geometry and wave function components supports the identification of one structure as a pseudospin partner band and the other as a wobbling band. This study reveals a novel coexistence mode between pseudospin partner bands and wobbling excitation, illustrating the diversity of rotational excitations in atomic nuclei.

In this work, we approach for the first time the task of performing a shell-model calculation of the matrix element for neutrinoless double-$\beta$ decay, within a fully-consistent framework where the expressions of the nuclear Hamiltonian and of the decay operators have been derived through chiral perturbation theory. More precisely, the effective shell-model Hamiltonian and all transition operators have been constructed by way of many-body perturbation theory, and then employed to calculate both spectroscopic properties of the nuclei involved in the decays under our consideration—namely ${}^{48}\mathrm{Ca}$, ${}^{76}\mathrm{Ge}$, and ${}^{82}\mathrm{Se}$—as well as the nuclear matrix elements of the electromagnetic and neutrinoless double-$\beta$ decays. We also present a study of the convergence properties of the calculated matrix elements in order to provide the elements for an estimate of the theoretical uncertainty.

Dilute gas-like configurations of $\alpha$ particles, often termed $\alpha$ condensates, have been proposed to exist in certain excited states of $\alpha$-conjugate nuclei. This work investigates the structural and decay properties of $\alpha$-condensate candidates within a few-body framework using leading-order cluster effective field theory interactions. To isolate dilute $N\alpha$ configurations, a constrained model space strategy is implemented inspired by Yamada and Schuck [Phys. Rev. C 69, 024309 (2004)], which constructs a description specifically for suprathreshold states. By explicitly decoupling the nonuniversal, compact configurations lying below the $N\alpha$ threshold, this framework describes $N\alpha$ condensates as quasibound states localized within the Coulomb barrier. The stochastic variational method is used to perform theoretical calculations for the proposed $N\alpha$ condensates ranging from ${}^8\mathrm{Be}$ to ${}^{20}\mathrm{Ne}$. The results not only map the formation of these novel cluster states in heavier $\alpha$-conjugate nuclei but also offer critical insights into the applicability of few-body universality to charged particles.

New dispersive-optical-model analyses of ${}^{54}\mathrm{Fe}$ and ${}^{90}\mathrm{Zr}$ together with updated results for ${}^{40}\mathrm{Ca}$ and ${}^{48}\mathrm{Ca}$ and the earlier result for ${}^{208}\mathrm{Pb}$ shed light on the behavior of neutron skins in nuclei. Starting with a negative skin for ${}^{40}\mathrm{Ca}$, a trend increasing somewhat stronger than linear emerges when the neutron skin of these nuclei is considered as a function of asymmetry, $(N-Z)/A$, and linked to the Green's function Monte Carlo results for asymmetric He nuclei. This general trend is consistent with the expectation that nuclei near the neutron drip line are expected to have very large neutron skins. The present analysis therefore motivates the question of which nuclei provide the most relevant link to neutron star physics.

New experimental data for the ${}^6\mathrm{He}(d,t){}^5\mathrm{He}$ one-neutron pickup reaction at 52 MeV were measured in inverse kinematics at the ACCULINNA-2 isotope in-flight separator at the Flerov Laboratory of Nuclear Reactions, Joint Institute for Nuclear Research, Dubna. The data were analyzed using two standard direct reaction models, the distorted wave Born approximation (DWBA) and coupled reaction channels. Single neutron wave functions for the ${}^6\mathrm{He}$ ground state obtained using three different nuclear structure methods, including the new cluster channel orthogonal function method, were tested and the dependence of the results of the reaction calculations on the properties of these wave functions was observed. The data were well reproduced by a DWBA calculation with the wave function obtained using the ab initio variational Monte Carlo method with no adjustable parameters.

The detailed excitation functions for ${}^{13}\mathrm{C}(\alpha ,\alpha )$ elastic scattering and the additional thick target inverse kinematics measurements were analyzed using the $R$ matrix approach. The analysis covered the excitation region in ${}^{17}\mathrm{O}$ from 8.6 up to 11.2 MeV. Information on single particle widths of the levels in the ${}^{17}\mathrm{O}$ nucleus and shell model calculations are in good agreement. A comparative analysis of the $\alpha$-cluster structures of ${}^{16}\mathrm{O}$ and ${}^{17}\mathrm{O}$ nuclei indicates their common underlying nature. However, the structure in ${}^{17}\mathrm{O}$ is more complex. In particular, each $\alpha$-cluster level with spin $J$ in ${}^{16}\mathrm{O}$ corresponds to several levels with spins $J=\pm 1/2$ in ${}^{17}\mathrm{O}$. Unexpectedly, the reduced-width-weighted centroids of the higher-spin members of these doublets always lie at lower excitation energies in ${}^{17}\mathrm{O}$.

Two-body breakup mechanisms of the weakly bound nucleus ${}^{10}\mathrm{B}$ were investigated through an exclusive coincidence measurement in the ${}^{10}\mathrm{B}+{}^{197}\mathrm{Au}$ system at a beam energy of 57 MeV. Coincident detection of two outgoing charged particles by an array of Si strip detector telescopes shows that the dominant two-body projectile breakup channels are ${}^6\mathrm{Li}+\alpha$ and ${}^9\mathrm{Be}+p$. Projectile excitation energies, reconstructed from the kinetic energies of the fragments, are found to reproduce the energies of specific excited states of ${}^{10}\mathrm{B}$ and thereby confirm the coexistence of both ${}^6\mathrm{Li}\text{−}\alpha$ and ${}^9\mathrm{Be}\text{−}p$ two-body cluster configurations for ${}^{10}\mathrm{B}$. Continuum discretized coupled channels (CDCC) calculations, considering each of the above two cluster configurations separately, were able to explain the experimental cross sections for both breakup as well as elastic scattering simultaneously.

We investigate the formation of light nuclei and hypernuclei in $\mathrm{Au}+\mathrm{Au}$ collisions at $\sqrt{s_{NN}}=3$ GeV within a coalescence framework embedded in the microscopic N-body parton-hadron-quantum-molecular dynamics (PHQMD) transport model. The Wigner phase-space distributions employed in the coalescence calculation are constructed from realistic $N$-body wave functions obtained by solving the Schrödinger equation in the hyperspherical harmonics formalism, providing a solid and parameter-free description of nuclear clusters and hypernuclei. By comparing calculated rapidity distributions with STAR data, we extract species-dependent coalescence times, revealing a non-universal formation pattern among different clusters. The resulting yields and kinematic distributions of light nuclei and hypernuclei are systematically analyzed and shown to be sensitive to the underlying wave-function structure and formation time. In addition, we explore cluster-nucleon formation channels for $A=4$ systems. These additional channels improve the description of ${}^4\mathrm{He}$ and ${}_{\mathrm{\Lambda }}{}^4\mathrm{H}$ yields and help address the underestimation of $A=4$ cluster production in theoretical approaches. Finally, we provide predictions for heavier hypernuclei, including ${}_{\mathrm{\Lambda }}{}^5\mathrm{He}$ and ${}_{\mathrm{\Lambda }\mathrm{\Lambda }}{}^5\mathrm{He}$, which are of interest for future experimental measurements.

We present a comparison of the ultrarelativistic quantum molecular dynamics model using a chiral mean field (CMF) equation of state with flow and pion production data in heavy ion collisions at low beam energies $E_{\mathrm{lab}}=0.2–2.0A$ GeV and varying isospin fraction. The CMF model parameters are constrained by known properties of the high density equation of state of quantum chromodynamics at varying isospin fractions. This allows us to calculate the equation of state as well as nuclear interactions for different physical systems like neutron stars and heavy ion collisions in a consistent way. It is found that heavy ion reactions at the upcoming FAIR facility will only have marginal sensitivity on the isospin dependence of the high-density equation of state. At lower beam energies, comparing to FOPI, HADES, and $S\pi \text{RIT}$ data, the sensitivity is higher but the observed deviations and systematic uncertainties of the existing data are larger than the sensitivity to the symmetry energy.

It is well known that nonrelativistic or semirelativistic constituent quark models can describe a large number of the meson and baryon properties with surprising accuracy. In this work, we study the Killingbeck potential using the WKB approximation in the constituent quark model. The implicit approximate solutions for the energy eigenvalues, together with analytical expressions for the root-mean-square radii and the probability densities at the origin, of the Schrödinger equation are obtained by using appropriate quantization conditions. Finally, we discuss applications to double-heavy-meson spectroscopy within the Killingbeck potential.

We investigate the thermal evolution of isolated neutron stars containing hyperon-mixed kaon-condensed matter, focusing on the role of proton superconductivity. The equation of state utilized for cooling calculation is based upon the minimal relativistic mean-field framework supplemented by chiral SU(3) dynamics for kaon condensation with an additional component on the three-baryon force, which ensures stiffness at high densities enough to meet astrophysical constraints on neutron-star masses and radii. We show that the nucleonic direct Urca processes operate at relatively low stellar masses $(M\gtrsim 1.3M_{\odot })$, erasing any observable signature of strangeness in the absence of superfluidity. However, if the proton ${}^1{\mathrm{S}}_0$ superconductivity works, because of suppression of fast neutrino cooling processes, the cooling scenario could become relevant with the strangeness, depending on the density regions of the pairing gap. In particular, if the proton superconductivity is so strong in high-density regions $(T_{c,p}\sim {10}^{10}\mathrm{K})$, the nucleon and hyperon direct Urca processes shut down, which makes the kaon-induced Urca processes dominant in massive neutron stars. This scenario is in good agreement with several cold isolated neutron stars identified recently, such as the Vela Jr., PSR $\mathrm{J}0205+6449$, and PSR B2334 $+$ 61. Hence, we suggest that strong proton superconductivity can render kaon condensation observationally visible through cold neutron-star observations, providing a potential signature of strangeness in dense matter.

The astrophysical $S$ factor for the radiative capture reaction ${}^3\mathrm{He}(\alpha ,\gamma ){}^7\mathrm{Be}$ plays a crucial role in big bang nucleosynthesis as well as stellar hydrogen burning. Its precise value is important in the context of primordial elemental abundances and the solar neutrino fluxes. In the present work we carry out a data-driven analysis of the $S$ factor using a Bayesian neural network. Unlike conventional neural networks, the Bayesian formulation produces a posterior distribution of $S$ factor curves consistent with experimental data and their uncertainties. From this distribution we extract the zero-energy astrophysical $S$ factor $S_{34}\left(0\right)=0.{544}_{-0.021}^{+0.022}\mathrm{keV}\mathrm{b}$. The posterior ensemble is further used to obtain reaction rates over the temperature range $T=0.005–4$ GK. The results are in agreement with existing calculations. This work provides an alternative framework for low-energy $S$ factor determination and uncertainty quantification in nuclear astrophysics.

I derive the volume dependence of bound states from a cluster-cluster picture with nucleon degrees of freedom. A constant factor called the geometric factor appears in the generalization from pointlike particles to clusters. I show that this factor is obtained explicitly when the underlying overlap integral is evaluated directly in the original many-body space. I achieve this by constructing Jacobi coordinates on the lattice with periodic boundary conditions. The derivation requires only the interaction between the clusters to be short ranged, with the nonperturbative Coulomb force included. The factor emerges as a geometric property of the many-body configuration space rather than a multiplicity of physical channels. I validate my derivation using many-body calculations; in particular, I find this factor to be essential in extracting asymptotic normalization constants from lattice calculations of the ${}^{16}\mathrm{O}$ ground state.