Recent observations by the ATOMKI Collaboration of anomalies in electron-positron pair production following proton capture on light nuclides has led to the postulation of a new boson with mass around 17 MeV. Here a team of scientists from the United States, Canada, and France presents the most detailed microscopic calculations to date of the proton capture reactions, and is unable to find a conventional explanation for the anomalies. While these calculations do not confirm the existence of the so-called X17 boson, they provide strong motivation for continued and independent experiments to investigate the ATOMKI results. Further refinements of the calculations may provide theoretical constraints for future data.

Nucleosynthesis during the Big Bang (BBN) can produce the lightest elements, including lithium, but the observed abundance of lithium in old stellar populations is much less than that predicted from BBN. To solve this so-called “lithium puzzle”, it had been postulated that a previously unobserved resonance in ${}^7$Be could deplete lithium through resonant proton capture on ${}^6$Li and thus account for the deficit. The authors performed a detailed study using information from all possible reactions that could be influenced by such a state, utilizing the stringent constraint imposed by the unitarity of the scattering matrix. They conclude that the postulated $3/2^{+}$ state in ${}^7$Be is highly unlikely, but they also suggest that additional radiative capture data are required to solve lingering discrepancies.

Nuclear masses are among the most significant observables to elucidate nuclear structure and its evolution with proton and neutron number as well as nucleosynthesis in the cosmos. This is especially so in exotic nuclei where masses will be among the first observables measured for new nuclei. Yet, such measurements are extremely difficult due to the short lifetimes and low production yields at rare isotope beam facilities. The present work describes the Rare-RI Ring facility, an isochronous storage ring at RIKEN, and the first commissioning measurements to establish its capabilities. The full identification of each ion before injection into the storage ring and the measurement time of about 1 ms are excellently suited for measuring masses of the most exotic nuclei, promising a breakthrough in the precision mass spectrometry of extremely rare short-lived radionuclides.

The ${}^{22}$Ne + $\alpha$ reaction has significant impact at the end of the core helium burning phase in red giant stars. Radiative $\alpha$ capture, ($\alpha$,$\gamma$), heats the plasma while the ($\alpha$,$n$) reaction provides neutrons for the weak $s$ process; their interplay determines the efficiency of the latter as a neutron source. The authors measure the strength of the resonance at $E_{\alpha ,\text{lab}}=830$ keV in the ${}^{22}$Ne($\alpha$,$n$)${}^{25}$Mg reaction. This resonance dominates the reaction rate for both the ${}^{22}$Ne($\alpha$,$n$)${}^{25}$Mg reaction and the competing ${}^{22}$Ne($\alpha$,$\gamma$)${}^{26}$Mg radiative capture at temperatures larger than 0.25 GK. As a crucial step, the authors characterize the stilbene neutron detector, where relevant information on the neutron energy is retained, and evaluate the possible sources of neutron background in their measurement. The results significantly improve the characterization of the astrophysically impactful 830 keV resonance.

Understanding nuclear properties away from the stability line and near the limits of the nuclear landscape relies heavily on theoretical calculations, because many unstable nuclei are difficult to reach in experiments. The authors apply two machine learning (ML) models to assess their performance in predicting nuclear mass excesses using available experimental data and a physics-based feature space. The models successfully reproduce known physical relationships and demonstrate a robust capability for extrapolation far beyond the training and test regions, offering results comparable to the model calculations. Incorporating techniques that enhance the interpretability of the ML models highlights their potential as powerful nuclear physics tools.

The authors perform rigorous Bayesian inference on a variety of measurements in relativistic Pb-Pb collisions at the LHC using a comprehensive multistage model combining QCD-based initial states with viscous hydrodynamics and a hadronic afterburner. In particular, they extracted systematic constraints on the temperature dependence of shear and bulk viscosities of quark-gluon plasma that are significantly more precise due to improved physical models and statistical methods. For the range of plasma temperature probed in heavy-ion collisions, they find that the specific bulk viscosity demanded by the data is strongly non-zero and temperature-dependent, whereas the specific shear viscosity shows a much weaker temperature dependence that is indistinguishable from a constant value even with improved statistical analysis. Importantly, the authors showcase the application of transfer learning to efficiently explore a range of model uncertainties wider than had been considered previously. This work represents a substantial advancement in constraining the shear and bulk viscosities of strongly interacting matter.

A chiral effective field theory (EFT) description of the nuclear interaction contains a power counting to organize the order-by-order contributions of the strong-interaction dynamics to nuclear observables. The truncation of the EFT expansion at finite order induces errors in predicted nucleon-nucleon scattering observables. These errors are correlated across scattering energies and angles, which robust uncertainty quantification needs to account for. This work reports a Bayesian analysis for neutron-proton scattering in a so-called $\mathrm{\Delta }$-full version of chiral EFT. The authors employ Gaussian processes to learn about the correlation structure of the truncation errors and find that the effective number of neutron-proton scattering data is reduced by approximately a factor of 4 due to the correlation structure of the EFT truncation error (shown in the figure for differential cross sections). The results are important for analyzing the predictive capabilities in $ab$-$initio$ nuclear theory.

Properties of neutron stars such as their masses and radii arise from the characteristics of highly asymmetric nuclear matter (with many more neutrons than protons) which is not accessible in experiments. The authors consider a family of neutron-star equations of state characterized by a nontrivial behavior of nuclear matter at high densities, including a steep rise and then decline (i.e., a sharp peak) in the speed of sound with density, which is compatible with ultraheavy neutron stars up to 2.5 solar masses. The symmetry-energy expansion is then applied to obtain equations of state applicable to the almost symmetric nuclear matter created in heavy-ion collisions in the laboratory. The authors find that the description incorporating a sharp peak in the speed of sound profile aligns well with experimental data. The systematic approach opens promising perspectives for further investigations bridging the physics of neutron stars and heavy-ion collisions.

This work studies the structure and dynamics of the inner crust of neutron stars where nuclear matter is expected to form slabs or so-called pasta phases. The authors report a first fully self-consistent calculation of the structure of the Coulomb lattice of nuclei immersed in a sea of dripped neutrons, taking fully into account, and on the same footing, both the band structure and superfluid effects. They employ a real-time method to extract the collective masses of a slab and of protons, which in turn quantify the conduction-neutron number density and the neutron effective mass, known as the entrainment effect. The results agree with recent self-consistent band calculations without superfluidity and demonstrate that the neutron effective mass is substantially reduced up to about 42% in the slab phase; superfluidity slightly enhances this anti-entrainment effect. The current one-dimensional formalism can be extended to two and three dimensions once the computational challenges of parallelization have been successfully addressed. This gives hope that the controversial situation concerning the entrainment effects in the inner crust of neutron stars can be resolved.

A first-of-its-kind measurement reveals the energy spectrum of the neutrons produced during the fission of plutonium, a common nuclear fuel component.

Microscopic optical potentials based on realistic nucleon-nucleon interactions are important for describing the scattering phenomenology involving nuclei far away from the valley of nuclear stability. The authors construct a new optical potential by combining the relativistic Brueckner-Hartree-Fock theory with a microscopic description of the density profile of the target nucleus. The new model provides good reproduction of proton scattering data on five target nuclei, opening up interesting perspectives for applications to exotic nuclei, including setting up a reliable framework to investigate isospin effects in nuclear structure from a scattering perspective.

At low temperatures (10–1000 K), negative muons injected into a mixture of deuterium (D) and tritium (T) can catalyze the $d+t$ fusion reaction into helium, which yields a neutron and 17.6 MeV energy. Following the catalyzed reaction, free muons can facilitate another fusion reaction, leading to a cyclic reaction known as muon-catalyzed fusion ($\mu$CF), a potential candidate for energy production. $\mu$CF has recently regained considerable research interest owing to several new developments and applications. For the nuclear reaction processes, the authors provide an elegant alternative to more complicated coupled-channels (CC) models (Kamimura $et$ $al.$, PRC 107, 034607) by replacing in the transition matrix the exact three-body wave function with a solution that uses a tailored $d$-$t$ optical potential. The calculated results reproduce well those of the full CC calculations, and the proposed tractable transition-matrix model promises applications to other $\mu$CF systems. The predicted low-energy negative muon spectrum will also be useful for the generation of an ultraslow negative muon beam by utilizing the $\mu$CF for various applications, such as a scanning negative muon microscope and an injection source for the muon collider.

Several past experiments such as SAGE, GALLEX, and BEST reported lower than expected neutrino capture rates on ${}^{71}$Ga. The origin of this so-called “gallium anomaly” could potentially indicate new neutrino physics, unless there was a more mundane explanation. Because the measured half-life of the electron-capture decay of ${}^{71}$Ge can be used to calculate the neutrino-capture cross section on ${}^{71}$Ga, the authors carried out three separate measurements to determine the half-life of ${}^{71}$Ge with high precision. Their new result of $11.468\pm 0.008$ days for the ${}^{71}$Ge half-life is consistent with the currently accepted value, but significantly more precise. It rules out an unexpectedly long ${}^{71}$Ge half-life as a potential explanation of the puzzling anomaly, leaving the anomaly’s origin an open question.

The modeling of relativistic collisions of nuclei is typically performed by numerically solving the equations of relativistic hydrodynamics. In a few rare cases, exact solutions to these equations exist. The authors derive two novel exact solutions which practitioners may use to verify their hydrodynamics algorithms. The work highlights the importance of an accurate description of the freeze-out configuration in systems characterized by large transverse and longitudinal flows and in collisions with large flow gradients, particularly in small systems.

This work reports the first experimental measurements made at a magnetic confinement fusion device (JET) of the $\mathrm{T}+\mathrm{T}\to \alpha +2n$ reaction, indicating the presence of an intermediate ${}^5$He state in the two-body resonant reaction $\mathrm{T}+\mathrm{T}\to {}^5\mathrm{He}+n$. Such measurements, in which two tritium nuclei (${}^3$H or T) fuse into helium (${}^4$He or $\alpha$), are relevant for modeling the neutron emission spectrum and estimating fuel content and confinement characteristics for magnetic confinement fusion. The results are also relevant for solar physics, as the mirror reaction ${}^3\mathrm{He}+{}^3\mathrm{He}\to {}^4\mathrm{He}+2p$ plays a role in the proton-proton chain of solar fusion.

The transition from the nucleon to the $\mathrm{\Delta }(1232)$ resonance is a sensitive test for models of the nucleon structure. A magnetic dipole ($M1$) quark spin-flip transition essentially dominates photoexcitation of the $\mathrm{\Delta }$, but smaller components in the nucleon and $\mathrm{\Delta }$ wave functions allow also electric quadrupole ($E2$) contributions. The ratio $E2/M1$ then provides fundamental information on both the spatial deformation of the nucleon or $\mathrm{\Delta }$, and on the corresponding $D$ states in their quark-model wave functions. The authors measured the $E2/M1$ ratio via single ${\pi }^0$ production from the proton with a circularly polarized photon beam and a longitudinally polarized proton target, exploiting the presence of interference terms between the measured amplitudes that enhance the effect of smaller contributions. This most precise experimental result to date for the $E2/M1$ ratio gives deep insight into the nucleon properties and provides a precision benchmark for all nonperturbative QCD models.

Efforts toward a precise determination of $V_{ud}$ and low-energy tests of the electroweak Standard Model have been ongoing for many years. The authors report a comprehensive re-evaluation of the $ft$ values in superallowed nuclear $\beta$ decays based on a fully data-driven analysis of the nuclear $\beta$-decay form factor. They utilize isospin relations to connect the nuclear charged weak distribution to the measurable charge distributions. The approach supersedes previous shell-model estimations and allows for a rigorous quantification of theory uncertainties in the phase-space factor, using experimental input rather than nuclear models. The work identifies the need for specific future experimental research to drive further understanding toward a regime of precision that is relevant for weak-interaction physics and thus for physics beyond the Standard Model.

Neutron transmission experiments can realize a high-sensitivity search for time-reversal invariance violation (TRIV) in nucleon-nucleon interactions through the same enhancement mechanism observed for large parity violating (PV) effects in neutron-induced compound nuclear processes. A recent polarized beam/polarized target measurement has now quantified the sensitivity for the best-known case, the 0.75 eV $p$-wave resonance in ${}^{139}$La. By determining the spin-dependent nuclear structure factor that relates TRIV and PV cross sections, this work shows that a future search for $P$-odd/$T$-odd interactions in forward transmission of polarized neutrons on polarized ${}^{139}$La would possess high TRIV sensitivity.

Neutrino experiments such as long-baseline oscillation measurements can determine properties of fundamental particles, but the present systematic uncertainties, e.g., in the neutrino-nucleus cross sections, must be significantly reduced. The authors determine the ${}^{16}$O spectral function from an $ab$-$initio$ calculation with realistic two- and three-body interactions that is benchmarked against earlier ${}^4$He results. They then obtain good results in the relativistic regime for quasi-elastic electron scattering as well as for neutrino scattering data from T2K. The predictions for both electron and neutrino scattering identify a particular need for low-energy electron-scattering data on ${}^{16}$O, for which a program is underway at MAMI in Germany. And being able to propagate the theoretical uncertainties to the final cross sections promises improved understanding of the anticipated more precise results from next-generation neutrino experiments.

The authors extend the proton-neutron finite-amplitude method, an iterative and efficient form of the Skyrme quasiparticle random-phase approximation that needs no diagonalization of the $pn$ QRPA matrix, for the coupling of quasiparticles to like-particle phonons. With this approach one can add beyond-QRPA correlations to computations of important nuclear properties such as Gamow-Teller strength and $\beta$-decay rates in deformed nuclei. The results show improved agreement with existing data for several deformed nuclei, a promising step toward a more reliable framework for large-scale $\beta$-decay calculations needed, e.g., for $r$-process modeling.

High-resolution experimental fusion excitation functions for ${}^{16,17,18}$O + ${}^{12}$C reveal a remarkable irregular behavior rooted in the structure of both the colliding nuclei and the quasimolecular composite system. Using a parameter-free time-dependent Hartree-Fock model, the authors assess the influence of the angular-momentum-dependent fusion barriers on fusion. They find that barrier penetrabilities taken directly from a density-constrained calculation provide a significantly improved description of the experimental data. The results expose the remaining deviations between the mean-field predictions and experimental fusion cross sections, and suggest this approach to garner insight into the impact of nuclear structure effects on fusion reactions.

A new theoretical analysis connects the results of high-energy particle experiments at the Large Hadron Collider with three-proton correlations inside nuclei.

The astrophysical origin for the chemical elements between the first and second $r$-process peaks is a matter of intense debate, with a number of nucleosynthesis processes at explosive stellar environments possibly contributing to their production. Modeling neutron-capture processes that would produce these elements requires reliable data on the trends of neutron separation energies of neutron-rich isotopes which are highly unstable and not readily accessible by experiment. This work describes the first application of an experimental technique, time-of-flight-magnetic-rigidity (ToF-B$\rho$), that is well-suited to measure masses of nuclei with very short half-lives in beams with relatively low intensities. The two-neutron separation energy deduced from the measured masses exhibits a smooth trend consistent with theoretical predictions within the range of experimental uncertainty, indicating that there is no sudden shape transition in these isotopes as hinted at by previous data. The successful application of the ToF-B$\rho$ technique to isotopes with $Z>28$ at the NSCL S800 spectrograph gives hope for a comprehensive program of mass measurements for isotopes relevant to $r$-process models with the same device at FRIB.

Neutrino mass is a key parameter in nuclear and particle physics and in cosmology. The Project 8 Collaboration developed an innovative method with potential to improve the current mass limits by more than an order of magnitude. Announced in a paper published last September (PRL 131, 102502; see also the Synopsis at https://physics.aps.org/articles/v16/s121), the method measures the frequency of radiation from tritium $\beta$-decay electrons spiraling in a magnetic field. In the current paper the authors provide the details of this unique measurement technique including the hardware and the role of simulations and precision spectroscopy that enabled their new direct mass measurement. This first, small-volume demonstration, along with the precision reached, shows a clear path to improve in future experiments on the conservative upper limit for the neutrino mass obtained here.

Optical potentials, either phenomenological or microscopic, are used in nuclear reactions to reduce the complexity of the quantum many-body scattering to a tractable one-body problem. In this work the authors extend to heavier nuclei a highly predictive microscopic approach based on $ab$-$initio$ self-consistent Green’s function (SCGF) calculations that use $NN$ and $3N$ chiral interactions as the only input. The computed elastic proton scattering off Ca and Ni isotope chains demonstrate the stability of the SCGF input, a method that requires only polynomial scaling of computational resources and reaches masses up to 140 nucleons or more. The predictive power of their optical potential promises interesting implications for studying nuclei away from stability, a frontier in nuclear science including nuclear astrophysics in the coming years.