When determining the authorship list for your next paper, be generous yet disciplined.

The American Physical Society (APS) is pleased to announce that it will begin sponsoring Astrobites, a daily astrophysical literature journal written by graduate students in astronomy. This mutually beneficial collaboration aims to enhance the dissemination of research, educational resources, and career insights in the field of astronomy and astrophysics.

A new Collection by the Physical Review journals celebrates the 50th anniversary of the discovery of asymptotic freedom in quantum chromodynamics (QCD)—the theoretical basis for the strong force of nature that binds quarks and gluons into hadrons.

To mark the 243rd American Astronomical Society meeting, Physical Review Letters, Physical Review C, and Physical Review D highlighted several significant papers in astrophysics to illustrate the type of research these journals seek to publish.

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.

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.

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.

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.

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.

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.

APS has selected 156 Outstanding Referees for 2024 who have demonstrated exceptional work in the assessment of manuscripts published in the Physical Review journals. A full list of the Outstanding Referees is available online.

APS has appointed Professor Joseph Kapusta, School of Physics and Astronomy, University of Minnesota as the Lead Editor of Physical Review C. Professor Kapusta takes the helm following the journal’s previous Lead Editor Benjamin F. Gibson.

A look back at Physical Review C’s first half century, and a salute to the talented authors and diligent referees who have made the journal a success.

Researchers look back at key contributions to the field of nuclear physics.

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