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1. The Nab experiment: A precision measurement of unpolarized neutron beta decay

   https://doi.org/10.1051/epjconf/201921904002  

   Fry, J.; Alarcon, R.; Baeßler, S.; Balascuta, S.; Palos, L. Barrón; Bailey, T.; Bass, K.; Birge, N.; Blose, A.; Borissenko, D.; et al (January 2019, EPJ Web of Conferences)

   Neutron beta decay is one of the most fundamental processes in nuclear physics and provides sensitive means to uncover the details of the weak interaction. Neutron beta decay can evaluate the ratio of axial-vector to vector coupling constants in the standard model, λ = g A / g V , through multiple decay correlations. The Nab experiment will carry out measurements of the electron-neutrino correlation parameter a with a precision of δ a / a = 10 −3 and the Fierz interference term b to δ b = 3 × 10 −3 in unpolarized free neutron beta decay. These results, along with a more precise measurement of the neutron lifetime, aim to deliver an independent determination of the ratio λ with a precision of δλ/λ = 0.03% that will allow an evaluation of V ud and sensitively test CKM unitarity, independent of nuclear models. Nab utilizes a novel, long asymmetric spectrometer that guides the decay electron and proton to two large area silicon detectors in order to precisely determine the electron energy and an estimation of the proton momentum from the proton time of flight. The Nab spectrometer is being commissioned at the Fundamental Neutron Physics Beamline at the Spallation Neutron Source at Oak Ridge National Lab. We present an overview of the Nab experiment and recent updates on the spectrometer, analysis, and systematic effects. 

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2. The Neutron Lifetime Discrepancy and Its Implications for Cosmology and Dark Matter

   https://doi.org/10.3390/sym16080956  

   Wietfeldt, Fred E (August 2024, Symmetry)

   Free neutron decay is the prototype for nuclear beta decay and other semileptonic weak particle decays. It provides important insights into the symmetries of the weak nuclear force. Neutron decay is important for understanding the formation and abundance of light elements in the early universe. The two main experimental approaches for measuring the neutron lifetime, the beam method and the ultracold neutron storage method, have produced results that currently differ by 9.8 ± 2.0 s. While this discrepancy probably has an experimental origin, a more exciting prospect is that it may be explained by new physics, with possible connections to dark matter. The experimental status of the neutron lifetime is briefly reviewed, with an emphasis on its implications for cosmology, astrophysics, and dark matter. 

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3. 4D-imaging of drip-line radioactivity by detecting proton emission from 54mNi pictured with ACTAR TPC

   https://doi.org/10.1038/s41467-021-24920-0  

   Giovinazzo, J.; Roger, T.; Blank, B.; Rudolph, D.; Brown, B. A.; Alvarez-Pol, H.; Arokia Raj, A.; Ascher, P.; Caamaño-Fresco, M.; Caceres, L.; et al (December 2021, Nature Communications)

   Abstract Proton radioactivity was discovered exactly 50 years ago. First, this nuclear decay mode sets the limit of existence on the nuclear landscape on the neutron-deficient side. Second, it comprises fundamental aspects of both quantum tunnelling as well as the coupling of (quasi)bound quantum states with the continuum in mesoscopic systems such as the atomic nucleus. Theoretical approaches can start either from bound-state nuclear shell-model theory or from resonance scattering. Thus, proton-radioactivity guides merging these types of theoretical approaches, which is of broader relevance for any few-body quantum system. Here, we report experimental measurements of proton-emission branches from an isomeric state in 54m Ni, which were visualized in four dimensions in a newly developed detector. We show that these decays, which carry an unusually high angular momentum, ℓ = 5 and ℓ = 7, respectively, can be approximated theoretically with a potential model for the proton barrier penetration and a shell-model calculation for the overlap of the initial and final wave functions. 

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4. Investigating the weak charge of 48Ca using a dispersive optical model

   https://doi.org/10.1016/j.physletb.2025.139371  

   Calleya, NL; Atkinson, MC; Dickhoff, WH (April 2025, Physics Letters B)

   Balantekin, B (Ed.)

   A new nonlocal dispersive-optical-model analysis has been carried out for neutrons and protons in 48Ca that reproduces the weak-form-factor measurement of CREX. In addition to elastic-scattering angular distributions, total and reaction cross sections, single-particle energies, the neutron and proton numbers, and the charge distribution, the CREX-measured weak form factor has been fit to extract the neutron and proton self-energies both above and below the Fermi energy. The resulting single-particle propagators yield a weak form factor of 𝐹𝑊 =0.125±0.05 and a neutron skin of 𝑅skin =0.152±0.05 fm, in good agreement with CREX. The rearrangement of the neutron distribution to accommodate such a thin neutron skin results in the high-momentum content of the neutrons exceeding that of the protons, in contrast to what is expected from high-energy two-nucleon knockout measurements by the CLAS collaboration and ab initio asymmetric matter calculations. The present analysis also emphasizes the importance of neutron experimental data in constraining weak charge observables necessary for a precise description of neutron densities. Notably, the neutron reaction cross section and further parity-violating experiments weak form factor measurements are essential to generate a unique way to determine the 48Ca neutron distribution in this framework. 

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5. Mass measurements show slowdown of rapid proton capture process at waiting-point nucleus 64Ge

   https://doi.org/10.1038/s41567-023-02034-2  

   Zhou, X.; Wang, M.; Zhang, Y. H.; Litvinov, Yu. A.; Meisel, Z.; Blaum, K.; Zhou, X. H.; Hou, S. Q.; Li, K. A.; Xu, H. S.; et al (August 2023, Nature Physics)

   Abstract X-ray bursts are among the brightest stellar objects frequently observed in the sky by space-based telescopes. A type-I X-ray burst is understood as a violent thermonuclear explosion on the surface of a neutron star, accreting matter from a companion star in a binary system. The bursts are powered by a nuclear reaction sequence known as the rapid proton capture process (rp process), which involves hundreds of exotic neutron-deficient nuclides. At so-called waiting-point nuclides, the process stalls until a slower β + decay enables a bypass. One of the handful of rp process waiting-point nuclides is 64 Ge, which plays a decisive role in matter flow and therefore the produced X-ray flux. Here we report precision measurements of the masses of 63 Ge, 64,65 As and 66,67 Se—the relevant nuclear masses around the waiting-point 64 Ge—and use them as inputs for X-ray burst model calculations. We obtain the X-ray burst light curve to constrain the neutron-star compactness, and suggest that the distance to the X-ray burster GS 1826–24 needs to be increased by about 6.5% to match astronomical observations. The nucleosynthesis results affect the thermal structure of accreting neutron stars, which will subsequently modify the calculations of associated observables. 

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