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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. On-line installation of the Superallowed Transition Beta-Neutrino Decay Ion Coincidence Trap

   https://doi.org/10.1051/epjconf/202430102002  

   Brodeur, M; Bardayan, DW; Bruce, O; Bualuan, R; Burdette, DP; Clark, JA; Gallant, AT; Gan, D; Guillet, D; Houff, AM; et al (January 2024, EPJ Web of Conferences)

   Bijker, R; Marín_Lámbarri, DJ; Yépez_Martínez, TC (Ed.)

   The Cabibbo-Kobayashi-Maskawa quark mixing matrix currently does not satisfy unitarity at the 2σ-level. This could be the result of an inaccurate value of one or both of its largest matrix elementsV_usandV_ud. In the case ofV_ud, the most precise measurement is obtained from thef t-value measurements of superallowed beta-transitions between 0⁺states. The accuracy of this determination can, in turn, be tested by extractingV_udin other transitions including superallowed transitions between mirror nuclei. The Superallowed Transition Beta-Neutrino Decay Ion Coincidence Trap (St. Benedict) is currently under construction at the Nuclear Science Laboratory of the University of Notre Dame to perform such a determination, with the goal of shedding more light on this tension with unitarity. St. Benedict will take a radioactive ion beam produced byTwinSol, thermalize it in a large volume gas catcher, then transport it in two separate differentially-pumped volumes using a radio-frequency (RF) carpet and a radio-frequency quadrupole (RFQ) ion guide before injecting it in an RFQ trap to create cool ion bunches for injection in the measurement Paul trap. In this paper, we detail the installation of the beam preparation components of St. Benedict, and present the results of the first RIBs successfully stopped and extracted from its gas catcher. 

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3. VLBA Astrometry of the Galactic Double Neutron Stars PSR J0509+3801 and PSR J1930–1852: A Preliminary Transverse Velocity Distribution of Double Neutron Stars and its Implications

   https://doi.org/10.3847/1538-4357/ad4883  

   Ding, Hao; Deller, Adam T; Swiggum, Joseph K; Lynch, Ryan S; Chatterjee, Shami; Tauris, Thomas M (July 2024, The Astrophysical Journal)

   Abstract The mergers of double neutron star (DNS) systems are believed to drive the majority of shortγ-ray bursts (SGRBs), while also serving as production sites of heavyr-process elements. Despite being key to (i) confirming the nature of the extragalactic SGRBs, (ii) addressing the poorly understoodr-process enrichment in the ultrafaint dwarf galaxies (UFDGs), and (iii) probing the formation process of DNS systems, the space velocity distribution of DNSs is still poorly constrained, due to the small number of DNSs with well-determined astrometry. In this work, we determine new proper motions and parallaxes of two Galactic DNSs, PSR J0509+3801 and PSR J1930−1852, using the Very Long Baseline Array, and we estimate the transverse velocitiesv_⊥of all 11 isolated Galactic DNSs having proper-motion measurements in a consistent manner. Our correlation analysis reveals that the DNSv_⊥is tentatively correlated with three parameters: spin period, orbital eccentricity, and companion mass. With the preliminaryv_⊥distribution, we obtain the following findings. First, the refinedv_⊥distribution is confirmed to agree with the observed displacements of the localized SGRBs from their host galaxy birth sites. Second, we estimate that around 11% and 25% of DNSs remain gravitationally bound to UFDGs with escape velocities of 15 and 25 km s⁻¹, respectively. Hence, the retained DNSs might indeed be responsible for ther-process enrichment confirmed so far in a few UFDGs. Finally, we discuss how a future ensemble of astrometrically determined DNSs may probe the multimodality of thev_⊥distribution. 

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4. Constraining Inputs to Realistic Kilonova Simulations through Comparison to Observed r-process Abundances

   https://doi.org/10.3847/1538-4357/acf3e0  

   Ristić, Marko; Holmbeck, Erika M.; Wollaeger, Ryan T.; Korobkin, Oleg; Champion, Elizabeth; O’Shaughnessy, Richard; Fryer, Chris L.; Fontes, Christopher J.; Mumpower, Matthew R.; Sprouse, Trevor M. (October 2023, The Astrophysical Journal)

   Kilonovae, one source of electromagnetic emission associated with neutron star mergers, are powered by the decay of radioactive isotopes in the neutron-rich merger ejecta. Models for kilonova emission consistent with the electromagnetic counterpart to GW170817 predict characteristic abundance patterns, determined by the relative balance of different types of material in the outflow. Assuming that the observed source is prototypical, this inferred abundance pattern in turn must matchr-process abundances deduced by other means, such as what is observed in the solar system. We report on analysis comparing the input mass-weighted elemental compositions adopted in our radiative transfer simulations to the mass fractions of elements in the Sun, as a practical prototype for the potentially universal abundance signature from neutron star mergers. We characterize the extent to which our parameter inference results depend on our assumed composition for the dynamical and wind ejecta and examine how the new results compare to previous work. We find that a dynamical ejecta composition calculated using the FRDM2012 nuclear mass and FRLDM fission models with extremely neutron-rich ejecta (Y_e= 0.035) along with moderately neutron-rich (Y_e= 0.27) wind ejecta composition yields a wind-to-dynamical mass ratio ofM_w/M_d= 0.47, which best matches the observed AT2017gfo kilonova light curves while also producing the best-matching abundance of neutron capture elements in the solar system, though, allowing for systematics, the ratio may be as high as of order unity. 

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5. Direct Evidence for r -process Nucleosynthesis in Delayed MeV Emission from the SGR 1806–20 Magnetar Giant Flare

   https://doi.org/10.3847/2041-8213/adc9b0  

   Patel, Anirudh; Metzger, Brian D; Cehula, Jakub; Burns, Eric; Goldberg, Jared A; Thompson, Todd A (April 2025, The Astrophysical Journal Letters)

   Abstract The origin of heavy elements synthesized through the rapid neutron capture process (r-process) has been an enduring mystery for over half a century. J. Cehula et al. recently showed that magnetar giant flares, among the brightest transients ever observed, can shock heat and eject neutron star crustal material at high velocity, achieving the requisite conditions for anr-process. A. Patel et al. confirmed anr-process in these ejecta using detailed nucleosynthesis calculations. Radioactive decay of the freshly synthesized nuclei releases a forest of gamma-ray lines, Doppler broadened by the high ejecta velocitiesv ≳ 0.1cinto a quasi-continuous spectrum peaking around 1 MeV. Here, we show that the predicted emission properties (light curve, fluence, and spectrum) match a previously unexplained hard gamma-ray signal seen in the aftermath of the famous 2004 December giant flare from the magnetar SGR 1806–20. This MeV emission component, rising to peak around 10 minutes after the initial spike before decaying away over the next few hours, is direct observational evidence for the synthesis of ∼10⁻⁶M_⊙ofr-process elements. The discovery of magnetar giant flares as confirmedr-process sites, contributing at least ∼1%–10% of the total Galactic abundances, has implications for the Galactic chemical evolution, especially at the earliest epochs probed by low-metallicity stars. It also implicates magnetars as potentially dominant sources of heavy cosmic rays. Characterization of ther-process emission from giant flares by resolving decay line features offers a compelling science case for NASA’s forthcoming COSI nuclear spectrometer, as well as next-generation MeV telescope missions. 

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