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AI summary · arXiv: 2610.36218v1 · nucl-ex

Beta decay of potassium-54 reveals the role of two-body currents in neutron-rich nuclei

Zhengyu Xu, Robert Grzywacz, Matthias Heinz, Thomas Papenbrock, Weijia Ong, James Allmond, Bertis Rasco, Krzysztof Rykaczewski,
and 79 more authorsHendrik Schatz, Bradley Sherrill, Oleg Tarasov, Peter Gysbers, Gaute Hagen, Petr Navratil, Samuel Ajayi, Honey Arora, Akaa Daniel Ayangeakaa, Hannah Berg, Jessica Berkman, Darren Bleuel, Konstantinos Bosmpotinis, Nico Braukman, Michael Carpenter, Giordano Cerizza, Aaron Chester, James Christie, Ian Cox, Heather L. Crawford, Benjamin Crider, Jacob Davis, Amelia Doetsch, Jeremias Garcia-Duarte, Alfredo Estrade, Aleksandra Fijalkowska, Christoforos Frantzis, Kei Fukushima, Tawfik Gaballah, Tim Gray, Erin Good, Kenny Haak, Shutaro Hanai, Jason Harke, Caley Harris, Adam Hartley, Marc Hausmann, Kirby Hermansen, Daniel Hoff, Donnie Hoskins, James Huffman, Rahul Jain, Marek Karny, Thomas King, Noritaka Kitamura, Kay Kolos, Elaine Kwan, Sean Liddick, Brenden Longfellow, Rebeka Lubna, Stephanie Lyons, Miguel Madurga, Mejdi Mogannam, Shree Neupane, Timilehin Ogunbeku, Gerard Owens-Fryar, Jayden Palomino, Mauricio Portillo, Mustafa Rajabali, Andrea Richard, Isaiah Richardson, Eleanor Ronning, Gregory Rose, Thomas Ruland, Nicholas Scielzo, Dustin Scriven, Darek Seweryniak, Kevin Siegl, Mallory Smith, Artemis Spyrou, Michał Stepaniuk, Adriana Sweet, Vandana Tripathi, Artemis Tsantiri, Sivahami Uthayakumaar, William Walters, Shane Watters, Marzena Wolinska-Cichocka, Rin Yokoyama
Original paper Submitted to arXiv on 26 Oct. 2026 Accepted for publication in Nature Communications License: NONEXCLUSIVE-DISTRIB 1.0 Also in: fr

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The essentials

The measured beta decay of neutron-rich potassium-54 agrees with ab initio calculations only when two-body weak currents are included, supporting their role in quenching Gamow-Teller strength.

Question

Do two-body weak currents explain the reduced Gamow-Teller strength observed in neutron-rich nuclei? The study tests this by measuring potassium-54 beta decay and comparing the result with nuclear-structure calculations.

Method

At FRIB, the authors identified about 4,600 potassium-54 nuclei over 98 hours and measured beta-delayed gamma rays and neutron time-of-flight spectra. They extracted the feeding and Gamow-Teller strength for a prominent state in calcium-54, then compared these data with multi-shell VS-IMSRG calculations, with and without two-body currents, and large-scale shell-model calculations.

Findings

  • Result The decay predominantly fed a neutron-unbound calcium-54 state at 6159(100) keV, with a feeding probability of 59(8)% and log ft = 4.4(1).
  • Result The measured Gamow-Teller strength for the main transition was 0.26 with asymmetric uncertainties.
  • Result For a 3− potassium-54 parent state, VS-IMSRG calculations with two-body currents gave a strength consistent with the measurement, whereas calculations without them predicted a substantially larger strength.
  • Authors’ interpretation The authors conclude that two-body currents produce significant quenching in this neutron-rich isotope, allowing agreement with experiment without phenomenological adjustment.
    See the passage in the paper
    This illustrates that also in this very neutron-rich isotope, two-body currents quench the matrix element significantly, giving good agreement with experiment without requiring phenomenological adjustments.
  • Authors’ interpretation The data and calculations favor a potassium-54 ground-state spin-parity assignment of (3,4)− and support persistence of the N=34 shell closure in potassium.

Limitations

  • The parent ground-state spin and parity could not be determined definitively, so calculations considered multiple possible assignments.
  • The VS-IMSRG calculations placed the predicted strength at higher excitation energy than observed, a discrepancy the authors attribute to the VS-IMSRG(2) approximation.
  • Limited statistics and unobserved gamma rays could have led to misidentified neutron channels and underestimated excitation energies for weaker beta feedings below 6 MeV.

Why it matters

The authors argue that this result demonstrates the importance of two-body currents in Gamow-Teller beta decay far from stability and provides a benchmark for ab initio predictions in exotic nuclei. They present the measurement as a milestone toward describing weak decays across neutron-rich regions of the nuclear chart.

Written by AI from the paper. Passages are checked word for word against the text we analysed; the rest may contain errors.

AI-generated keywords: two-body weak currents Gamow–Teller decay potassium-54 calcium-54 neutron-rich nuclei

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  • Original abstract
The paper investigates whether two-body weak currents can explain the reduced Gamow–Teller (GT) strengths often predicted by nuclear calculations, particularly for nuclei with a large neutron excess. These currents modify the decay operator so that it can act on two interacting nucleons. Although theory has linked them to the apparent quenching of beta decay, experimental tests in very neutron-rich nuclei have been scarce. The authors studied the beta decay of potassium-54 to calcium-54. This provides a comparatively simple test case: calcium-54 is doubly magic, and the decay was expected to be dominated by a strong transition to a highly excited state. At the Facility for Rare Isotope Beams, the researchers identified about 4,600 potassium-54 ions over 98 hours of beam time. They implanted the ions in a detector and measured the subsequent beta-delayed gamma rays and neutrons. Both types of spectroscopy were needed because the principal decay populates a neutron-unbound state in calcium-54, which can be studied only through its neutron emissions. Using neutron time-of-flight peaks and gamma-ray data, the team inferred the feeding of calcium-54 states and estimated the transition strength. The strongest observed feeding was to a state at 6,159(100) keV. Its beta-feeding probability was 59(8)%, and its comparative partial half-life was log ft = 4.4(1), consistent with a Gamow–Teller transition. The deduced strength was B(GT) = 0.26, with uncertainties of +0.08 and −0.04. The neutron data were mainly consistent with decay to the ground and first excited states of calcium-53. The gamma-ray spectrum also suggested a beta-delayed two-neutron emission probability of at least 5%, while the branch leaving calcium-54 bound appeared negligible. The authors compared the measurement with ab initio valence-space in-medium similarity renormalization group calculations, which used a multi-shell space to represent the relevant cross-shell excitations, and with large-scale shell-model calculations. The ab initio calculations considered potassium-54 ground-state spins of 2−, 3−, and 4−. The 2− option did not produce a single transition strong enough to match the data. For a 3− parent, the predicted strength was 0.25 with two-body currents and 0.43 without them; for a 4− parent, the corresponding values were 0.14 and 0.35. The 3− prediction with two-body currents agrees particularly well with the measured strength. The shell-model results also favored 3− or 4− over 2−. Together, the comparisons support a likely ground-state assignment of 3− or 4−, although they do not decisively distinguish between the two. The central conclusion is that two-body currents substantially reduce the calculated strength, bringing it into agreement with experiment without an additional empirical adjustment. The work therefore provides evidence for the importance of these currents in beta decay far from stability and supports the persistence of the neutron shell closure at neutron number 34 in potassium-54. The authors note several limitations. The parent spin remains uncertain, the statistics are limited, and some weaker apparent feeding below 6 MeV may be misassigned because of undetected gamma rays or confusion between one- and two-neutron emissions. On the theoretical side, the predicted excitation energies are higher than the measured ones, a known issue with the approximation used. Calculations with larger valence spaces were not feasible or did not converge reliably. These uncertainties temper the detailed interpretation, but the measured strength of the principal transition and its agreement with calculations that include two-body currents remain the paper’s key results.
Created on 02 Nov. 2026

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Weekly letter Presented in "Particles, nuclei and gravitation this week" of 2 November 2026. Particles, nuclei and gravitation this week: Testing the hidden machinery of matter
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