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First full Dalitz plot measurement in neutron β decay using the Nab spectrometer and implications for new physics

Francisco M. Gonzalez1,*, Jin Ha Choi2,3, Himal Acharya4, Skylar Clymer5, Andrew Hagemeier6, David G. Mathews1,4, August Mendelsohn7, Austin Nelsen4, Hitesh Rahangdale8 et al. (Nab Collaboration)

Hitesh Rahangdale8, Love Richburg8, Ricardo Alarcon5, Ariella Atencio9, Stefan Baeßler6,1, Thomas Bailey2,3, Libertad Barrón Palos10, Noah Birge8, Dennis Borissenko6, Michael Bowler6, Leah J. Broussard1, Albert T. Bryant6, Jimmy Caylor8, Tim Chupp11, Christopher Crawford4, R. Alston Croley4, Micah Cruz8, Chris Cude-Woods2, George Dodson12, Wenjiang Fan6, Deion Fellers13, Nadia Fomin8, Emil Frlež6, Matthew Frost14, Jason Fry15, Duncan Fuehne16, Michael T. Gericke7, Michelle Gervais4, Corey Gilbert1, Ferenc Glück17, Rebecca Godri8, Geoff L. Greene8,1, William Greene8, Josh Hamblen18, Paul Harmston8,†, David Harrison7, Leendert Hayen19,2, Carter Hedinger6, Chelsea Hendrus11, Sean Hollander1, Kavish Imam8, Erik B. Iverson14, Aaron Jezghani20, Chenyang Jiang14, Wolfgang Klassen7,‡, Huangxing Li6, Nick Macsai7, Mark Makela21, Russell Mammei22, Ricky Marshall6, Madelyn Martinez5, Mark McCrea4, Pat McGaughey21, Sean McGovern6, David McLaughlin6, Jacqueline Mirabal21, Paul Mueller1, Andrew Mullins4, William Musk6, Jordan O'Kronley8, Seppo I. Penttilä1, D. Elliot Perryman8,§, Josh Pierce23, Jason A. Pioquinto6, Dinko Počanić6, Hunter Presley8, John Ramsey1, Glenn Randall5, Zachary Raney6, Jackson Ricketts18, Grant Riley21,8, Americo Salas-Bacci6, Sepehr Samiei6, Alexander Saunders1, Wolfgang Schreyer1, E. Mae Scott8,∥, Thomas Shelton4, Aryaman Singh6, Alexander Smith6, Erick Smith21, Eric Stevens6, R. J. Taylor2,3, Leonard Tinius6, Isaiah Wallace8, Jonathan Wexler2,3, W. Scott Wilburn21, A. R. Young2,3, and B. Zeck2,3 (Nab Collaboration)

  • 1Physics Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA
  • 2Department of Physics, North Carolina State University, Raleigh, North Carolina 27695, USA
  • 3Triangle Universities Laboratory, Duke University, Durham, North Carolina 27708, USA
  • 4Department of Physics and Astronomy, University of Kentucky, Lexington, Kentucky 40506, USA
  • 5Department of Physics, Arizona State University, Tempe, Arizona 85287, USA
  • 6Department of Physics, University of Virginia, Charlottesville, Virginia 22904, USA
  • 7Department of Physics, University of Manitoba, Winnipeg, MB R3T 2N2, Canada
  • 8Department of Physics and Astronomy, University of Tennessee, Knoxville, Tennessee 37996, USA
  • 9Department of Physics, Drexel University, Philadelphia, Pennsylvania 19104, USA
  • 10Universidad Nacional Autónoma de México, México, D.F. 04510, México
  • 11Department of Physics, University of Michigan, Ann Arbor, Michigan 48109, USA
  • 12Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA
  • 13Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
  • 14Neutron Technologies Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA
  • 15Department of Physics, Geosciences, and Astronomy, Eastern Kentucky University, Richmond, Kentucky 40475, USA
  • 16Department of Physics, University of Chicago, Chicago, Illinois 60637, USA
  • 17Karlsruhe Institute of Technology, IAP, 76021 Karlsruhe, POB 3640, Germany
  • 18Department of Physics and Astronomy, University of Tennessee at Chattanooga, Chattanooga, Tennessee 37403, USA
  • 19LPC Caen, ENSICAEN, Université de Caen, CNRS/IN2P3, Caen, France
  • 20Partnership for an Advanced Computing Environment, Georgia Tech, Atlanta, Georgia 30332, USA
  • 21Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA
  • 22Department of Physics, University of Winnipeg, Winnipeg, MB R3B 2E9, Canada
  • 23Neutron Scattering Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA

  • *Contact author: gonzalezfm@ornl.gov
  • †Present address: Department of Physics, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, USA.
  • ‡Present address: Department of Physics, The University of British Columbia, Vancouver, BC V5T 1Z4, Canada.
  • §Present address: Institut Laue-Langevin, Grenoble, France.
  • ∥Present address: Centre College, 600 West Walnut Street, Danville, Kentucky 40422, USA.
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Phys. Rev. C 113, 035501 – Published 4 March, 2026

DOI: https://doi.org/10.1103/ksmp-zxsl

Abstract

Precision measurements of observables in neutron β decay are used to test the standard model description of the weak interaction and search for evidence of new physics. The Nab experiment at the Fundamental Neutron Physics Beamline at the Spallation Neutron Source was constructed to measure correlations in neutron decay by utilizing an asymmetric spectrometer and novel detection system to accurately reconstruct the proton momentum and electron energy for each β decay. This work describes the detection of neutron β-decay products in the Nab spectrometer and presents the first full Dalitz plot representation of the phase space of neutron β decay for all electrons >100 keV. In addition, new constraints are placed on a possible excited neutron state, hypothesized to explain the disagreement between the appearance and disappearance neutron lifetime techniques.

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Phys. Rev. C 113, 035501– Published 4 March, 2026
  • Received 21 August 2025
  • Accepted 2 December 2025
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