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LUNAR: A Monte Carlo generator for bound-nucleon decay in liquid argon

J. A. Nowak*

  • *Contact author: j.nowak@lancaster.ac.uk
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Phys. Rev. D 114, 073007 – Published 27 October, 2026

DOI: https://doi.org/10.1103/mw7s-jl1c

Abstract

The search for nucleon decay in liquid-argon time-projection chambers requires a quantitative description of how the bound nuclear environment reshapes the decay-product kinematics. We present LUNAR, a fast, openly available Monte Carlo generator dedicated to two-body decays of protons and neutrons bound in argon-40, the target nucleus of the DUNE far detector. The parent nucleon is drawn from a selectable nuclear ground state (ten momentum distributions ranging from mean-field Fermi gases to argon spectral functions) and bound off the mass shell by one of three removal-energy prescriptions, including the momentum-dependent optical potential of Juszczak et al. The two-body decay is performed off shell and boosted to the laboratory frame, and the daughter meson is then propagated out of the nucleus by a semiclassical intranuclear cascade with an optional formation zone for the freshly produced meson. We use the generator to separate the distinct roles of Fermi motion and binding in shaping the observable meson spectrum, to quantify final-state interactions channel by channel, and to translate present Super-Kamiokande limits into expected DUNE event yields for the full set of standard decay modes. Final-state interactions leave the supersymmetry-favored p→K+ν¯ signal, and the neutral-kaon modes, essentially intact while roughly halving the pion, η, and antikaon rates, an effect that dominates over the ∼10% spread induced by the choice of nuclear model. For the pion channels, the modeling uncertainty of the cascade itself exceeds that spread. The code is released to the community as a lightweight, extensible tool for signal efficiency and systematics studies.

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Phys. Rev. D 114, 073007– Published 27 October, 2026
  • Received 1 July 2026
  • Revised 17 August 2026
  • Accepted 11 September 2026
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