- Physical Review Journals
All JournalsPhysics Magazine

Physical Review D

Reuse & Permissions

It is not necessary to obtain permission to reuse this article or its components as it is available under the terms of the Creative Commons Attribution 4.0 International license. This license permits unrestricted use, distribution, and reproduction in any medium, provided attribution to the author(s) and the published article's title, journal citation, and DOI are maintained. Please note that some figures may have been included with permission from other third parties. It is your responsibility to obtain the proper permission from the rights holder directly for these figures.

Export citation

Export citation

Choose format for download:

Download Citation
  • Open Access

Predicting the outcome of collisional neutrino flavor conversion

Julien Froustey*

  • *Contact author: jfroustey@berkeley.edu
PDF

Phys. Rev. D 112, 023029 – Published 18 July, 2025

DOI: https://doi.org/10.1103/syxg-sqfn

Abstract

Collisional flavor instabilities, driven by differing neutrino and antineutrino reaction rates, are expected to occur in dense astrophysical environments like supernovae and neutron star mergers, but have yet to be incorporated in large-scale simulations. We derive analytical expressions for the asymptotic state resulting from a homogeneous and isotropic instability, and apply these predictions to two representative conditions from a neutron star merger simulation. We emphasize the importance of using a collision term that allows for both damping of flavor coherence and relaxation back to the classical steady state. When this classical configuration is collisional unstable, the resulting asymptotic state reflects a compromise between classical relaxation and flavor conversion, defining a “quantum” equilibrium with nonzero coherence. This analysis highlights the possibility of a tradeoff between classical and quantum effects, an important feature with regard to the inclusion of flavor oscillation physics into global simulations.

View figure in article

Physics Subject Headings (PhySH)

Article Text

References (88)

  1. H. T. Janka, T. Melson, and A. Summa, Physics of core-collapse supernovae in three dimensions: A sneak preview, Annu. Rev. Nucl. Part. Sci. 66, 341 (2016).
  2. A. Burrows and D. Vartanyan, Core-collapse supernova explosion theory, Nature (London) 589, 29 (2021).
  3. K. Kyutoku, M. Shibata, and K. Taniguchi, Coalescence of black hole-neutron star binaries, Living Rev. Relativity 24, 5 (2021).
  4. D. Radice, S. Bernuzzi, and A. Perego, The dynamics of binary neutron star mergers and GW170817, Annu. Rev. Nucl. Part. Sci. 70, 95 (2020).
  5. A. Mezzacappa, Toward realistic models of core collapse supernovae: A brief review, IAU Symp. 362, 215 (2020).
  6. K. Kiuchi, General Relativistic Magnetohydrodynamics Simulations for Binary Neutron Star Mergers, in New Frontiers in GRMHD Simulations (Springer Nature, Singapore, 2025).
  7. A. Mezzacappa, E. Endeve, O. E. Bronson Messer, and S. W. Bruenn, Physical, numerical, and computational challenges of modeling neutrino transport in core-collapse supernovae, Living Rev. Comput. Astrophys. 6, 4 (2020).
  8. F. Foucart, Neutrino transport in general relativistic neutron star merger simulations, Living Rev. Comput. Astrophys. 9, 1 (2023).
  9. T. Fischer, G. Guo, K. Langanke, G. Martinez-Pinedo, Y.-Z. Qian, and M.-R. Wu, Neutrinos and nucleosynthesis of elements, Prog. Part. Nucl. Phys. 137, 104107 (2024).
  10. X. Wang and R. Surman, Neutrinos and heavy element nucleosynthesis, in Handbook of Nuclear Physics, edited by I. Tanihata, H. Toki, and T. Kajino (Springer Nature Singapore, Singapore, 2023), p. 119.
  11. H. Duan, G. M. Fuller, and Y.-Z. Qian, Collective neutrino oscillations, Annu. Rev. Nucl. Part. Sci. 60, 569 (2010).
  12. A. Mirizzi, I. Tamborra, H.-T. Janka, N. Saviano, K. Scholberg, R. Bollig, L. Hudepohl, and S. Chakraborty, Supernova neutrinos: Production, oscillations and detection, Riv. Nuovo Cimento 39, 1 (2016).
  13. S. Chakraborty, R. Hansen, I. Izaguirre, and G. Raffelt, Collective neutrino flavor conversion: Recent developments, Nucl. Phys. B908, 366 (2016).
  14. I. Tamborra and S. Shalgar, New developments in flavor evolution of a dense neutrino gas, Annu. Rev. Nucl. Part. Sci. 71, 165 (2021).
  15. F. Capozzi and N. Saviano, Neutrino flavor conversions in high-density astrophysical and cosmological environments, Universe 8, 94 (2022).
  16. M. C. Volpe, Neutrinos from dense environments: Flavor mechanisms, theoretical approaches, observations, and new directions, Rev. Mod. Phys. 96, 025004 (2024).
  17. S. Richers and M. Sen, Fast flavor transformations, in Handbook of Nuclear Physics, edited by I. Tanihata, H. Toki, and T. Kajino (Springer Nature, Singapore, 2022), p. 117.
  18. L. Johns, S. Richers, and M.-R. Wu, Neutrino oscillations in core-collapse supernovae and neutron star mergers, arXiv:2503.05959.
  19. C. J. Stapleford, C. Fröhlich, and J. P. Kneller, Coupling neutrino oscillations and simulations of core-collapse supernovae, Phys. Rev. D 102, 081301 (2020).
  20. J. Ehring, S. Abbar, H.-T. Janka, G. Raffelt, and I. Tamborra, Fast neutrino flavor conversion in core-collapse supernovae: A parametric study in 1D models, Phys. Rev. D 107, 103034 (2023).
  21. J. Ehring, S. Abbar, H.-T. Janka, G. Raffelt, and I. Tamborra, Fast neutrino flavor conversions can help and hinder neutrino-driven explosions, Phys. Rev. Lett. 131, 061401 (2023).
  22. K. Mori, T. Takiwaki, K. Kotake, and S. Horiuchi, Three-dimensional core-collapse supernova models with phenomenological treatment of neutrino flavor conversions, Publ. Astron. Soc. Jpn. 77, L9 (2025).
  23. X. Li and D. M. Siegel, Neutrino fast flavor conversions in neutron-star postmerger accretion disks, Phys. Rev. Lett. 126, 251101 (2021).
  24. O. Just, S. Abbar, M.-R. Wu, I. Tamborra, H.-T. Janka, and F. Capozzi, Fast neutrino conversion in hydrodynamic simulations of neutrino-cooled accretion disks, Phys. Rev. D 105, 083024 (2022).
  25. R. Fernández, S. Richers, N. Mulyk, and S. Fahlman, Fast flavor instability in hypermassive neutron star disk outflows, Phys. Rev. D 106, 103003 (2022).
  26. Y. Qiu, D. Radice, S. Richers, and M. Bhattacharyya, Neutrino flavor transformation in neutron star mergers, arXiv:2503.11758.
  27. H. Nagakura and M. Zaizen, Time-dependent and quasisteady features of fast neutrino-flavor conversion, Phys. Rev. Lett. 129, 261101 (2022).
  28. Z. Xiong, M.-R. Wu, G. Martinez-Pinedo, T. Fischer, M. George, C.-Y. Lin, and L. Johns, Evolution of collisional neutrino flavor instabilities in spherically symmetric supernova models, Phys. Rev. D 107, 083016 (2023).
  29. H. Nagakura and M. Zaizen, Connecting small-scale to large-scale structures of fast neutrino-flavor conversion, Phys. Rev. D 107, 063033 (2023).
  30. H. Nagakura, Roles of fast neutrino-flavor conversion on the neutrino-heating mechanism of core-collapse supernova, Phys. Rev. Lett. 130, 211401 (2023).
  31. H. Nagakura, Global features of fast neutrino-flavor conversion in binary neutron star mergers, Phys. Rev. D 108, 103014 (2023).
  32. S. Shalgar and I. Tamborra, Neutrino quantum kinetics in a core-collapse supernova, J. Cosmol. Astropart. Phys. 09 (2024) 021.
  33. M. Myers, T. Cooper, M. Warren, J. Kneller, G. McLaughlin, S. Richers, E. Grohs, and C. Frohlich, Neutrino flavor mixing with moments, Phys. Rev. D 105, 123036 (2022).
  34. E. Grohs, S. Richers, S. M. Couch, F. Foucart, J. P. Kneller, and G. C. McLaughlin, Neutrino fast flavor instability in three dimensions for a neutron star merger, Phys. Lett. B 846, 138210 (2023).
  35. E. Grohs, S. Richers, S. M. Couch, F. Foucart, J. Froustey, J. P. Kneller, and G. C. McLaughlin, Two-moment neutrino flavor transformation with applications to the fast flavor instability in neutron star mergers, Astrophys. J. 963, 11 (2024).
  36. J. Froustey, S. Richers, E. Grohs, S. D. Flynn, F. Foucart, J. P. Kneller, and G. C. McLaughlin, Neutrino fast flavor oscillations with moments: Linear stability analysis and application to neutron star mergers, Phys. Rev. D 109, 043046 (2024).
  37. J. Froustey, J. P. Kneller, and G. C. McLaughlin, Quantum maximum entropy closure for small flavor coherence, Phys. Rev. D 111, 063022 (2025).
  38. J. P. Kneller, J. Froustey, E. B. Grohs, F. Foucart, G. C. McLaughlin, and S. Richers, Quantum closures for neutrino moment transport, Phys. Rev. D 111, 063046 (2025).
  39. E. Grohs, S. Richers, J. Froustey, F. Foucart, J. P. Kneller, and G. C. McLaughlin, Advection algorithms for quantum neutrino moment transport, Phys. Rev. D 111, 083018 (2025).
  40. H. Nagakura, L. Johns, and M. Zaizen, Bhatnagar-Gross-Krook subgrid model for neutrino quantum kinetics, Phys. Rev. D 109, 083013 (2024).
  41. S. Bhattacharyya and B. Dasgupta, Fast flavor depolarization of supernova neutrinos, Phys. Rev. Lett. 126, 061302 (2021).
  42. S. Bhattacharyya and B. Dasgupta, Elaborating the ultimate fate of fast collective neutrino flavor oscillations, Phys. Rev. D 106, 103039 (2022).
  43. M. Zaizen and H. Nagakura, Simple method for determining asymptotic states of fast neutrino-flavor conversion, Phys. Rev. D 107, 103022 (2023).
  44. M. Zaizen and H. Nagakura, Characterizing quasisteady states of fast neutrino-flavor conversion by stability and conservation laws, Phys. Rev. D 107, 123021 (2023).
  45. H. Nagakura and M. Zaizen, Basic characteristics of neutrino flavor conversions in the postshock regions of core-collapse supernova, Phys. Rev. D 108, 123003 (2023).
  46. S. Abbar, M.-R. Wu, and Z. Xiong, Physics-informed neural networks for predicting the asymptotic outcome of fast neutrino flavor conversions, Phys. Rev. D 109, 043024 (2024).
  47. Z. Xiong, M.-R. Wu, M. George, and C.-Y. Lin, Robust integration of fast flavor conversions in classical neutrino transport, Phys. Rev. Lett. 134, 051003 (2025).
  48. S. Richers, J. Froustey, S. Ghosh, F. Foucart, and J. Gomez, Asymptotic-state prediction for fast flavor transformation in neutron star mergers, Phys. Rev. D 110, 103019 (2024).
  49. M. George, Z. Xiong, M.-R. Wu, and C.-Y. Lin, Evolution and the quasistationary state of collective fast neutrino flavor conversion in three dimensions without axisymmetry, Phys. Rev. D 110, 123018 (2024).
  50. K. A. Lund, P. Mukhopadhyay, J. M. Miller, and G. C. McLaughlin, Angle-dependent in situ fast flavor transformations in post-neutron-star-merger disks, Astrophys. J. Lett. 985, L9 (2025).
  51. T. Wang and A. Burrows, The effect of the fast-flavor instability on core-collapse supernova models, arXiv:2503.04896.
  52. L. Johns, Collisional flavor instabilities of supernova neutrinos, Phys. Rev. Lett. 130, 191001 (2023).
  53. Z. Xiong, L. Johns, M.-R. Wu, and H. Duan, Collisional flavor instability in dense neutrino gases, Phys. Rev. D 108, 083002 (2023).
  54. Y.-C. Lin and H. Duan, Collision-induced flavor instability in dense neutrino gases with energy-dependent scattering, Phys. Rev. D 107, 083034 (2023).
  55. I. Padilla-Gay, I. Tamborra, and G. G. Raffelt, Neutrino fast flavor pendulum. II. Collisional damping, Phys. Rev. D 106, 103031 (2022).
  56. J. Liu, M. Zaizen, and S. Yamada, Systematic study of the resonancelike structure in the collisional flavor instability of neutrinos, Phys. Rev. D 107, 123011 (2023).
  57. M. Zaizen, S. Richers, H. Nagakura, H. Suzuki, and C. Kato, Inspecting neutrino flavor instabilities during proto-neutron star cooling phase in supernova: I. Spherically symmetric model, arXiv:2407.20548.
  58. S. Shalgar and I. Tamborra, Do neutrinos become flavor unstable due to collisions with matter in the supernova decoupling region?, Phys. Rev. D 109, 103011 (2024).
  59. J. Liu, H. Nagakura, R. Akaho, A. Ito, M. Zaizen, and S. Yamada, Universality of the neutrino collisional flavor instability in core-collapse supernovae, Phys. Rev. D 108, 123024 (2023).
  60. R. Akaho, J. Liu, H. Nagakura, M. Zaizen, and S. Yamada, Collisional and fast neutrino flavor instabilities in two-dimensional core-collapse supernova simulation with Boltzmann neutrino transport, Phys. Rev. D 109, 023012 (2024).
  61. J. Liu, H. Nagakura, R. Akaho, A. Ito, M. Zaizen, S. Furusawa, and S. Yamada, Muon-induced collisional flavor instability in core-collapse supernova, Phys. Rev. D 110, 043039 (2024).
  62. Z. Xiong, M.-R. Wu, M. George, C.-Y. Lin, N. K. Largani, T. Fischer, and G. Martinez-Pinedo, Fast neutrino flavor conversions in a supernova: Emergence, evolution, and effects, Phys. Rev. D 109, 123008 (2024).
  63. H. Nagakura, K. Sumiyoshi, S. Fujibayashi, Y. Sekiguchi, and M. Shibata, Neutrino flavor instabilities in a binary neutron star merger remnant: Roles of a long-lived hypermassive neutron star, arXiv:2504.20143.
  64. D. F. G. Fiorillo, I. Padilla-Gay, and G. G. Raffelt, Collisions and collective flavor conversion: Integrating out the fast dynamics, Phys. Rev. D 109, 063021 (2024).
  65. L. Johns and S. Rodriguez, Collisional flavor pendula and neutrino quantum thermodynamics, arXiv:2312.10340.
  66. M. Zaizen, Spectral diversity in collisional neutrino-flavor conversion: Flavor equipartition or swap, Phys. Rev. D 111, 103029 (2025).
  67. C. Kato, H. Nagakura, and L. Johns, Collisional flavor swap with neutrino self-interactions, Phys. Rev. D 109, 103009 (2024).
  68. G. Sigl and G. Raffelt, General kinetic description of relativistic mixed neutrinos, Nucl. Phys. B406, 423 (1993).
  69. S. A. Richers, G. C. McLaughlin, J. P. Kneller, and A. Vlasenko, Neutrino quantum kinetics in compact objects, Phys. Rev. D 99, 123014 (2019); 109, 129902(E) (2024).
  70. Y. Zhang and A. Burrows, Transport equations for oscillating neutrinos, Phys. Rev. D 88, 105009 (2013).
  71. S. Navas et al. (Particle Data Group), Review of particle physics, Phys. Rev. D 110, 030001 (2024).
  72. F. Foucart, P. C.-K. Cheong, M. D. Duez, L. E. Kidder, H. P. Pfeiffer, and M. A. Scheel, Robustness of neutron star merger simulations to changes in neutrino transport and neutrino-matter interactions, Phys. Rev. D 110, 083028 (2024).
  73. E. O’Connor, An open-source neutrino radiation hydrodynamics code for core-collapse supernovae, Astrophys. J. Suppl. Ser. 219, 24 (2015).
  74. P. Virtanen et al., scipy 1.0: Fundamental algorithms for scientific computing in python, Nat. Methods 17, 261 (2020).
  75. D. F. G. Fiorillo and G. G. Raffelt, Fast flavor conversions at the edge of instability in a two-beam model, Phys. Rev. Lett. 133, 221004 (2024).
  76. D. F. G. Fiorillo and G. G. Raffelt, Theory of neutrino fast flavor evolution. Part II. Solutions at the edge of instability, J. High Energy Phys. 12 (2024) 205.
  77. D. F. G. Fiorillo and G. G. Raffelt, Theory of neutrino slow flavor evolution. Part I. Homogeneous medium, J. High Energy Phys. 04 (2025) 146.
  78. D. F. G. Fiorillo and G. G. Raffelt, Collective flavor conversions are interactions of neutrinos with quantized flavor waves, Phys. Rev. Lett. 134, 211003 (2025).
  79. L. Johns, Thermodynamics of oscillating neutrinos, arXiv:2306.14982.
  80. R. Bollig, H. T. Janka, A. Lohs, G. Martinez-Pinedo, C. J. Horowitz, and T. Melson, Muon creation in supernova matter facilitates neutrino-driven explosions, Phys. Rev. Lett. 119, 242702 (2017).
  81. T. Fischer, G. Guo, G. Martinez-Pinedo, M. Liebendörfer, and A. Mezzacappa, Muonization of supernova matter, Phys. Rev. D 102, 123001 (2020).
  82. E. Loffredo, A. Perego, D. Logoteta, and M. Branchesi, Muons in the aftermath of neutron star mergers and their impact on trapped neutrinos, Astron. Astrophys. 672, A124 (2023).
  83. H. H.-Y. Ng, C. Musolino, S. D. Tootle, and L. Rezzolla, Accurate muonic interactions in neutron star mergers and impact on heavy-element nucleosynthesis, Astrophys. J. Lett. 985, L36 (2025).
  84. M. A. Pajkos and E. R. Most, Influence of muons, pions, and trapped neutrinos on neutron star mergers, Phys. Rev. D 111, 043013 (2025).
  85. J. Liu, H. Nagakura, M. Zaizen, L. Johns, and S. Yamada, Asymptotic states of fast neutrino-flavor conversions in the three-flavor framework, Phys. Rev. D 111, 123004 (2025).
  86. S. Bhattacharyya, M.-R. Wu, and Z. Xiong, Role of matter inhomogeneity on fast flavor conversion of supernova neutrinos, arXiv:2504.11316.
  87. J. Froustey, C. Pitrou, and M. C. Volpe, Neutrino decoupling including flavour oscillations and primordial nucleosynthesis, J. Cosmol. Astropart. Phys. 12 (2020) 015.
  88. J. Froustey and C. Pitrou, Primordial neutrino asymmetry evolution with full mean-field effects and collisions, J. Cosmol. Astropart. Phys. 03 (2022) 065.

Outline

Information

Phys. Rev. D 112, 023029– Published 18 July, 2025
  • Received 30 May 2025
  • Accepted 30 June 2025
Crossmark - Check for updates button
Creative Commons Logo

Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI. Funded by SCOAP3.

Published by the American Physical Society

Sign In to Your Journals Account

Filter

Filter

Article Lookup

Enter a citation