- 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

X17 with chiral couplings

Max H. Fieg1,2,*, Toni Mäkelä1,†, Tim M. P. Tait1,‡, and Miša Toman1,§

  • *Contact author: mfieg@fnal.gov
  • †Contact author: tmakela@uci.edu
  • ‡Contact author: ttait@uci.edu
  • §Contact author: mtoman@uci.edu
PDF

Phys. Rev. D 113, 115045 – Published 17 June, 2026

DOI: https://doi.org/10.1103/nnvg-5w2c

Abstract

In recent years, the ATOMKI collaboration has performed a series of measurements of excited nuclei, observing a resonant excess of electron-positron pairs at large opening angles compared to the Standard Model prediction. The excess has been hypothesized to be due to the production of a new spin-1 or spin-0 particle, X17, with a mass of about 17 MeV. Recently, the PADME experiment has reported an excess in the e+e− cross section at center-of-mass energies near 17 MeV, perhaps further hinting at the existence of a new state. Studies of the spin-1 case have hitherto focused on either vector or axial-vector couplings to quarks and leptons, whereas UV theories more naturally produce both vector and axial-vector (i.e., chiral) couplings, analogous to the Standard Model weak interactions. We consider the ATOMKI anomalies in the context of an X with chiral couplings to quarks and explore the parameter space that can explain the ATOMKI anomalies, contrasting them with experimental constraints. We find that it is possible to accommodate the reported ATOMKI signals. However, the 99% confidence level region is in tension with null results from searches for atomic parity violation and direct searches for new low mass physics coupled to electrons. This tension is found to be driven by the magnitude of the reported excess in the transition of C12(17.23), which drives the best-fit region toward excluded couplings.

View figure in article

Physics Subject Headings (PhySH)

Article Text

References (39)

  1. A. J. Krasznahorkay et al., Observation of anomalous internal pair creation in Be8: A possible indication of a light, neutral boson, Phys. Rev. Lett. 116, 042501 (2016).
  2. A. J. Krasznahorkay et al., New results on the Be8 anomaly, Proc. Sci. BORMIO2017 (2017) 036.
  3. A. J. Krasznahorkay et al., New results on the Be8 anomaly, J. Phys. Conf. Ser. 1056, 012028 (2018).
  4. A. J. Krasznahorkay, M. Csatlós, L. Csige, J. Gulyás, T. J. Ketel, A. Krasznahorkay, I. Kuti, Á. Nagy, B. M. Nyakó, N. Sas, and J. Timár, On the creation of the 17 MeV X boson in the 17.6 MeV M1 transition of Be8, EPJ Web Conf. 142, 01019 (2017).
  5. A. J. Krasznahorkay et al., New anomaly observed in C12 supports the existence and the vector character of the hypothetical X17 boson, Phys. Rev. C 106, L061601 (2022).
  6. A. J. Krasznahorkay et al., New evidence supporting the existence of the hypothetic X17 particle, arXiv:1910.10459.
  7. T. T. Anh et al., Checking the Be8 anomaly with a two-arm electron positron pair spectrometer, Universe 10, 168 (2024).
  8. K. Afanaciev et al. (MEG II Collaboration), Search for the X17 particle in Li7(p,e+e−)8Be processes with the MEG II detector, Eur. Phys. J. C 85, 763 (2025).
  9. F. Arias-Aragón, G. G. di Cortona, E. Nardi, and C. Toni, Combined evidence for the X17 boson after PADME results on resonant production in positron annihilation, arXiv:2504.11439.
  10. J. Cederkäll, Y. Hiçy𝚤lmaz, E. Lytken, S. Moretti, and J. Rathsman, Hunting the elusive X17 in CEνNS at the ESS, arXiv:2509.15128.
  11. F. Bossi et al., Search for a new 17 MeV resonance via e+e− annihilation with the PADME Experiment, arXiv:2505.24797.
  12. J. L. Feng, B. Fornal, I. Galon, S. Gardner, J. Smolinsky, T. M. P. Tait, and P. Tanedo, Protophobic fifth-force interpretation of the observed anomaly in Be8 nuclear transitions, Phys. Rev. Lett. 117, 071803 (2016).
  13. J. Kozaczuk, D. E. Morrissey, and S. R. Stroberg, Light axial vector bosons, nuclear transitions, and the Be8 anomaly, Phys. Rev. D 95, 115024 (2017).
  14. J. L. Feng, B. Fornal, I. Galon, S. Gardner, J. Smolinsky, T. M. P. Tait, and P. Tanedo, Particle physics models for the 17 MeV anomaly in beryllium nuclear decays, Phys. Rev. D 95, 035017 (2017).
  15. J. L. Feng, T. M. P. Tait, and C. B. Verhaaren, Dynamical evidence for a fifth force explanation of the ATOMKI nuclear anomalies, Phys. Rev. D 102, 036016 (2020).
  16. D. Barducci and C. Toni, An updated view on the ATOMKI nuclear anomalies, J. High Energy Phys. 02 (2023) 154; 07 (2023) 168(E).
  17. C. J. G. Mommers and M. Vanderhaeghen, Constraining the axial-vector X17 interpretation with 12C data, Phys. Lett. B 858, 139031 (2024).
  18. O. Seto and T. Shimomura, Atomki anomaly in gauged U(1)R symmetric model, J. High Energy Phys. 04 (2021) 025.
  19. L. Delle Rose, S. Khalil, S. J. D. King, S. Moretti, and A. M. Thabt, Atomki anomaly in family-dependent U(1)′ extension of the standard model, Phys. Rev. D 99, 055022 (2019).
  20. B. Puliçe, A Family-nonuniversal U(1)′ model for excited beryllium decays, Chin. J. Phys. (Taipei) 71, 506 (2021).
  21. Y. Kahn, G. Krnjaic, S. Mishra-Sharma, and T. M. P. Tait, Light weakly coupled axial forces: Models, constraints, and projections, J. High Energy Phys. 05 (2017) 002.
  22. M. Hostert and M. Pospelov, Pion decay constraints on exotic 17 MeV vector bosons, Phys. Rev. D 108, 055011 (2023).
  23. A. Anastasi et al., Limit on the production of a low-mass vector boson in e+e−→Uγ, U→e+e− with the KLOE experiment, Phys. Lett. B 750, 633 (2015).
  24. D. Banerjee et al. (NA64 Collaboration), Search for a hypothetical 16.7 MeV gauge boson and dark photons in the NA64 experiment at CERN, Phys. Rev. Lett. 120, 231802 (2018).
  25. D. Banerjee et al. (NA64 Collaboration), Improved limits on a hypothetical X(16.7) boson and a dark photon decaying into e+e− pairs, Phys. Rev. D 101, 071101 (2020).
  26. L. Di Luzio, P. Paradisi, and N. Selimovic, Hunting for a 17 MeV particle coupled to electrons, Nucl. Phys. B1021, 117177 (2025).
  27. E. Peets, Compatibility of the updated (g−2)μ, (g−2)e and PADME-favored couplings with the preferred region of ATOMKI X17, arXiv:2601.05288.
  28. W. Altmannshofer et al. (PIONEER Collaboration), PIONEER: Studies of rare pion decays, arXiv:2203.01981.
  29. P. B. Denton and J. Gehrlein, Neutrino constraints and the ATOMKI X17 anomaly, Phys. Rev. D 108, 015009 (2023).
  30. W. Horiuchi and Y. Suzuki, Four-body calculation for excited states of He4, Int. J. Mod. Phys. A 24, 2134 (2009).
  31. A. J. Krasznahorky, A. Krasznahorkay, M. Csatlós, J. Timár, M. Begala, A. Krakó, I. Rajta, I. Vajda, and N. J. Sas, An update of the hypothetical X17 particle, Universe 10, 409 (2024).
  32. A. J. Krasznahorkay, A. Krasznahorkay, M. Csatlós, L. Csige, J. Timár, M. Begala, A. Krakó, I. Rajta, and I. Vajda, Observation of the X17 anomaly in the decay of the giant dipole resonance of Be8, arXiv:2308.06473.
  33. A. M. Baldini et al. (MEG II Collaboration), The design of the MEG II experiment, Eur. Phys. J. C 78, 380 (2018).
  34. R. Eichler and C. Grab, The SINDRUM-I experiment, SciPost Phys. Proc. 5, 007 (2021).
  35. J. R. Batley et al. (NA48/2 Collaboration), Search for the dark photon in π0 decays, Phys. Lett. B 746, 178 (2015).
  36. S. G. Porsev, K. Beloy, and A. Derevianko, Precision determination of electroweak coupling from atomic parity violation and implications for particle physics, Phys. Rev. Lett. 102, 181601 (2009).
  37. V. A. Dzuba, V. V. Flambaum, and Y. V. Stadnik, Probing low-mass vector bosons with parity nonconservation and nuclear anapole moment measurements in atoms and molecules, Phys. Rev. Lett. 119, 223201 (2017).
  38. S. Pastore, R. B. Wiringa, S. C. Pieper, and R. Schiavilla, Quantum Monte Carlo calculations of electromagnetic transitions in Be8 with meson-exchange currents derived from chiral effective field theory, Phys. Rev. C 90, 024321 (2014).
  39. National Nuclear Data Center. http://www.nndc.bnl.gov/ensdf.

Outline

Information

Phys. Rev. D 113, 115045– Published 17 June, 2026
  • Received 23 March 2026
  • Accepted 8 May 2026
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