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arxiv: 2606.17320 · v2 · pith:FE2CFR26new · submitted 2026-06-15 · ✦ hep-ex

Improved limits on a new Z' in B-L scenarios with the NA64 experiment at CERN

The NA64 Collaboration: Yu. M. Andreev , A. Antonov , M. A. Ayala Torres , D. Banerjee , B. Banto Oberhauser , V. Bautin , J. Bernhard , P. Bisio
show 47 more authors
A. Celentano N. Charitonidis P. Crivelli A. V. Dermenev S. V. Donskov R. R. Dusaev T. Enik V. N. Frolov S. V. Gertsenberger S. Girod S. N. Gninenko M. H\"osgen Y. Kambar A. E. Karneyeu G. Kekelidze B. Ketzer D. V. Kirpichnikov M. M. Kirsanov V. A. Kramarenko L. V. Kravchuk N. V. Krasnikov S. V. Kuleshov V. E. Lyubovitskij V. Lysan A. Marini L. Marsicano V. A. Matveev R. Mena Fredes R. Mena Yanssen L. Molina Bueno M. Mongillo D. V. Peshekhonov V. A. Polyakov B. Radics K. Salamatin V. D. Samoylenko H. Sieber D. Shchukin O. Soto V. O. Tikhomirov I. Tlisova A. N. Toropin M. Tuzi P. V. Volkov I. V. Voronchikhin J. Zamora-Sa'a A. S. Zhevlakov
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Pith reviewed 2026-07-01 07:37 UTC · model grok-4.3

classification ✦ hep-ex
keywords Z' bosonB-L gauge symmetryfixed-target experimentelectron beamdark matterneutrino massesgauge boson limits
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The pith

NA64 sets the most stringent lab limits on g_{B-L} for sub-GeV Z' masses with 2016-2022 electron data

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper establishes new upper bounds on the coupling g_{B-L} of a hypothetical Z' gauge boson in U(1)_{B-L} extensions of the Standard Model by analyzing the full NA64 fixed-target dataset. With (9.4 ± 0.5) × 10^{11} electrons on target collected from 2016 to 2022, the search includes the resonant e^{+}e^{-} annihilation channel that improves reach between 200 and 300 MeV. For the unbroken B-L case these limits surpass those from neutrino-scattering experiments and become the strongest laboratory constraints below 1 GeV. The same data also constrain scenarios in which the Z' decays invisibly to dark-matter particles. The results tighten the allowed parameter space linking neutrino mass generation to possible dark-matter candidates.

Core claim

Using the complete 2016–2022 electron-beam exposure of (9.4 ± 0.5) × 10^{11} electrons on target, the NA64 collaboration extracts new exclusion limits on the B-L gauge coupling g_{B-L} that exceed previous laboratory bounds from neutrino experiments for the unbroken U(1)_{B-L} model across the sub-GeV mass range; the resonant e^{+}e^{-} annihilation channel supplies additional sensitivity between 200 and 300 MeV, while invisible-mode results directly limit models in which the Z' couples to dark matter.

What carries the argument

The resonant e^{+}e^{-} annihilation production channel for the Z' in electron-nucleus collisions, which enhances sensitivity in the 200–300 MeV mass window.

If this is right

  • For unbroken U(1)_{B-L}, g_{B-L} is now bounded more tightly than by any prior laboratory measurement in the sub-GeV regime.
  • In dark-matter-coupled scenarios the invisible-decay limits translate directly into excluded regions of parameter space.
  • The three-fold increase in statistics relative to earlier NA64 analyses directly improves the coupling reach across the full mass range examined.
  • Models that generate neutrino masses through a B-L breaking scale are now subject to stronger laboratory constraints below 1 GeV.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • The same dataset could be reanalyzed with different background assumptions to test robustness of the resonant-channel contribution.
  • Combining these limits with cosmological bounds on extra gauge bosons would further restrict viable B-L model space.
  • Higher-intensity electron beams at future facilities could extend the same technique to still smaller couplings.

Load-bearing premise

The resonant e^{+}e^{-} annihilation production channel is modeled correctly and does not suffer from unaccounted systematic effects or background contributions that would alter the derived limits.

What would settle it

Observation of a Z' signal in a neutrino-scattering experiment at a value of g_{B-L} below the NA64 exclusion curve would contradict the reported bounds.

Figures

Figures reproduced from arXiv: 2606.17320 by A. Antonov, A. Celentano, A. E. Karneyeu, A. Marini, A. N. Toropin, A. S. Zhevlakov, A. V. Dermenev, B. Banto Oberhauser, B. Ketzer, B. Radics, D. Banerjee, D. Shchukin, D. V. Kirpichnikov, D. V. Peshekhonov, G. Kekelidze, H. Sieber, I. Tlisova, I. V. Voronchikhin, J. Bernhard, J. Zamora-Sa'a, K. Salamatin, L. Marsicano, L. Molina Bueno, L. V. Kravchuk, M. A. Ayala Torres, M. H\"osgen, M. M. Kirsanov, M. Mongillo, M. Tuzi, N. Charitonidis, N. V. Krasnikov, O. Soto, P. Bisio, P. Crivelli, P. V. Volkov, R. Mena Fredes, R. Mena Yanssen, R. R. Dusaev, S. Girod, S. N. Gninenko, S. V. Donskov, S. V. Gertsenberger, S. V. Kuleshov, T. Enik, The NA64 Collaboration: Yu. M. Andreev, V. A. Kramarenko, V. A. Matveev, V. A. Polyakov, V. Bautin, V. D. Samoylenko, V. E. Lyubovitskij, V. Lysan, V. N. Frolov, V. O. Tikhomirov, Y. Kambar.

Figure 1
Figure 1. Figure 1: FIG. 1 [PITH_FULL_IMAGE:figures/full_fig_p002_1.png] view at source ↗
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Figure 2. Figure 2: FIG. 2 [PITH_FULL_IMAGE:figures/full_fig_p003_2.png] view at source ↗
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Figure 3. Figure 3: FIG. 3 [PITH_FULL_IMAGE:figures/full_fig_p003_3.png] view at source ↗
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Figure 4. Figure 4: FIG. 4 [PITH_FULL_IMAGE:figures/full_fig_p005_4.png] view at source ↗
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Figure 5. Figure 5: FIG. 5 [PITH_FULL_IMAGE:figures/full_fig_p005_5.png] view at source ↗
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Figure 7. Figure 7: FIG. 7 [PITH_FULL_IMAGE:figures/full_fig_p006_7.png] view at source ↗
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Figure 6. Figure 6: FIG. 6 [PITH_FULL_IMAGE:figures/full_fig_p006_6.png] view at source ↗
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Figure 8. Figure 8: FIG. 8 [PITH_FULL_IMAGE:figures/full_fig_p007_8.png] view at source ↗
read the original abstract

Extensions of the Standard Model featuring an additional $U(1)_{B-L}$ gauge symmetry provide a compelling framework linking the origin of neutrino masses to possible dark matter candidates. The associated gauge boson, $Z'$, couples directly to Standard Model fermions and can be produced in fixed-target experiments through electron-nucleus interactions. In this work, we present new constraints on the coupling constant $g_{B-L}$ obtained with the NA64 experiment using the full electron-beam dataset collected between 2016 and 2022, corresponding to $(9.4\pm0.5)\times10^{11}$ electrons on target. The analysis includes the resonant $e^{+}e^{-}$ annihilation production channel, which enhances sensitivity in the mass range $m_{Z'}\in[200,300]$ MeV. The larger dataset provides approximately three times the statistics of previous analyses, thereby improving sensitivity. For the unbroken $U(1)_{B-L}$ case, the new limits exceed those from dedicated neutrino-scattering experiments, providing the most stringent laboratory bounds on $g_{B-L}$ for sub-GeV masses of the new boson. In scenarios where the $Z'$ couples to dark matter, the decay width is dominated by invisible channels, and the corresponding exclusion limits can be directly derived from the NA64 invisible-mode analysis.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

1 major / 0 minor

Summary. The paper reports new upper limits on the B-L gauge coupling g_{B-L} of a Z' boson using the full NA64 electron-beam dataset of (9.4 ± 0.5) × 10^{11} electrons on target collected 2016–2022. The analysis incorporates the resonant e^{+}e^{-} annihilation production channel, which is stated to enhance sensitivity for m_{Z'} ∈ [200, 300] MeV. For unbroken U(1)_{B-L}, the resulting limits are claimed to exceed those from dedicated neutrino-scattering experiments and constitute the most stringent laboratory bounds for sub-GeV masses; invisible-decay scenarios are addressed via the existing NA64 invisible-mode analysis.

Significance. If the resonant-channel modeling, efficiencies, and background subtraction are shown to be robust, the result would tighten laboratory constraints on B-L extensions relevant to neutrino-mass generation and dark-matter candidates, tripling the prior statistics and adding a new production mode. The quantified EoT uncertainty and explicit separation of visible versus invisible cases are strengths that support reproducibility of the limit-setting procedure.

major comments (1)
  1. Abstract: the central claim that the resonant e^{+}e^{-} channel yields the most stringent bounds for unbroken U(1)_{B-L} in the 200–300 MeV window is load-bearing and rests on unshown modeling of the production cross section, acceptance, efficiency, and irreducible backgrounds (e.g., radiative or misidentification processes). Without explicit equations, efficiency curves, or systematic tables for this channel, it is impossible to verify that the reported improvement over neutrino-scattering experiments survives plausible variations in these quantities.

Simulated Author's Rebuttal

1 responses · 0 unresolved

We thank the referee for the careful and constructive review. We address the major comment below and will revise the manuscript to incorporate the requested details.

read point-by-point responses

  1. Referee: Abstract: the central claim that the resonant e^{+}e^{-} channel yields the most stringent bounds for unbroken U(1)_{B-L} in the 200–300 MeV window is load-bearing and rests on unshown modeling of the production cross section, acceptance, efficiency, and irreducible backgrounds (e.g., radiative or misidentification processes). Without explicit equations, efficiency curves, or systematic tables for this channel, it is impossible to verify that the reported improvement over neutrino-scattering experiments survives plausible variations in these quantities.

    Authors: We agree that the abstract claim regarding the resonant e^{+}e^{-} annihilation channel requires supporting details to be verifiable. The manuscript describes the inclusion of this channel and the overall dataset, but does not provide explicit equations for the production cross section, acceptance/efficiency curves, or dedicated systematic tables for backgrounds in this mode. We will revise by adding these elements (formulas, figures, and a systematics table) in a new subsection or appendix. This will allow assessment of robustness under variations and confirmation of the improvement over neutrino-scattering limits. revision: yes

Circularity Check

0 steps flagged

No circularity: experimental limits extracted from data counts and efficiencies

full rationale

The paper reports new upper limits on g_{B-L} from the NA64 fixed-target experiment using a measured dataset of (9.4±0.5)×10^11 electrons on target. Limits are obtained by comparing predicted signal yields (production cross-section times acceptance times efficiency) against observed event counts in visible and invisible channels. No derivation step reduces a claimed prediction to a fitted parameter by construction, nor does any central result rely on a self-citation chain whose content is unverified within the paper. The resonant-channel inclusion is a standard physics modeling choice whose validity is external to the reported bounds. The result is therefore self-contained against external benchmarks (prior neutrino-scattering limits) and receives score 0.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 0 invented entities

The central claim rests on standard assumptions of the NA64 detector performance and background modeling that are typical for fixed-target experiments but not detailed in the abstract.

axioms (1)
  • domain assumption Standard Model predictions for backgrounds and signal efficiencies in electron-nucleus interactions are accurate when calibrated with control data.
    The limit extraction depends on this modeling to distinguish signal from background in both visible and invisible channels.

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Works this paper leans on

54 extracted references · 32 canonical work pages · 11 internal anchors

  1. [1]

    0 Branching Ratio Z ′ → ν ¯ν Z ′ → hadr Z ′ → e− e+ Z ′ → µ − µ + B − L 200 400 600 800 mZ′ [MeV]

  2. [2]

    0 Branching Ratio Z ′ → ν ¯ν Z ′ → hadr Z ′ → e− e+ B − 3Le 200 400 600 800 mZ′ [MeV]

  3. [3]

    0 Branching Ratio Z ′ → ν ¯ν Z ′ → hadr Z ′ → e− e+ Z ′ → µ − µ + B − 2Le − Lµ 200 400 600 800 mZ′ [MeV]

  4. [4]

    0 Branching Ratio Z ′ → ν ¯ν Z ′ → hadr Z ′ → e− e+ Z ′ → µ − µ + B − Le − 2Lµ FIG. 2. Branching fractions of Z ′ decay channels for mZ′ < 1 GeV in the unbroken symmetry case (Dirac neutrinos) described by Eq. 1 for various B − L-type models. 10− 3 10− 2 10− 1 100 101 102 103 mZ′ [MeV]

  5. [5]

    ECAL vs HCAL

    0 BR(Z ′ → invis) Invisible BR B − L B − 3Le B − 2Le − Lµ B − Le − 2Lµ FIG. 3. Invisible branching ratio as a function of mediator mass for different B − L-type models in the unbroken sym- metry (Dirac neutrinos). II. Z ′ SEARCHES A T NA64 NA64 is a fixed-target experiment exploiting the high- energy beams in the North Area facility at CERN. In particular, ...

    work page doi:10.13039/501100011033/feder 2022

  6. [6]

    The Physics of Heavy Z' Gauge Bosons

    P. Langacker, Rev. Mod. Phys. 81, 1199 (2009) , arXiv:0801.1345 [hep-ph]

    work page internal anchor Pith review Pith/arXiv arXiv 2009

  7. [7]

    Unbroken B-L Symmetry

    J. Heeck, Phys. Lett. B 739, 256 (2014) , arXiv:1408.6845 [hep-ph]

    work page internal anchor Pith review Pith/arXiv arXiv 2014

  8. [8]

    $Z^\prime$ portal dark matter in the minimal $B-L$ model

    S. Okada, Adv. High Energy Phys. 2018, 5340935 (2018), arXiv:1803.06793 [hep-ph]

    work page internal anchor Pith review Pith/arXiv arXiv 2018

  9. [9]

    Serendipity in dark photon searches

    P. Ilten, Y. Soreq, M. Williams, and W. Xue, JHEP 06, 004 (2018) , arXiv:1801.04847 [hep-ph]

    work page internal anchor Pith review Pith/arXiv arXiv 2018

  10. [10]

    Okada, S

    N. Okada, S. Okada, and Q. Shafi, Phys. Lett. B 810, 135845 (2020) , arXiv:2003.02667 [hep-ph]

    work page arXiv 2020

  11. [11]

    Y. M. Andreev et al. (NA64), Phys. Rev. Lett. 129, 161801 (2022) , arXiv:2207.09979 [hep-ex]

    work page arXiv 2022

  12. [12]

    K. Asai, A. Das, J. Li, T. Nomura, and O. Seto, Phys. Rev. D 106, 095033 (2022) , arXiv:2206.12676 [hep-ph]

    work page arXiv 2022

  13. [13]

    K. Asai, A. Das, J. Li, T. Nomura, and O. Seto, Phys. Rev. D 109, 075026 (2024) , arXiv:2307.09737 [hep-ph]

    work page arXiv 2024

  14. [14]

    S. K. A., A. Das, G. Lambiase, T. Nomura, and Y. Orikasa, Eur. Phys. J. C 84, 1224 (2024) , arXiv:2308.14483 [hep-ph]

    work page arXiv 2024

  15. [15]

    S. K. A., S. Das, A. Das, and S. Mandal, Phys. Rev. D 112, 115045 (2025) , arXiv:2508.10734 [hep-ph]

    work page arXiv 2025

  16. [16]

    Y. Y. Komachenko and M. Y. Khlopov, Sov. J. Nucl. Phys. 51, 692 (1990)

    1990

  17. [17]

    Fayet, Phys

    P. Fayet, Phys. Lett. B 95, 285 (1980)

    1980

  18. [18]

    Fayet, Nucl

    P. Fayet, Nucl. Phys. B 187, 184 (1981)

    1981

  19. [19]

    Fayet, Nucl

    P. Fayet, Nucl. Phys. B 347, 743 (1990)

    1990

  20. [20]

    Fayet, Phys

    P. Fayet, Phys. Rev. D 70, 023514 (2004) , arXiv:hep- ph/0403226

    work page arXiv 2004

  21. [21]

    Scalar Dark Matter candidates

    C. Boehm and P. Fayet, Nucl. Phys. B 683, 219 (2004) , arXiv:hep-ph/0305261

    work page internal anchor Pith review Pith/arXiv arXiv 2004

  22. [22]

    Fayet, Phys

    P. Fayet, Phys. Rev. D 75, 115017 (2007) , arXiv:hep- ph/0702176

    work page arXiv 2007

  23. [23]

    X. G. He, G. C. Joshi, H. Lew, and R. R. Volkas, Phys. Rev. D 43, 22 (1991)

    1991

  24. [24]

    X.-G. He, G. C. Joshi, H. Lew, and R. R. Volkas, Phys. Rev. D 44, 2118 (1991)

    1991

  25. [25]

    Fileviez Perez et al

    P. Fileviez Perez et al. , (2022), arXiv:2208.00010 [hep- ph]

    work page arXiv 2022

  26. [26]

    Acharya et al

    H. Acharya et al. (LEGEND), Phys. Rev. Lett. 136, 022701 (2026) , arXiv:2505.10440 [hep-ex]

    work page arXiv 2026

  27. [27]

    Abe et al

    K. Abe et al. (XMASS), Phys. Lett. B 809, 135741 (2020), arXiv:2005.11891 [hep-ex]

    work page arXiv 2020

  28. [28]

    X. P. Geng et al. (CDEX), Phys. Rev. D 107, 112002 (2023), arXiv:2210.01604 [hep-ex]

    work page arXiv 2023

  29. [29]

    Atzori Corona, M

    M. Atzori Corona, M. Cadeddu, N. Cargioli, F. Dordei, C. Giunti, Y. F. Li, E. Picciau, C. A. Ternes, and Y. Y. Zhang, JHEP 05, 109 (2022) , arXiv:2202.11002 [hep-ph]

    work page arXiv 2022

  30. [30]

    Neutrino-Electron Scattering: General Constraints on Z' and Dark Photon Models

    M. Lindner, F. S. Queiroz, W. Rodejohann, and X.-J. 9 Xu, JHEP 05, 098 (2018) , arXiv:1803.00060 [hep-ph]

    work page internal anchor Pith review Pith/arXiv arXiv 2018

  31. [31]

    Constraints on Dark Photon from Neutrino-Electron Scattering Experiments

    S. Bilmis, I. Turan, T. M. Aliev, M. Deniz, L. Singh, and H. T. Wong, Phys. Rev. D 92, 033009 (2015) , arXiv:1502.07763 [hep-ph]

    work page internal anchor Pith review Pith/arXiv arXiv 2015

  32. [32]

    The Dark Photon

    M. Fabbrichesi, E. Gabrielli, and G. Lanfranchi (2020) SpringerBrief in Physics, arXiv:2005.01515 [hep-ph]

    work page internal anchor Pith review Pith/arXiv arXiv 2020

  33. [33]

    See Appendix A for expressions of the partial widths

  34. [34]

    H. W. Atherton, C. Bovet, N. Doble, G. von Holtey, L. Piemontese, A. Placci, M. Placidi, D. E. Plane, M. Reinharz, and E. Rossa, Precise Measurements of Particle Production by 400-GeV/ c Protons on Beryllium Targets, Tech. Rep. (CERN, 1980)

    1980

  35. [35]

    H. W. Atherton, P. Coet, N. T. Doble, and D. E. Plane, Electron and photon beams in the SPS experimental ar- eas, Tech. Rep. (CERN, Geneva, 1985)

    1985

  36. [36]

    Prebibaj, G

    T. Prebibaj, G. Arduini, H. Bartosik, and M. A. Fraser, SPS Operation and Future Proton Sharing Scenarios for the ECN3 facility , Tech. Rep. (CERN, 2023)

    2023

  37. [37]

    Banerjee, J

    D. Banerjee, J. Bernhard, M. Brugger, N. Charitonidis, N. Doble, L. Gatignon, and A. Gerbershagen, (2021), dedicated to Giorgio Brianti on the 50th anniversary of his founding the SPS Experimental Areas Group of CERN-Lab II and hence initiating the present Enterprise

    2021

  38. [38]

    Y. M. Andreev et al. , Nucl. Instrum. Meth. A 1057, 168776 (2023) , arXiv:2305.19411 [hep-ex]

    work page arXiv 2023

  39. [39]

    Observing a light dark matter beam with neutrino experiments

    P. deNiverville, M. Pospelov, and A. Ritz, Phys. Rev. D84, 075020 (2011) , arXiv:1107.4580 [hep-ph]

    work page internal anchor Pith review Pith/arXiv arXiv 2011

  40. [40]

    Volkov, P

    V. Volkov, P. Volkov, T. Enik, G. Kekelidze, V. Kra- marenko, V. Lysan, D. Peshekhonov, A. Solin, and A. Solin, Phys. Part. Nucl. Lett. 16, 847 (2019)

    2019

  41. [41]

    Performance of Multiplexed XY Resistive Micromegas detectors in a high intensity beam

    D. Banerjee et al. , Nucl. Instrum. Meth. A 881, 72 (2018), arXiv:1708.04087 [physics.ins-det]

    work page internal anchor Pith review Pith/arXiv arXiv 2018

  42. [42]

    High purity 100 GeV electron identification with synchrotron radiation

    E. Depero et al., Nucl. Instrum. Meth. A 866, 196 (2017), arXiv:1703.05993 [physics.ins-det]

    work page internal anchor Pith review Pith/arXiv arXiv 2017

  43. [43]

    D. P. Woodruff, Synchrotron Radiation: Sources and Ap- plications to the Structural and Electronic Properties of Materials (Cambridge University Press, 2021)

    2021

  44. [44]

    Therefore, unless stated otherwise, any cuts on the total HCAL energy deposition refer exclusively to the first three modules

    In the following, the HCAL4 module is treated indepen- dently from the other modules. Therefore, unless stated otherwise, any cuts on the total HCAL energy deposition refer exclusively to the first three modules

  45. [45]

    Agostinelli et al

    S. Agostinelli et al. (GEANT4), Nucl. Instrum. Meth. A 506, 250 (2003)

    2003

  46. [46]

    Allison et al

    J. Allison et al. (GEANT4), IEEE Trans. Nucl. Sci. 53, 270 (2006)

    2006

  47. [47]

    Allison et al

    J. Allison et al. (GEANT4), Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment 835, 186 (2016)

    2016

  48. [48]

    B. B. Oberhauser et al. , Comput. Phys. Commun. 300, 109199 (2024) , arXiv:2401.12573 [hep-ph]

    work page arXiv 2024

  49. [49]

    Y. M. Andreev et al. (NA64), Phys. Rev. Lett. 131, 161801 (2023) , arXiv:2307.02404 [hep-ex]

    work page arXiv 2023

  50. [50]

    Berlin, N

    A. Berlin, N. Blinov, G. Krnjaic, P. Schuster, and N. Toro, Phys. Rev. D 99, 075001 (2019) , arXiv:1807.01730 [hep-ph]

    work page arXiv 2019

  51. [51]

    Y. M. Andreev et al. , JHEP 07, 212 (2024) , arXiv:2404.06982 [hep-ex]

    work page arXiv 2024

  52. [52]

    Abreu et al

    H. Abreu et al. (F ASER), Phys. Lett. B 848, 138378 (2024), arXiv:2308.05587 [hep-ex]

    work page arXiv 2024

  53. [53]

    Y. M. Andreev et al. (NA64), Phys. Rev. D 110, 112015 (2024), arXiv:2409.10128 [hep-ex]

    work page arXiv 2024

  54. [54]

    A6 must also be included in the total width

    If the coupling to DM is considered, then the partial width in Eq. A6 must also be included in the total width