A new analysis constrains the properties of a hypothetical X17 particle using experimental measurements of muon g-2 and electron g-2. It does not rule out the X17 particle hypothesis, although it does meaningfully constrain the properties it can have if it does exist.
We combine the current experimental muon g−2 world average, which incorporates the final Fermilab result, with the latest electron g−2 determinations based on cesium and rubidium measurements to set 95% CL exclusion contours for a pure vector mediator coupled to leptons. We explicitly test the assumption that the electron and muon coupling magnitudes are equal by comparing this restricted case with the case of independent electron and muon couplings and quantify the impact on the allowed parameter space.
In the minimal visible dark-photon model, both leptons constrain the same kinetic mixing and are analyzed through a combined χ2 analysis. We compare the resulting g−2 bounds with existing accelerator direct-search exclusions and model-dependent astrophysical and cosmological constraints. From the accelerator comparison, we identify a region in the (mA′,|ϵ|) parameter space near 17~MeV, close to the reported X17 mass, that remains allowed by the direct-search contours displayed here but is excluded by the cesium-based electron g−2 constraint. The rubidium-based fit does not exclude this interval.
For an X17 boson with independent lepton couplings, we constrain the electron and muon couplings separately. Electron-only direct searches leave two disconnected allowed regions near the reported X17 mass: a newly reopened low-coupling interval and a higher-coupling region above the NA64 excluded band. The cesium-based electron g−2 constraint closes the higher-coupling region, while the rubidium-based constraint reduces its extent; neither affects the newly reopened low-coupling interval.
Using the current experimental muon g−2 world average, we obtain a new g−2-based exclusion region for the muon coupling, with no significant preference for a nonzero coupling.
Raoul Serao, Antonio Capolupo, "Electron and Muon g−2 Constraints on Light Vector Bosons: Dark Photons and the X17 Boson" arXiv:2608.24677 (August 25, 2026).
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ReplyDeleteThe X17 anomaly: The neutron channel 33He(n,X17)44He explored at CERN / n_TOF
Pierfrancesco Mastinu1* and Carlo Gustavino2**
1 INFN -LNL, Legnaro (PD), ITALY
2 INFN -ROMA1, Roma, ITALY
Published online: 5 August 2026
Abstract
Significant anomalies have been observed in the relative angular emission of electron-positron pairs in the 7Li(p, e+e−)8 Be, 3H(p, e+e−)4He and 11B(p, e+e−)12C nuclear reactions that have been portrayed as the signature of a new boson call X17 of mass around MX17 16.8 MeV/c2, mediating an exotic, unknown interaction. It has been proposed that X17 could be a vector boson, mediator of a fifth “protophobic” force, i.e. characterized by a strong suppression of the coupling to protons compared to neutrons. Under this assumption, the X17 discovery could help to explain, at least partially, the long standing anomaly on the muon (electron) magnetic moment. Till now, few experiments repeated the ATOMKI measurements, but still a clear confirmation (or rejection) of the existence of the X17 is pending. At n_TOF, we would like to measure the reaction induced by neutrons, 3He(n, X17)4He, which produces the same 4He* state as the 3H(p, X17)4He. In this paper, after a brief presentation of the physics motivations, the new proposed reaction will be analyzed. The detection setup is based on 4 Micro Pattern Gaseous Detectors (MPGDs) which work in Time-Projection-Chamber (TPC) mode to reconstruct the X17 decay leptons trajectories. Special attention was paid in order to measure X17 angular distribution, fondamental to get information about its quantum number and particular R&D care has been devoted to reducing the amount of inert material around the high pressure 3He gas-cell to be used as a target for the neutron-induced reaction. A specific design of the Carbon fibre gas-cell, aiming at reducing the background produced by scattered neutrons as well as inhibiting the production of external pair creation (EPC) has been developed.
2 X17 search: detection setup
The study of the 4He∗ de-excitation produced through
the 3He(n, e− e+)4He is under investigation by using the
n_TOF EAR2 facility at CERN[11]. This facility pro-
vides a pulsed, broadband neutron beam, in which the neu-
tron energy is deduced with the time of flight (ToF) tech-
nique. The wide energy range of EAR2 neutron beam al-
lows to excite simultaneously the first three levels of 4He∗.
The experimental setup is shown in Fig 2. The detection
setup is composed of 4 Micro-Pattern Gaseous Detectors
(MPGDs) which work in Time-Pojection-Chamber mode
equipped by a mesh of X-Y read-out strips placed on tw
4 Conclusions
X17 is offering an exciting challenge for experimental nu-
clear physicists, our experimental goal was to perform the
first measurement of the 3He(n,e+ e−)4He process at the
n_TOF facility in the second half of 2025, to shed light on
the X17 boson claim and to investigate its quantum num-
bers. The tests and first measurements showed unexpected
beam induced background level that spoiled the scintillator
bars performance, we realized that more tests are needed
and since the beginning of February 2026 we are work-
ing with our set up on the neutron beam, trying to solve
the background problems. The final measurements are
scheduled starting on the first of July 2026. The detection
setup was designed to be well suited for proton-induced
reactions as well, for instance it can be used to repeat the
ATOMKI 3He experiment with improved kinematic recon-
struction. In these latter case, the specular structure of the
3He and 3H nuclei and the different numbers of neutrons
and protons involved in the two reactions suggest to in-
vestigate in the next future the isospin dependency of the
X17-nucleon interaction, as well as the alleged “protopho-
bicity”.
References