I continue to be skeptical that an X17 particle exists. There is no sign of it at the LHC so far, and it has not been replicated at a different experiment from ATOMKI that has seen anomalies.
The ATOMKI anomalies motivate the possibility of a new particle, the X17, with a mass of 17 MeV and weak couplings to nuclei and electrons. Such a particle may be produced at the LHC, travel 480 m in the far-forward direction, and decay to e+e− in FASER.
In this paper, previous FASER searches for dark photons and axion-like particles (ALPs) are combined and interpreted as searches for a candidate X17 particle, the protophobic vector boson. The searches use data collected from pp collisions at s√=13.6 TeV in LHC Run 3 from 2022 for the dark photon search and from 2022-2023 for the ALP search.
No excess events are found above the expected background, and the combined null results exclude a previously-unconstrained "low-coupling window" with electron coupling 6 × 10^−5 < ϵe < 1.2 × 10^−4. A "high-coupling window" remains.
FASER Collaboration, "Constraints on a Long-Lived X17 Vector Boson with FASER" arXiv:2611.03904 (November 2, 2026).
Another preprint has more X17 constraint discussion here.
The FASER Collaboration is an international scientific team of 125 researchers across 27 institutions in 11 countries operating the Forward Search Experiment (FASER) at CERN
ReplyDeletethat is a lot of physicst taking x17 seriously
There is no sign of it at the LHC so far, and it has not been replicated at a different experiment from ATOMKI that has seen anomalies.
Neutral and Charged Decay Modes of the X17 Boson Observed in dCu Collisions at a Momentum of 3.8 GeV/c per Nucleon
Kh. U. Abraamyan(
Dubna, JINR and Yerevan State U.
)
, Ch. Austin(
Unlisted, UK
)
, M.I. Baznat(
IAP, Chisinau
)
, K.K. Gudima(
IAP, Chisinau
)
, M.A. Kozhin(
Dubna, JINR
)
Show All(7)
Sep 15, 2026
6 pages
Published in:
Phys.Part.Nucl. 57 (2026) 5, 1325-1330
Published: Sep 15, 2026
Abstract: (Springer)
Earlier (see [1]) we reported the observation of structures at about 17 and 38 MeV/c22 in the invariant mass spectra of photon pairs. The results of testing of the observed signals, including the results of the Monte Carlo simulation support the conclusion that the observed signals are the consequence of detection of the particles with masses of about 17 and 38 MeV/c22 decaying into a pair of photons. A comparative analysis of two series of measurements with two scintillation counters switched on in coincidence and without them, all other conditions being equal, are performed. Thus, together with the decay mode of X17 into two photons, the charged mode (e+e−e+e−) of the decay of X17 is also confirmed. According to preliminary estimates, the partial widths of these decays are comparable.
peer review journal
The hypothetical X17 boson has been studied via neutral and charged decay modes in deuteron-copper (dCu) collisions at a momentum of 3.8 GeV/c per nucleon
Collision System: Deuteron-copper (dCu) collisions
with statistical significance exceeding 6σ for the 17 MeV/c² peak
has some citation