Where is the X17?

FASER Briefings

FASER looks for the X17, didn’t find it. Anomaly remains.

5 November 2026 | FASER Collaboration

The ATOMKI Anomalies have puzzled physicists for over a decade. The first was first observed in 2015 at the ATOMKI research institute in Hungary, which, when looking at electron-positron pairs produced from de-excitations of Beryllium-8 nuclei, found a significant excess of pairs traveling in directions separated by a large angle. In the decade that followed, similar excesses have been reported in experiments involving Helium-4 and Carbon-12 nuclei. The Standard Model does not yet provide a satisfactory explanation for these excesses, but the results can be explained by a new force carrier particle with a mass of approximately 17 MeV: the X17.

Other experiments have also tried to find the X17. The MEG II experiment looked for it, but did not find it, while the PADME experiment reported hints of an excess that could be caused by the X17, adding to the puzzle. If such a particle exists, it could also be produced in large numbers at the Large Hadron Collider (LHC), but given the particle’s small mass, it would be produced in the forward direction, largely missing the acceptance of detectors like ATLAS and CMS. 

The ForwArd Search ExpeRiment (FASER) specializes in looking for such lightly, feebly-interacting particles from its location 480 meters downstream of the ATLAS interaction point (IP1).  The X17 can be produced at IP1 and then travel down the beam pipe and through 100 meters of rock and concrete to reach FASER. Once inside FASER, it can decay into an electron-positron pair. Such signatures, where a neutral particle enters FASER and spontaneously decays into an electron and a positron, are very unusual, and are not easily mimicked by Standard Model particles. In fact, most Standard Model particles (aside from muons and neutrinos) cannot even make the journey to FASER. 


To look for the X17, FASER combined two of its existing searches that looked for similar particles: dark photons and axion-like particles, or ALPs. The combination allowed FASER to track the decay particles using the tracking stations if the X17 decayed before them, while also being sensitive to X17’s that decayed after the tracking stations by using the calorimeter to measure the energy of the decay products. Since the previous searches did not find dark photons or ALPs, the combination analysis also did not find the X17. But this is still valuable, since the results can significantly reduce the possible properties the X17 could have. 

Given FASER’s location, the analysis was very sensitive to very feebly-interacting X17’s that live for a long time before decaying.  In the end, FASER excluded X17’s with electron couplings εe < 2×10-4.  This did not completely disprove the existence of the X17.  But now there is only a narrow window at higher electron couplings that is allowed, and FASER’s results now focus attention on this last hiding place for the X17.

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November 5, 2026