The viable parameter space for the hypothetic X17 particle (with a mass of about 17 MeV) proposed to explain some unexpected nuclear physics is very nearly null.
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% CL 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 12C(17.23), which drives the best-fit region towards excluded couplings.
Max H. Fieg, Toni Mรคkelรค, Tim M.P. Tait, Miลกa Toman, "The X17 with Chiral Couplings" arXiv:2602.11263 (February 11, 2026).
authors also note
ReplyDelete"Recently, the topic has been reinvigorated [9, 10] by the
observation of the PADME positron annihilation experi-
ment, entirely dissimilar to the nuclear experiments,
hinting at an excess compatible with the X17 [11
Further investigations for the origin of the ATOMKI
excesses are clearly needed. The X must couple to elec-
trons, and future PADME measurements with greater in-
tegrated luminosity can either confirm or refute the new
particle interpretation."
in other words, despite theoretical issues PADME results are encouraging, hoping for 5 sigma measurements with greater in-
tegrated luminosity
https://padme.lnf.infn.it/wp-content/uploads/sites/37/2025/09/PADME-vulcano-2025-valente-final.pdf
Optimized setup for Run IV
New data set being acquired [Run IV]
- Set the target closer to the calorimeter, increase acceptance
- Residual dipole magnetic field reduced [<1G]
- Improved readout of target position
- New detectors
expectations for Run IV:
- ×2 acceptance increase
- ×2 statistics increase
- 2.5 days for data collection, 3000 e+/spill as in Run III
- Points divided into 2 scans as in Run III
26
Run IV-part 1 data already on tape: 18 energy scan points collected (∼2×1010 PoTs each)
equally separated by 1.5 MeV in the range Ebeam = (269.5, 295) MeV; ๐ = (16.60, 17.36) MeV
Run IV-part 2 already scheduled for autumn 2025: add 18-20 scan points, offset by 0.75 MeV
Run IV-part 1 data already on tape: 18 energy scan points collected (∼2×1010 PoTs each)
equally separated by 1.5 MeV in the range Ebeam = (269.5, 295) MeV; ๐ = (16.60, 17.36) MeV
Run IV-part 2 already scheduled for autumn 2025: add 18-20 scan points, offset by 0.75 MeV
new data taking with an upgraded detector is ongoing: Jun-Nov 2025,
possible extension beginning of 2026
What if the Run IV result will confirm the excess, but not
conclusively, e.g. getting a significance still be below 5๐?
How to improve?
- The dominant uncertainty in the Run IV result will be most likely the
systematics, even though the improved geometry and the new detectors
should improve wrt Run III, in particular if ๐๐, ๐พ๐พ final states can be used
separately
- From the statistics point of view the main limitation is the maximum length
of the Frascati BTF positron beam
- ≈400 ns is the maximum obtained beam pulse length, with an acceptable momentum
spread, due to the not-flat shape of the accelerating voltage of the LINAC [given by the
SLED compression of the RF power for doubling the energy gain of the S-band cavities].
- On the other side, the density of positrons in the beam × density of the electrons in
the target = the maximum number of annihilations that can be handled by the PADME
detectors with acceptable levels of pile-up is ≈102 ns-1 for a 0.1 mm thick C target