At Hungary's Institute for Nuclear Research, ATOMKI, physicists noticed something odd: in the decays of excited atomic nuclei, pairs of electrons and positrons sometimes fly apart at an unusual angle. It's as if an invisible mediator particle briefly appears and disappears between them. This hint has been dubbed the ATOMKI anomaly, and the hypothetical particle is called the X(17) boson, because its mass is around 17 megaelectronvolts (about 34 times heavier than an electron).
To test whether such a particle could exist and how it interacts with electrons, the authors of the new study gathered data from a wide range of experiments — from ultra-precise measurements of the electron's magnetic properties to accelerator experiments where particle beams hit targets. They derived constraints on the strength of X(17)'s interaction with electrons. It turns out that many possibilities, such as scalar or pseudoscalar (where the particle has no spin)—are incompatible with observations due to parity conservation. The pure axial-vector option (spin 1, but with special behavior under mirror reflection) is ruled out with 98% confidence.
This result doesn't prove X(17) exists, but it sets clear boundaries for future searches. If the particle is there, it must be a vector boson and interact with electrons in a strictly defined way. Now experimentalists know what to look for and can tune their detectors to the right signal.
as written by the authors arXiv:2601.08567
The Atomki anomaly puts forward the hypothesis of an $$X(17)$$ particle to explain its observation. Utilizing experimental results from the Atomki experiments, measurements of the electron's anomalous magnetic moment, beam dump experiments, the KLOE-2 experiment, the PADME experiment, and the parity-violating M{\o}ller scattering experiment, we derive constraints on the $$Xee$$ coupling of the $$X(17)$$ boson to electrons. It is found that the scalar and pseudoscalar models can be excluded by Atomki experiments due to the parity conservation, and the pure axial-vector model is excluded at 98\% C.L. Meanwhile, the analyses in both pure vector and the vector $$\pm$$ axial-vector models consistently show that the $$Xee$$ coupling is of the vector type and has an almost fixed value, $$\left(6.78 \pm 0.042\right) \times 10^{-4} \lesssim |\varepsilon_e^v| \lesssim \left(6.93 \pm 1.66\right) \times 10^{-4}$$ in unit of electric charge $$e$$.
A mass of 17 MeV is about 34 times the electron mass, but thousands of times less than the proton mass. Such a light particle could be produced in nuclear transitions, as in the ATOMKI experiments.
The idea of a mediator particle with small mass and weak coupling is reminiscent of 'dark photons' from science fiction — invisible carriers of new forces that could explain dark matter.