Light dark sector via thermal decays of Dark Matter: the case of a 17 MeV particle coupled to electrons

Abstract

Recent experimental observations, most notably those reported by the ATOMKI and Positron Annihilation into Dark Matter Experiment (PADME) collaborations, have hinted anomalies that may indicate the presence of a new resonance with a mass around $17\,\text{MeV}$, potentially interacting with both nucleons and electrons. Since 2020, ATOMKI has observed this resonance in nuclear transitions from excited to ground states in ${}^{8}\mathrm{Be}$, ${}^{4}\mathrm{He}$, and ${}^{12}\mathrm{C}$. More recently, in 2025, PADME, operating at the Laboratori Nazionali di Frascati, has also hinted a similar excess, in this case in the $e^{+}e^{-}$ final-state events originating from positron annihilation on fixed-target atomic electrons of Carbonium. This concordance strengthens the case for a common underlying origin, potentially involving a new boson, conventionally referred to as $X_{17}$. Despite these intriguing developments, the global experimental landscape remains highly dynamic, particularly in light of recent MEG~II constraints, and a definitive confirmation or exclusion of the $X_{17}$ hypothesis is still lacking. Within this evolving and exciting context, this thesis investigates whether a hypothetical $17\,\text{MeV}$ particle, coupled to electrons as suggested by the PADME observations, could function as a mediator between the Standard Model and previously unexplored hidden sectors. Such a mediator could, in principle, offer a novel pathway toward addressing one of the principal outstanding inconsistencies of the Standard Model: the nature and origin of dark matter.

Figures (17)

• Figure 1: Rotation curves of spiral galaxies taken from Cirelli:2024ssz. On the left, the original rotation curve of Andromeda by Rubin and Ford (1970). On the right, a recent rotation curve for the Milky Way adapted from refId0. Both are plotted in function of the distance with respect to the center of the galaxy
• Figure 2: Possible mass range for DM candidates. In this work, we focus specifically on the MeV region.
• Figure 3: Schematic representation of the three main experimental strategies to probe particle DM: indirect detection, direct detection, and collider searches. They correspond respectively to SM final states from DM, DM–SM scattering, and DM production from SM collisions.The symbol $\chi$ denotes the dark–matter particle. Image taken from https://doi.org/10.1002/andp.201500114.
• Figure 4: Temperature evolution of the effective numbers of degrees of freedom. Image taken from Ref. Cirelli:2024ssz.
• Figure 5: Evolution of the DM abundance $Y$ in a freeze-out scenario. (i) For $z \ll 1$ ($T \gg m_\chi$) the abundance tracks its equilibrium value and remains approximately constant. (ii) Around $z \sim 1$ the DM becomes non–relativistic, annihilations become inefficient, and the abundance rapidly departs from equilibrium. (iii) For $z \gg 1$ annihilations freeze out and the abundance settles to its final asymptotic value, determining the relic density. Figure adapted from Ref. Cirelli:2024ssz.