Hydrogen Portal to Exotic Radioactivity

We show that in a special class of dark sector models, the hydrogen atom can serve as a portal to new physics, through its decay occurring in abundant populations in the Sun and on Earth. The large fluxes of hydrogen decay daughter states can be detected via their decay or scattering. By constructing two models for either detection channel, we show that the recently reported excess in electron recoils at xenon1t could be explained by such signals in large regions of parameter space unconstrained by proton and hydrogen decay limits.

Figure 1

The two scenarios considered in this work that could explain the XENON1T electron recoil excess. In the first, H atoms decay on Earth (primarily in the oceans) to produce a dark baryon that propagates through the Earth and decays in a detector to another fermion and a photon. In the second, $e^{-}p$ capture in the Sun produces a fast-moving long-lived dark sector particle that scatters on detector electrons.

Figure 2

Lower limits, as derived in Ref. [9], on the H lifetime as functions of $Q$ and the total mass of the decay final state in our two scenarios. Top: Limits from the low energy Borexino spectrum [17] (red) and the search for $n\to \chi \gamma$ [16] (purple) in the neutron-mixing model of scenario 1. Bottom: Limits from Ref. [17] and a study of ${}^{14}\mathrm{C}$ purity [20] on the EFT interaction of scenario 2. Also shown with a green club and blue spade are the benchmark (${\tau }_{\mathrm{H}}$, $Q$) in scenario 1 and 2, respectively, that give rise to the spectra in Fig. 3.

Figure 3

Event rates in xenon1t in the scenarios of decay (green) and scattering (blue) of hydrogen daughters, seen to significantly reduce tensions in the data for the benchmark points indicated in Fig. 2. Here the red curve denotes the known backgrounds taken from Ref. [1]. The dashed curves depict the signal only while the solid curves depict the signal plus background. See text for further details.