Relaxion stars and their detection via atomic physics

Abhishek Banerjee,Gilad Perez,Dmitry Budker,Hyungjin Kim,Joshua Eby

doi:10.1038/s42005-019-0260-3

Abstract

The cosmological relaxion can address the hierarchy problem, while its coherent oscillations can constitute dark matter in the present universe. We consider the possibility that the relaxion forms gravitationally bound objects that we denote as relaxion stars. The density of these stars would be higher than that of the local dark matter density, resulting in enhanced signals in table-top detectors, among others. Furthermore, we raise the possibility that these objects may be trapped by an external gravitational potential, such as that of the Earth or the Sun. This leads to formation of relaxion halos of even greater density. We discuss several interesting implications of relaxion halos, as well as detection strategies to probe them. Here, we show that current and near-future atomic physics experiments can probe physical models of relaxion dark matter in scenarios of bound relaxion halos around the Earth or Sun.

Highlights

The cosmological relaxion can address the hierarchy problem, while its coherent oscillations can constitute dark matter in the present universe
Our results show that if light scalars like relaxions exist in selfgravitating configurations, they can be probed through transient encounters with the Earth only in the mass range of roughly mφ ≳ 4 ́ 10À8 eV; at smaller relaxion masses, either the encounter rate is much lower than 1 yr−1, or the density of the selfgravitating object is lower than the background dark matter (DM) density
In this work, we consider the effect ofscalar-field DM, e.g. relaxion DM, in atomic physics experiments. We propose that such DM can form gravitationally bound objects denoted as boson stars, and suggest that these stars can be formed around the Earth or the Sun leading to relaxion halos with density well above that of the local DM

Summary

Results

Various theoretical and experimental efforts have been put forward to probe effective variation of fundamental constants induced by a coherently oscillating background DM field As it can be seen from Eq (4), the effect is strongest when the mass is the lightest, mφ ’ 10À21 eV, which is marginally allowed by the observation of large-scale structures of the universe[37,38] or measured rotational velocities in galaxies[39]. Substituting this expression to Eq (3), one can compute the variation of fundamental constants, but the resulting effect is small; in the range mφ ≳ 10À15 eV, the sensitivity estimates discussed above suggest it is difficult to compete with the bounds that arise from fifth-force experiments[18,19,24]. Some generic properties of boson stars are reviewed in Supplementary Note 1

M2Pl 7π m2φ R4?

Discussion

Code availability