Is the effect of the Sun's gravitational potential on dark matter particles observable?

TL;DR

This paper investigates how the Sun's gravitational potential influences dark matter detection signals, showing that gravitational focusing affects the interpretation of annual modulation data and is challenging to observe directly.

Contribution

It provides the first detailed analysis of Solar gravitational focusing effects on dark matter direct detection signals, including implications for current and future experiments.

Findings

GF reduces the allowed parameter space for DAMA/LIBRA modulation.

Detecting GF effects in future experiments is difficult, even with large exposures.

GF is more relevant for high-mass dark matter and in inelastic scattering scenarios.

Abstract

We consider the effect of the Sun's gravitational potential on the local phase space distribution of dark matter particles, focusing on its implication for the annual modulation signal in direct detection experiments. We perform a fit to the modulation signal observed in DAMA/LIBRA and show that the allowed region shrinks if Solar gravitational focusing (GF) is included compared to the one without GF. Furthermore, we consider a possible signal in a generic future direct detection experiment, irrespective of the DAMA/LIBRA signal. Even for scattering cross sections close to the current bound and a large exposure of a xenon target with 270 ton yr it will be hard to establish the presence of GF from data. In the region of dark matter masses below 40 GeV an annual modulation signal can be established for our assumed experimental setup, however GF is negligible for low masses. In the high mass region, where GF is more important, the significance of annual modulation itself is very low. We obtain similar results for lighter targets such as Ge and Ar. We comment also on inelastic scattering, noting that GF becomes somewhat more important for exothermic scattering compared to the elastic case.