Discriminating Dark Matter Origins with Directional Detection

Abstract

Scenarios where dark matter is boosted to relativistic velocities provide a promising probe of sub-GeV dark matter. Cosmic-ray upscattered and supernova-produced dark matter generate relativistic fluxes peaked toward the Galactic Centre, an anisotropy that offers a strong directional signature and is not mimicked by any terrestrial or cosmic background. We determine how many directional recoil events are required in a gas time-projection chamber to distinguish various scenarios for the origin of dark matter particles arriving in the solar system, which are otherwise indistinguishable without directionality. We find that standard halo dark matter particles can be distinguished from boosted populations with as few as $\mathcal{O}(20)$ events under reasonable track reconstruction performance and background conditions.

Figures (6)

• Figure 1: The directional event rates from cosmic ray boosted dark matter (left), supernova-produced dark matter (middle), and halo dark matter (right), each for two different DM mass values. The rates are displayed in galactic coordinates, with the galactic plane running horizontally.
• Figure 2: Recoil spectra for all DM models. The spectra extend to much higher energies for the high-mass cases than for the low-mass cases, as expected kinematically. The three rates for each set (solid and dashed lines) are normalised such that they generate the same number of events above a threshold of 5 keV.
• Figure 3: The directionality of normalised scattering rate with galactic longitude, for high mass (left) and low mass (right) CRDM, SNDM and halo DM at fixed galactic latitude. The angular spectra for the choice of SNDM and CRDM parameters demonstrate very similar shapes, as expected, with peaks lining up at the galactic centre $l_{\textrm{GC}} = 0\degree$. The halo DM peak exhibits a perpendicular arrival direction, coming from $l_{\textrm{Cygnus}} \sim 90\degree$.
• Figure 4: The median significance, $S_{50}$, at which an isotropic background could be rejected as a function of the number of detected signal events. The top three panels show the higher-mass halo DM, SNDM and CRDM models, while the bottom panels show the lower-mass cases. Detector performance degrades from left to right.
• Figure 5: The interquartile ranges for the median reconstructed longitude from $10^5$ Monte Carlo experiments as a function of the number of recoil events. The left panel shows the higher-mass models, while the right shows the lower-mass models. Degrading detector performance is indicated by increasing transparency, with halo DM in green and SNDM in orange. The darkest colour shows the case when we do not include any of the performance limitations discussed in the text, which we use for comparison only. The median direction approaches the true source direction with an increasing number of events, as expected, while the width of the bands quantifies how accurate the reconstructed median direction would be in at least 50% of hypothetical experiments. We consider the two models to be distinguishable as long as the two regions do not overlap. We omit the CRDM case in this figure as it overlaps with the SNDM case.