The median recoil direction as a WIMP directional detection signal

TL;DR

This paper explores how the median recoil direction in WIMP detection can confirm dark matter signals, determining the number of events needed to reject isotropy with high confidence, considering backgrounds and speed distribution features.

Contribution

It introduces a method to use the median recoil direction to confirm WIMP signals and quantifies the required events for statistical significance under various conditions.

Findings

31 events needed for zero background to reject isotropy

Number of events increases with higher isotropic background

Features in the speed distribution affect median recoil direction analysis

Abstract

Direct detection experiments have reached the sensitivity required to detect dark matter WIMPs. Demonstrating that a putative signal is due to WIMPs, and not backgrounds, is a major challenge however. The direction dependence of the WIMP scattering rate provides a potential WIMP `smoking gun'. If the WIMP distribution is predominantly smooth, the Galactic recoil distribution is peaked in the direction opposite to the direction of Solar motion. Previous studies have found that, for an ideal detector, of order 10 WIMP events would be sufficient to reject isotropy, and rule out an isotropic background. We examine how the median recoil direction could be used to confirm the WIMP origin of an anisotropic recoil signal. Specifically we determine the number of events required to reject the null hypothesis that the median direction is random (corresponding to an isotropic Galactic recoil distribution) at 95% confidence. We find that for zero background 31 events are required, a factor of roughly 2 more than are required to simply reject isotropy. We also investigate the effect of a non-zero isotropic background. As the background rate is increased the number of events required increases, initially fairly gradually and then more rapidly, once the signal becomes subdominant. We also discuss the effect of features in the speed distribution at large speeds, as found in recent high resolution simulations, on the median recoil direction.