Gusts in the Headwind

By Dr Grace Lawrence MRSV‍

‍2023 RSV Young Scientist Research Prizes (Physical Sciences) - 1st prize winner

‍Dark matter. The elusive and non-luminous substance that makes up a quarter of our cosmos, and permeates every corner of our universe. However, despite a plethora of indirect evidence, its true nature remains one of the greatest astrophysical mysteries of the 21st century.

Direct detection, an exciting experimental detection technique, offers researchers the opportunity to directly detect and characterise dark matter through its interactions with particles we can detect. Direct detection experiments search for the signature left when a dark matter particle recoils off the nucleus of a target atom in a process known as nuclear recoil.

While we cannot directly observe them, every galaxy has its own halo of dark matter, on which the ordinary, visible matter hangs. Our solar system circumnavigates the centre of our galaxy, while Earth orbits the Sun. As we do this, our apparent motion through the Milky Way’s halo of dark matter creates the Headwind Effect. The Earth’s changing velocity over a year with respect to this headwind traces out an oscillating signal called ‘annual modulation’, peaking in the middle of the year.

Annual modulation and direct detection theory are underpinned by the Standard Halo Model, a simplified way to mathematically model our galaxy and its dark matter content. With annual modulation triggered by the headwind effect, I asked the question, ‘what if there were gusts of dark matter buffeting the headwind?’ I investigated how the inherent variation of dark matter across a galaxy can inform and impact dark matter experiments.¹

^‍In preparation for direct detection of dark matter here on Earth, I first generated predictions of what we would find. I found that, despite clear evidence in the simulations of large variations and structures within the dark matter field, the annual modulation signals showed little variation. In fact, predictions made using the Standard Halo Model, remained consistent compared to supercomputer simulations, giving confidence to the existing theory and infrastructure in this field. Importantly, large variations in the time of year that the signal will peak were recorded, potentially complicating the way that annual modulation signals are interpreted. Direct detection experiments sit at the intersection of astronomy, particle physics, nuclear physics, and in the case of the Australian SABRE experiment, at the bottom of the active Stawell gold mine. The SABRE (Sodium Iodide with Active Background REjection) experiment is part of a dual-hemisphere effort to detect dark matter on opposite sides of the globe to distinguish between astrophysical and local seasonal effects. As the headwind of dark matter passes through the Earth, a small percentage of the dark matter particles will interact with the experiment, via nuclear recoil, transferring energy into the detector. SABRE uses radio-pure sodium-iodide crystals as their detector target, chosen for their atomic size and ability to convert high percentages of particle energy into light.

Searching the heavens from underground may seem counterintuitive, but an underground physics lair is a key criterion in the potential success of direct detection as cosmic radiation threatens to saturate or contaminate dark matter signals. By quantifying the impact of real dark matter structure, derived from simulations, on the signals expected for SABRE and other Earth-based experiments, we have significantly enhanced the preparedness of direct detection experiments to uncover a dark matter detection and guide the spotlight on this dark phenomenon.

‍References:

1. Lawrence et al. (2022). Gusts in the headwind: uncertainties in direct dark matter detection. Monthly Notices of the Royal Astronomical Society, 524 (2), 2606–2623. doi.org/10.1093/mnras/stac2447

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