WoU-MMA: Ice Characterization and Calibration to Enable Ultra-High Energy Neutrino Astronomy

Since their invention four hundred years ago, optical telescopes have been the primary tools used in astronomy. Over the past 25 years, scientists have started to focus more on subatomic particles, along with visible light, that come from cosmic sources. Previous experiments have shown that exploring new aspects of the universe can lead to unexpected discoveries of astronomical objects. One particularly fascinating subatomic particle is the neutrino. Neutrinos can arrive on Earth from sources that are too far away to be seen with regular telescopes. As a result, several neutrino telescopes have recently been set up in remote areas around the world, and researchers are currently developing their scientific capabilities. This approach is similar to what Galileo did when he built his telescope; shortly after it was invented, he made the remarkable discovery in 1610 of the four moons orbiting Jupiter, rather than Earth. By improving the images captured by neutrino telescopes, we may uncover equally exciting and transformative cosmic sources that could reshape our understanding of the universe.

Over the past three decades, our research group firstly demonstrated the feasibility of detecting Ultra-High Energy Neutrinos (UHEN) via in-ice radio-frequency (RF) methods, the characterization of the RF properties of polar ice has been since then an ongoing effort. Foundational measurements of the RF attenuation length and the depth-dependent refractive index (n(z)) has confirmed the suitability of cold polar ice as both a neutrino target and as an effective RF transmission medium. However, subsequent studies of RF propagation along both vertical and horizontal paths revealed unexpected ±6 dB variations in signal strength and even detected signals in configurations for which propagation should have been forbidden. Additional complexities emerged from 2018 deep pulsing experiments conducted from the SPICE borehole at the South Pole to the ARA radio receiver array, which revealed puzzling differences in amplitude and frequency content between direct (D) and refracted (R) signal paths. Aggregated calibration data highlight persistent discrepancies between theoretical models and observed behavior, particularly for receivers located in the upper 100 meters of the ice sheetknown as the firnwhere density gradients are most pronounced. This region, while ideal for deploying radio receivers using existing drilling technology, presents significant modeling challenges. Moreover, to enable multi-messenger astrophysics, it is essential to accurately reconstruct the incoming neutrino’s direction to correlate it with known astrophysical sources. While a confirmed UHEN detection may be within reach by the end of the decade, our current understanding of RF signal propagation in ice remains insufficient for precise neutrino astronomy. The discoveries of new sources will be enabled by an extended, targeted calibration campaign of the telescopes to be conducted on-site in Greenland, and off-site in our domestic laboratories.