Around the globe, physicists are striving to identify dark matter (DM) particles and their interactions with observable matter employing a range of tactics and detectors. Given these particles do not emit, reflect or absorb light, they have hitherto been exceedingly challenging to detect, particularly via standard experimental procedures.

Scientists from TRIUMF, University of Minnesota, University of California Berkeley, and Stanford University recently put forth a new strategy that could facilitate the detection of these enigmatic particles, not accounted for by the standard model. This strategy, outlined in a paper published in Physical Review Letters, seeks to identify indicators suggesting the annihilation of dark matter into visible matter within expansive neutrino detectors.

“Terrestrial DM (DM particles that are captured by the Earth through collisions with Earth’s constituents) that interacts robustly with conventional baryonic matter may boast a tantalizingly substantial density, nearly 15 orders of magnitude greater than the Galactic DM density (~ 0.3 GeV/cm3),” said Anupam Ray, a key scientist involved in the study, during an interview.

The pressing question then became: how to identify such plentiful DM particles within the Earth’s volume? Given their minute kinetic energy (~ 0.03 eV), detecting them through typical direct detection experiments seems virtually unattainable, as these tests lack sensitivity to such low energy deposition. Therefore, the team started to brainstorm innovative methods to identify these DM particles.

Rather than scouting for indicators suggesting the scattering of DM particles, a common tactic in most direct detection endeavors to date, Ray and his associates proposed observing their annihilation signals. These signals transpire when DM particles are annihilated, or when they collide with other particles and are consequently destroyed, releasing energy in the process.

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Annihilation signals differ from scattering signals in that they are not limited to modest amounts of kinetic energy, perhaps making them simpler to recognize and detect. With terrestrial DM particles believed to be prolific, the scientists recommended hunting for them by seeking out signals pointing to their annihilation within large-volume neutrino detectors, such as Super-Kamiokande. This massive Cherenkov detector, nestled beneath Mount Ikeno in Japan, is employed to investigate neutrinos sourced from the sun, supernovae, the atmosphere, and other origins.

“Terrestrial DMs that strongly interact with traditional baryonic matter are profusely available within any sizeable volume neutrino detectors, such as Super-Kamiokande,” Ray explained. “If they annihilate within the Super-Kamiokande fiducial volume, it could trigger observable traces. Super-K can readily scan these annihilation products, and from these scans, one could yield unparalleled sensitivity to DM parameters. It’s critical to emphasize that, even if these robustly interacting terrestrial DM particles constitute a very small fraction of the overall DM density (there’s no reason to presume DM is composed of a single species), our proposed technique can deliver world-class sensitivity to the DM parameters.”

The recent research by this group presents a new method that could assist in investigating terrestrial, robustly interacting DM particles, which are postulated to be highly prevalent yet have remained exceedingly challenging to observe. Even if these specific particles represent only a minuscule portion of the current DM density, this new approach could perform exceptionally well, thereby aiding the continued quest for DM.

“We are now eager to delve into the neutrino signatures from robustly interacting terrestrial DM,” Ray added. “In this study, we are not sensitive to a comparatively heavy DM mass (say a DM mass of 10 GeV or more). Because, as the DM becomes heavier, they concentrate towards the Earth’s core, and as a result, their number density within the Super-Kamiokande volume is significantly reduced, leading to a negligible signal. Nevertheless, through utilizing the neutrino signal, we remain optimistic about probing the heavy DM parameter space.”

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