Neutrinos are elusive particles. Although they are the most abundant particles in the Universe, they are very small and light (about 500 000 times smaller than electrons), chargeless, and they barely interact with matter: about 65 million neutrinos from the Sun pass through our thumbnail every second without disturbing us. These physical properties make them special for our field since they can travel from very distant galaxies without being affected by matter, or deflected by magnetic fields.
Neutrinos span a large range of energies. The lowest-energy particles, being the most abundant, come from the Big Bang, the Earth’s core and nuclear reactors (man-made and natural, like the Sun). On the other hand, neutrinos with the highest energies (energies close to the kinetic energy of a baseball traveling at 100 mph!) can be a product of the interaction of ultra-high energy (UHE) cosmic rays (such as protons) with particles of light (photons) remaining from the Big Bang. We do not know how and where these UHE cosmic rays are produced, a long-standing problem in our field; but they are most likely produced outside our galaxy, since we have not found something that energetic in our galaxy, the Milky Way. Because of the considerable distance to the sources, the UHE neutrino flux (number of neutrinos passing through certain area and time) at the earth’s surface is small, about one particle every ten years hitting the Earth. This, plus the fact that they barely interact with matter, makes studying UHE neutrinos very challenging. We have not seen one yet. These are the particles that the Askaryan Radio Array (ARA) experiment is looking for.
The ARA experiment, located in Antarctica (a radio quiet continent), consists of antennas buried under the Antarctic ice at about 200 m, and it looks for radio signals that are product of the interaction of neutrinos with ice. However, ARA mainly detects noise signals produced by the ice itself (thermal), and by humans.
ARA detects about a billion signals per year, and there is a small chance that one of them is a neutrino signal. My research focuses on studying new analysis techniques, and improving existing ones, that can help us clean the data from thermal and human-made noise. We use information such as the power content of the signal, its reconstructed source location, and how similar it is to what we would expect a neutrino signal look like to impose cuts that will filter out noise events.
So far, from the data we have analyzed, we have not seen anything. However, that has allowed us to constraint the UHE neutrino flux, which, in turn, could rule out proposed theories whose UHE neutrino flux predictions fall above our constraints. On the other hand, in the case that we see a neutrino, this will be a milestone for our field as it will be the first time that a neutrino at the highest energies is observed. This will help shape the next generation of experiments looking for these very energetic particles, allowing us to detect more events and to consequently being able to shine light on the long-standing mystery of the origin of UHE cosmic rays.