The Pierre Auger Observatory provides a laboratory for studying fundamental physics at energies far beyond those available at particle colliders. It can study hadrons (particles composed of quarks), photons, and, in principle, ultrahigh energy cosmic neutrinos. Cosmic neutrinos can provide valuable information because they are so energetic and their interactions are uncluttered by the strong and electromagnetic forces. These neutrinos are either produced at the same sites as ultrahigh energy (charged) cosmic rays, or originate from cosmic ray interactions, particularly with the cosmic microwave background.
Much about the cosmic neutrino flux is unknown, but Auger hopes to address the question of whether or not the neutrino-nucleon cross-section matches the standard model prediction beyond the TeV-scale. If it does not match, then new non-perturbative interactions may be revealed. To probe this topic, the rate of downgoing neutrinos of all flavors is compared with the rate of upgoing tau neutrinos. More specifically, upgoing tau neutrinos cause "Earth-skimming events." When a tau neutrino interacts in the Earth's crust, a tau lepton is produced and may decay just above the surface, where it can be observed. Downgoing air showers triggered by neutrino interactions in the atmosphere are called "quasi-horizontal events" if they arrive at the detector from just above the horizon. These slightly downgoing neutrinos are separated from the large background of charged cosmic ray air showers by the existence of an electromagnetic component at the time of observation.
A higher-than-expected rate of quasi-horizontal events could be attributed to either a higher cosmic neutrino flux or a larger neutrino-nucleon cross-section for interactions. However, these two possibilities cause opposite effects to occur to the rate of Earth-skimming tau neutrino events. If the cosmic neutrino flux is higher than expected, the Earth-skimming neutrino rate will also be higher. However, if the neutrino-nucleon cross-section is higher than expected, the Earth-skimming neutrino rate will be lower than predicted since the hadronic secondaries produced in the new-physics processes will interact before being able to escape the Earth's crust.
Neutrino interactions in the Earth and in the atmosphere and the Auger detector response were studied using simulations and detailed models of the local terrain. If 1 Earth-skimming and 10 quasi-horizontal events are observed, new physics would be needed to explain the observations since the standard model would be ruled out at the 99% confidence level. In the most optimistic case, a decade or so would be required to observe new physics phenomena.