Unusual Near-Horizon Cosmic-Ray-Like Events Observed by the Fourth Flight of ANITA

Abstract

Despite decades of experimental observations, the astrophysical sources producing the measured flux of ultrahigh energy cosmic rays (UHECRs) have yet to be identified. Neutrinos, extremely weakly interacting neutral particles, are expected to be produced inside the astrophysical accelerators responsible for the production of UHECRs, and during the propagation of UHECRs to Earth. As neutral weakly-interacting particles, ultrahigh energy neutrinos are perhaps the best probe of the hadronic and leptonic processes governing these extreme astrophysical environments beyond the local universe. Yet, despite two decades of experimental searches, ultrahigh energy neutrinos have never been definitively detected. ANITA-IV, the fourth flight of the ANtarctic Impulsive Transient Antenna (ANITA), observed four anomalous events extremely close to the horizon. In this dissertation, I present the possibility that one or more of these anomalous “near horizon” events are indeed ultrahigh energy tau neutrinos detected via the unique Earth-skimming “tau air shower channel”. I develop the first “end-to-end” simulation of ANITA-IV’s sensitivity to these unique events and I use this simulation to determine whether these events are observationally consistent with tau-lepton-induced extensive air showers and, if they are, what are the constraints on the implied flux from populations of diffuse and point-like neutrino sources. Finally, I perform a blind search for any statistically significant associations between these four anomalous events and catalogs of astrophysical sources that are considered to be possible ultrahigh-energy neutrino and cosmic ray emitters. I find that these events are not observationally inconsistent with ultrahigh energy tau neutrinos, but that the implied (diffuse) flux and (point-like) fluence necessary to explain these events is in strong tension with limits set by other observatories, such as the Pierre Auger Observatory and IceCube. After unblinding the results of my search for associations between these events and catalogs of sources that have the potential to be UHECR sources, I find no statistically significant associations with any of the considered sources.