The pros and cons of beyond standard model interpretations of ANITA events

Understand

The Antarctic Impulsive Transient Antenna (ANITA) experiment has observed two air shower events with energy $\sim 500~{\rm PeV}$ emerging from the Earth with exit angles $\sim 30^\circ$ above the horizon.

• As was immediately noted by the ANITA Collaboration, these events (in principle) could originate in the atmospheric decay of an upgoing $\tau$-lepton produced through a charged current interaction of a $\nu_\tau$ inside the Earth.
• However, the relatively steep arrival angles of these perplexing events create tension with the standard model (SM) neutrino-nucleon interaction cross section.
• Deepening the conundrum, the IceCube neutrino telescope and the Pierre Auger Observatory with substantially larger exposures to cosmic $\nu_\tau$'s in this energy range have not observed any events.