Characteristics of Four Upward-Pointing Cosmic-Ray-like Events Observed with ANITA

Abstract

We report on four radio-detected cosmic-ray (CR) or CR-like events observed with the Antarctic Impulsive Transient Antenna (ANITA), a NASA-sponsored long-duration balloon payload. Two of the four were previously identified as stratospheric CR air showers during the ANITA-I flight. A third stratospheric CR was detected during the ANITA-II flight. Here, we report on characteristics of these three unusual CR events, which develop nearly horizontally, 20-30 km above the surface of Earth. In addition, we report on a fourth steeply upward-pointing ANITA-I CR-like radio event which has characteristics consistent with a primary that emerged from the surface of the ice. This suggests a possible τ-lepton decay as the origin of this event, but such an interpretation would require significant suppression of the standard model τ-neutrino cross section.

Upward-Pointing Cosmic-Ray-like Events Observed with ANITA

“…The ANITA collaboration recently reported the observation of four upward-pointing cosmicray-like events observed during the first flight [1]. Three of these events were consistent with cosmic-ray events above the geometric horizon but below the horizontal.…”

“…The relative probability of a τ-lepton air shower to an anthropogenic origin is ∼ 550, which favors the former hypothesis but does not exclude the latter with high confidence. The analysis described here is discussed in more detail in [1]. The data recently obtained with the third and fourth flights of ANITA, with ongoing blind analyses designed to address this disambiguation, will potentially confirm or falsify the τ-lepton decay hypothesis.…”

“…In the top panel of Figure 2 we show the electric field peak in the 200-1200 MHz band for a 10 17 eV hadronic shower as a function of the view angle θ view for various emergence angles with the τ lepton decay altitude set to 0 km above the Antarctic ice sheet. We used the magnetic field at the location of the candidate event from [1]. In the bottom panel of Figure 2, we estimate the square of the electric field peak times the distance to the ANITA payload at 37 km altitude multiplied by the solid angle corresponding to the full-width half max of θ view .…”

“…In the original publication [1] it was pointed out that the emergence angle of the event is 25.4 • with a ∼ 1 • uncertainty, meaning that the trajectory through the Earth has an attenuation coefficient of 4 × 10 −6 at 1 EeV, making it unlikely that such an event could have been observed with ANITA's exposure. In addition, standard model (SM) neutrino interaction cross-sections and τ lepton energy loss rates should lead to many more of these events observed closer to the horizon.…”

“…The upward-going τ lepton and reflected down-going cosmic ray hypotheses each predict a different location of air shower maximum resulting in different predictions for the polarization. The radio impulse shape, polarization, and air shower geometry of these events is discussed in detail in [1].…”