Can transition radiation explain the ANITA event 3985267?

We investigate whether transition radiation from a particle shower crossing the interface between Earth and air and induced by an Earth-skimming neutrino can explain the upward event announced recently by the ANITA Collaboration. While the properties of the observed signal can in principle be explained with transition radiation, a conservative upper limit on the experiment’s aperture for this kind of signal shows that the flux necessary for a successful explanation is in tension with the current best limits from the Pierre Auger Observatory, the IceCube neutrino detector, and the ANITA balloon. We also show that in this scenario, the direction of the incoming neutrino is determined precisely to within a few degrees, combining the polarization properties of the observed events with the Earth opacity to ultrahigh energy neutrinos.

Figure 1

Sketch of the emission geometry. The neutrino moves along a trajectory (gray arrow) described by a zenith angle ${\theta }_{\mathrm{sh}}$ from the vertical; the emission point is at the intersect of this trajectory with the Earth’s surface. The observer is located at distance $d$ from the emission point at zenith angle ${\theta }_o$. Projections of neutrino and observer directions into the horizontal plane are displayed with dashed lines; the angle between them is ${\varphi }_o$.

Figure 2

Approximate degree of polarization of the signal $A_V/A_H$ defined in Eq. (6) as a function of the neutrino inclination ${\theta }_{\mathrm{sh}}$ and the observer position ${\varphi }_o$ (see Fig 1); using fixed ${\theta }_o=63.2°$ determined by the data. Only a portion of the parameter space in agreement with the measured values $A_V/A_H\approx 0.2$ is shown.

Figure 3

Contributions to the conservative upper limit on the exposure for TR detection from neutral current interaction (solid) and charged current interaction of ${\nu }_{\tau }$, ${\nu }_{\mu }$ (dashed) and ${\nu }_e$ (dot dashed); each curve represents a contribution of a single flavor.

Figure 4

Conservative upper limit on the exposure of the two combined ANITA flights for neutrino detection using TR (black), compared with exposures of other experiments (gray): standard analysis of ANITA using Askaryan radiation [26] (dot-dashed), Pierre Auger Observatory [7] (dashed), IceCube [4, 27] (solid).