First-principle calculation of birefringence effects for in-ice radio detection of neutrinos

Abstract

The detection of high-energy neutrinos in the EeV range requires new detection techniques to cope with the small expected flux. The radio detection method, utilizing Askaryan emission, can be used to detect these neutrinos in polar ice. The propagation of the radio pulses has to be modeled carefully to reconstruct the energy, direction, and flavor of the neutrino from the detected radio flashes. Here, we study the effect of birefringence in ice, which splits up the radio pulse into two orthogonal polarization components with slightly different propagation speeds. This provides useful signatures to determine the neutrino energy and is potentially important to determine the neutrino direction to degree precision. We calculated the effect of birefringence from first principles where the only free parameter is the dielectric tensor as a function of position. Our code, for the first time, can propagate full RF waveforms, taking interference due to changing polarization eigenvectors during propagation into account. The model is available open-source through the NuRadioMC framework. We compare our results to in-situ calibration data from the ARA and ARIANNA experiments and find good agreement for the available time delay measurements. This indicates a significant improvement of the prediction power of birefringence effects compared to previous models. Finally, the implications and opportunities for neutrino detection are discussed.

Simulation study for an in-situ calibration system for the measurement of the snow accumulation and the index-of-refraction profile for radio neutrino detectors

“…We further note that birefringence was found to have an important effect at the South Pole [29][30][31], i.e., that the index-of-refraction depends on the polarization and propagation direction of the radio pulse. The birefringence asymmetry is at the per-mill level at the South Pole and even smaller at the other sites considered for in-ice radio detection of neutrinos.…”

Results from the ARIANNA high-energy neutrino detector

“…Results from the ARIANNA high-energy neutrino detector code quickly became the de-facto community standard for in-ice neutrino detection, with a large community of developers continuously improving and enhancing its capabilities (see e.g. [19][20][21][22]).…”

“…One limitation of this study was that not the complete phase space of possible trajectories through the ice could be probed, in particular, mostly horizontally propagating signals. More experimental data probing the effects of propagation through ice on the radio signals will be useful, in particular, to probe the effects of birefringence [22,24].…”

Using NuRadioMC to study the performance of UHE radio neutrino detectors

“…Using NuRadioMC to study the performance of UHE radio neutrino detectors S. Bouma In addition, polar ice can be a birefringent medium, due to alignment of the ice crystals with the ice flow or gravitational stress. Code to simulate birefringence in NuRadioMC has been developed recently [12], and is expected to be included in the main release version soon.…”