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Radio detection of the PeV - EeV cosmic-neutrino flux

Periodic Reporting for period 1 - RadNu (Radio detection of the PeV - EeV cosmic-neutrino flux)

Reporting period: 2019-02-01 to 2020-07-31

The RadNu project aims to show the proof-of-principle of the radar detection technique to probe cosmic-neutrino-induced particle cascades. The final goal is to install an in-nature radar detection set-up with the sensitivity to detect 1-10 cosmic neutrinos in the currently unexplored PeV-EeV energy range. The detection of this flux allows us to explore the inner engines and physics of the highest energy environments in our universe. This will be done by the development of cutting-edge radar technologies as well as advanced analysis and reconstruction techniques.
RadNu

Currently the RadNu team at the host institute consists of:

Full time RadNu: PI, 2 Postdocs (postdoc 2 starting October 1, 2020), 2 PhD students
Part time RadNu (external funds): 1 Professor, 1 PostDoc, 2 PhD students

Main results and ongoing research:

SLAC beam test experiment and data analysis

The strong suggestion of a scattered signal found during our experiment in May 2018 in combination with the SLAC facility detector upgrade had the second beam test experiment moved forward to Oktober 2018. The analysis for the scatter found in the May 2018 experiment was finalized within the RadNu project and published in Phys. Rev. D. The Oktober 2018 experiment was performed successfully, and except the HDPE target that was used instead of ice fulfilled all conditions given in the RadNu proposal. Large part of the data analysis of this second run was performed within the first year of the RadNu ERC project. The improved experimental set-up, in combination with an improved knowledge on the noise environment lead to a major breakthrough with the first ever detection of a radar reflection of a high-energy particle cascade. This result was published in [2] and got a broad attention in various media (see www.radarechotelescope.org).

Modeling radio signals from particle beams and cascades moving between different media

Studies were performed to predict the coherent transition radiation signal, expected not only in a beam test experiment like the one performed at SLAC, but also in the natural process of a cosmic-ray-induced air shower hitting a large altitude ice sheet. This signal is expected to pose a possible background for our in-nature radar detection set-up. The results of our calculations showed that the transition radiation from cosmic-ray air showers moving from air to ice cannot be ignored, and more interestingly could form an explanation for two so-far unexplained events detected by the ANITA neutrino detector. These results were published in [3].

The signals from cosmic-ray induced particle cascades should provide a detectable signal in our proposed radar set-up, but also in the ARA and RNO-G Askaryan radio detectors searching for the direct radio emission from a neutrino induced particle cascade in ice. As member of the ARA and RNO-G collaborations, the signal from a cosmic-ray particle cascade moving between different media is being modeled an an analysis to search for these cosmic-ray signals is ongoing. First promising results are expected to be submitted to a journal in winter 2020/2021.

Radar Echo Telescope (RET)

The detection of a radar scatter at the Oktober 2018 SLAC experiment lead to the formation of the RET collaboration (www.radarechotelescope.org). Studies performed within the RadNu project show that in favorable circumstances a cosmic-ray particle cascade will hit the ice close to its maximum development, containing enough charge and energy to induce an in-ice particle cascade. The in-ice part of the cascade subsequently was found to have favorable properties for its radar detection, which initiated the RET-CR experiment as an intermediate detector toward the RET-N radar neutrino detector. These results are foreseen to be submitted to a journal in the fall of 2020.

The initial goal of the RET-CR experiment is to provide the proof-of-principle for the radar detection technique to probe a particle cascade in ice, by investigating cosmic-ray-induced cascades moving from air to ice. A cosmic-ray air shower is easily detected by a surface set-up, that can subsequently trigger the in-ice radar system. The radar detection of the air shower core moving into the ice is therefore the perfect way to show the proof-of-principle of the detection method in nature. Sensitivity studies for the RET-CR detector have been performed, and surface detector preparations are ongoing. The surface set-up will be tested at the host institute (VUB) in the fall of 2020. The RET-CR instrument paper is foreseen to be submitted to a peer-reviewed journal in the fall of 2020.

Radar signal modeling and reconstruction

The radar scattering model developed by the PI has been expanded during the first period of the RadNu project. Ionization charge collisions and their effect on radar scattering were studied, and a more realistic particle cascade model has been implemented. Besides the detailed model, basic signal properties were investigated using a simplified relativistic line-cascade model. Several novel and useful signal features were identified. The improved model and the observed signal features are currently written down for publication.

Besides these modeling efforts also energy and directional reconstruction are currently under investigation by applying Machine Learning algorithms to simulation.
Beyond the state of the art results:

The first ever detection of a radar scatter from a high-energy particle cascade

The Oktober 2018 SLAC beam test experiment was performed successfully, and fulfilled all conditions given in the RadNu proposal, except for the target, for which it was decided to use a High-Density-PolyEthyleen (HDPE) targed. Large part of the data analysis was performed within the first year of the RadNu ERC project. The improved experimental set-up, in combination with an improved knowledge on the noise environment at SLAC lead to a major breakthrough with the first ever detection of a radar reflection of a high-energy particle cascade. This result was pubished in [1] and got a broad attention in the media (see www.radarechotelescope.org).

Coherent transition radiation: implications for the anomalous ANITA events

Studies were performed to predict the background signal from coherent transition radiation, expected not only in a beam test experiment like the one performed at SLAC, but also in the natural process of a cosmic-ray-induced air shower hitting a large altitude ice sheet. The results of our calculations showed that the transition radiation from cosmic-ray air showers moving from air to ice cannot be ignored, and more interestingly could form an explanation for two so-far unexplained events detected by the ANITA neutrino detector. Our investigations showed that the ANITA signals likely originate from the transition radiation of a high-energy cosmic-ray particle cascade hitting a large altitude air-ice boundary. These results were published in [2].

[1] S. Prohira, K.D. de Vries, et. al., Phys.Rev.Lett. 124 (2020) 9, 091101

[3] Krijn D. de Vries, Steven Prohira, Phys.Rev.Lett. 123 (2019) 9, 091102
Radar detection of a neutrino-induced particle cascade in ice