We aim to detect neutrinos with energies in excess of 10 PeV using the radar echo method. When a high energy neutrino interacts in a dense material, like ice, it will produce a dense, relativistic cascade of charged particles. As this cascade moves through the medium, it ionizes the material, freeing electrons from their atoms. A short-lived cloud of free charge is produced that can reflect radio waves. The radar echo method is simple: a transmitting antenna broadcasts a radio signal into a volume, and a receiving antenna monitors that same volume. If one of these neutrinos interacts within this volume, the incident radio will be reflected from the ionization cloud to the receiver (think radar bouncing off of an airplane--but on a much, much shorter timescale [a few nanoseconds]).
The NSF and ERC supported first iteration of the Radar Echo Telescope, the Radar Echo Telescope for Cosmic Rays (RET-CR) will aim to detect cascades in the ice produced by high energy cosmic rays. These are charged particles that interact in the upper atmosphere, creating a cascade similar to a neutrino-induced cascade (described above) but on a much longer length scale (air is 1000 times less dense than ice, which is why neutrinos don't interact in the air). When this atmospheric cascade hits the ground (which happens if the primary particle is energetic enough and the ground elevation is high enough) it will produce a much denser cascade within the ground. RET-CR will have a transmitter and receiver buried near the surface of a high elevation ice sheet. We will detect the in-ice cascade from these high energy cosmic ray air showers that impact the ice. we'll use this to develop the technology to eventually target neutrinos with RET-N, the Radar Echo Telescope for neutrinos.
Why try to detect such high-energy neutrinos? Neutrinos are the only observed particle with beyond-standard-model properties. They have yet to be fully understood, and as such, studying them could lead to a deeper understanding of nature. Those at the highest energies are also intersting for another reason: they act as cosmic messengers. Neutrinos only interact via the weak force, and so can travel vast cosmic distances undeflected and unimpeded. Therefore, a detection at earth will point directly back to its source. Associating neutrinos with sources may help astronomers understand the sources of the highest energy cosmic rays, and the most energetic processes in the universe. And the key to this is to first detect them.
Doing so is difficult-the flux is very low, and neutrinos very rarely interact with matter (though they interact more readily at the energies RET probes). Radio techniques help us to instrument massive volumes, in order to increase the chances of detection.
The long and storied history of this method can be found on the "history" tab of this page.
Our newly released (04/21) instrument paper, The Radar Echo Telescope for Cosmic Rays: Pathfinder Experiment for a Next-Generation Neutrino Observatory, (arxiv:2104.00459)
S. Prohira, K. D. de Vries, P. Allison, J. Beatty, D. Besson, A. Connolly, N. van Eijndhoven, C. Hast, C.-Y. Kuo, U. A. Latif, T. Meures, J. Nam, A. Nozdrina, J. P. Ralston, Z. Riesen, C. Sbrocco, J. Torres, and S. Wissel, Observation of Radar Echoes from High-Energy Particle Cascades, Phys.Rev.Lett. 124 (2020) 9, 091101– Published 6 March 2020 (arxiv:1910.12830)
S. Prohira, K. D. de Vries, D. Besson, A. Connolly, C. Hast, U. Latif, T. Meures, A. Nozdrina, J. P. Ralston, Z. Riesen, D. Saltzberg, J. Torres, S. Wissel, and X. Zuo, Suggestion of coherent radio reflections from an electron-beam induced particle cascade, Phys. Rev. D 100, 072003 – Published 14 October 2019 (arxiv:1810.09914)
The radar echo method dates back to 1940, when P.M.S. Blackett and A.C.B. Lovell theorized  that they could use newly invented radar technology to detect ultra-high energy cosmic rays in the atmosphere. The concept of the extensive-air-shower (EAS), desribed by Auger in 1938 , was central to their investigation. In an EAS, the energy of a primary cosmic ray particle is distributed into a cascade of subsequent particles after the first interaction, and these subsequent particles move relativistically down toward the surface of earth. As they go, they will ionize a trail through the atmosphere by kicking out electrons from air molecules as they fly by. Blackett and Lovell thought that this ionization would be dense enough to reflect radio waves, and might be responsible for "sporadic radio reflexions" that had been observed in the atmosphere above 10 km by several different radar systems. While it was commonly believed that these were due to meteorites (which produce ionization trails in a similar manner), it was not ruled out that EAS could also contribute to the signals they observed. The idea is simple: set up a radar system that transmits radio waves into the atmosphere, and if an EAS occurs in the illuminated region of the sky, it will reflect these radio waves back to your receiver.
As chronicled in a fascinating history  by Lovell, they began operation in 1941 of some portable radar systems at Jodrell Bank, Manchester, UK, which was at the time just an empty field. From their 1940 paper, they had calculated that they should see a measurable rate of radar reflections from cosmic rays with a modest radar system. However, also in 1941, a letter came to Blackett from T.L. Eckersley, showing that they had a mistake in their calculations---a big one. It showed that they had not accurately considered collisional damping in their expressions (meaning, the free ionized electrons are not truly free, they bang into other things, mostly neutral molecules, and therefore their ability to reflect ratio is diminished) and that it could drastically reduce the power of the returned signal. This letter and the objection it raised (which was correct) would have stopped their experimentation---had they seen it. As it happens, this letter was missed by Blackett and Lovell in the frenzy of WWII, and work at Jodrell Bank continued to advance for some time, until 1945.
It was around this time that they became aware of the letter, and they began investigating the issue of collisional damping raised by Eckersley. What Lovell found was that it largely invalidated their current investigations, a conclusion he made to Blackett around 1946. However, Lovell showed that the method might still be viable with a large enough antenna for very high-energy cosmic rays. So they constructed what turned out to be the highest gain radio antenna on earth at the time. Alas, after definitive observations of meteor showers in the fall/winter of 1946 , all of the reflections that they observed turned out to be consistent with meteor reflections (in agreement with previous experiments ), and the prospect of EAS detection via radar was abandoned at Jodrell Bank. However, they had inadvertently created a radio antenna of immense importance to the burgeoning field of radio astronomy, and fortuitously started an observatory that continues to this day. Lovell, in his history, contends that if they had not missed Eckersley's letter and had instead abandoned their investigations in 1941 per the catastrophic collisional damping term, the observatory at Jodrell Bank would likely never had come to fruition at all.
In the 1960's there was some more investigation , but the results were unpublished. It is assumed that, similarly to the investigation by Blackett and Lovell, the reflections observed were solely attributable to meteors. In 2000 the method was once again revisited by Gorham , and then by a pilot study MARIACHI, which lead to a full scale experiment, the Telescope Array RAdar experiment . However, it seems that the original issue that Eckersley raised all those decades ago still plagued this theory (and another issue-the short electron recombination and attachment rates at EAS altitudes). As shown by several authors, and by the eventual non-detection by TARA, in-air detection of cosmic rays via radar is not feasible. 
But what about in-ice?
In 2012-2013, several groups [10,11] theorized and investigated whether or not a high-energy neutrino interacting in a dense material---producing an EAS-like cascade, but on a much smaller length scale---could be detectable via radar. This cascade, and subsequent ionization, would be orders of magnitude more dense than that of an EAS of equivalent energy in air (because ice is so much more dense than air), and therefore might allow for a radar reflection by the same principles. Chiba et al. demonstrated a proof of concept in the laboratory, and subsequent theory papers [12,13] explored the method in greater detail (with  incorporating Eckersley's original concern from first principles). Finally, experiment T576 at SLAC made an observation of a radar echo from a particle cascade (albeit, from an electron bunch in a plastic target, rather than a nuetrino in ice) in good agreement with the latest theoretical predictions. 
This brings us to the present. We have developed the concept for the Radar Echo Telescope in hopes of moving the radar echo idea out of the lab and into the field, to seek the elusive radar echo in nature. We think it has promise to be to the field of neutrino astronomy what Blackett and Lovell, and the many after them, hoped that it could be for EAS and cosmic rays. But of course, the history of the field keeps us humble...could there be something we have missed? Only more experimentation, and experimentation in nature, can give us the definitive answer.References:  Blackett, P. M. S., & Lovell, A. C. B. (1941). Radio echoes and cosmic ray showers. Proceedings of the Royal Society of London. Series A. Mathematical and Physical Sciences, 177(969), 183-186. link  Auger, P., Ehrenfest, P., Maze, R., Daudin, J., & Fréon, R. A. (1939). Extensive cosmic-ray showers. Reviews of modern physics, 11(3-4), 288. link  Lovell, A. C. B. (1993). Reminiscences and discoveries: The Blackett-Eckersley-Lovell correspondence of World War II and the origin of Jodrell Bank. Notes and Records of the Royal Society of London, 47(1), 119-131. link  Lovell, A. C. B., Banwell, C. J., & Clegg, J. A. (1947). Radio echo observations of the Giacobinids meteors, 1946. Monthly Notices of the Royal Astronomical Society, 107, 164.  Appleton, E. V., & Piddington, J. H. (1938). The reflexion coefficients of ionospheric regions. Proceedings of the Royal Society of London. Series A-Mathematical and Physical Sciences, 164(919), 467-476.link  Matano, T., Nagano, M., Suga, K., & Tanahashi, G. (1968). Tokyo large air shower project. Canadian Journal of Physics, 46(10), S255-S258.  Gorham, P. On the possibility of radar echo detection of ultrahigh-energy cosmic ray induced and neutrino induced extensive air showers. Astroparticle Physics, 15, 177.  Abbasi, R., Othman, M. A. B., Allen, C., Beard, L., Belz, J., Besson, D., ... & Hanlon, W. (2014). Telescope array radar (TARA) observatory for ultra-high energy cosmic rays. Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 767, 322-338.  Abbasi, R. U., Abe, M., Othman, M. A. B., Abu-Zayyad, T., Allen, M., Anderson, R., ... & Besson, D. (2017). First upper limits on the radar cross section of cosmic-ray induced extensive air showers. Astroparticle Physics, 87, 1-17.  Chiba, M., Kamijo, T., Yabuki, F., Yasuda, O., Akiyama, H., Chikashige, Y., ... & Utsumi, M. (2012). Radar for detection of ultra-high-energy neutrinos reacting in a rock salt dome. Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 662, S222-S225.  de Vries, K. D., Hanson, K., & Meures, T. (2015). On the feasibility of RADAR detection of high-energy neutrino-induced showers in ice. Astroparticle Physics, 60, 25-31.  de Vries, K. D., Coppin, P., O'Murchadha, A., Scholten, O., Toscano, S., & van Eijndhoven, N. (2018). On the Radar detection of high-energy neutrino-induced cascades in ice; From Radar scattering cross-section to sensitivity. arXiv preprint arXiv:1802.05543.  Prohira, S., & Besson, D. (2019). Particle-level model for radar based detection of high-energy neutrino cascades. Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 922, 161-170.  Prohira, S., De Vries, K. D., Allison, P., Beatty, J., Besson, D., Connolly, A., ... & Meures, T. (2020). Observation of Radar Echoes from High-Energy Particle Cascades. Physical Review Letters, 124(9), 091101.
9/2020: RET-CR has been awarded 4 years of funding by the National Science Foundation (NSF)! During this period we'll develop and deploy RET-CR to test out the radar echo method in nature.OSU press release
6/2020:Some recent news about the T576 experiment, performed at SLAC, where we observed radar echoes from electron-beam induced particle cascades:Physics Today Physics World Phys. Rev. Focus OSU press release VUB press release (Dutch) SLAC news
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