Limits on the flux of ultrahigh-energy neutrinos from radio astronomical observations

Abstract

We discuss results from current experiments and prospects for future experiments aimed at detecting cosmic ultra-high-energy hadrons and neutrinos using radio astronomical methods proposed earlier by Dagkesamanski$\overset{\lower0.5em\hbox{$\smash{\scriptscriptstyle\smile}$}}{l}$ and Zheleznykh. The target for the hadrons and neutrinos is the Moon, and the experiments are designed to detect nanosecond flashes of radio emission from electromagnetic and hadronic cascades produced by high-energy particles in the lunar soil (regolith) using the largest available radio telescopes. We present the results of numerical simulations of the generation of such radio impulses in neutrino reactions, taking into account the requirements for the emergence of the radio emission from the lunar soil. These simulations enable us to correctly relate the detection rate for the radio impulses to the flux of ultra-high-energy neutrinos. The results of the first searches for nanosecond radio flares from the Moon using the 64-m Kalyazin Radio Astronomical Observatory of the Astro Space Center are reported. Experimental limits on the diffuse flux of cosmic neutrinos with energies exceeding 10²⁰ eV obtained from these and similar observations carried out in the USA using 70-m and 34-m radio telescopes are compared with theoretical predictions of the neutrino flux for various astrophysical models. Enhancing the sensitivity of such experiments and increasing their duration, especially monitoring the Moon simultaneously with several radio telescopes, could provide important results about the nature of ultra-high-energy cosmic rays in the relatively near future.