Constraints on the Diffuse Flux of Ultra-High Energy Neutrinos from Four Years of Askaryan Radio Array Data in Two Stations

Abstract: The Askaryan Radio Array (ARA) is an ultra-high energy (UHE, $&gt;10<sup>{17}$ eV) neutrino detector designed to observe neutrinos by searching for the radio waves emitted by the relativistic products of neutrino-nucleon interactions in Antarctic ice. In this paper, we present constraints on the diffuse flux of ultra-high energy neutrinos between $10<sup>{16}-10<sup>{21}$ eV resulting from a search for neutrinos in two complementary analyses, both analyzing four years of data (2013-2016) from the two deep stations (A2, A3) operating at that time. We place a 90 % CL upper limit on the diffuse all flavor neutrino flux at $10<sup>{18}$ eV of $EF(E)=5.6\times10<sup>{-16}$ $\textrm{cm}<sup>{-2}$\textrm{s}^{-1}sr<sup>−1\textrm{sr}<sup>{-1}. This analysis includes four times the exposure of the previous ARA result, and represents approximately 1/5 the exposure expected from operating ARA until the end of 2022.