Constraints on the diffuse flux of ultrahigh energy neutrinos from four years of Askaryan Radio Array data in two stations

Abstract

The Askaryan Radio Array (ARA) is an ultrahigh energy (UHE, $>{10}^{17}\text{ }\text{ }\mathrm{eV}$) 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 ultrahigh energy neutrinos between ${10}^{16}$ and ${10}^{21}\text{ }\text{ }\mathrm{eV}$ 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 ${10}^{18}\text{ }\text{ }\mathrm{eV}$ of $EF(E)=5.6\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}16}\text{ }\text{ }{\mathrm{cm}}^{\ensuremath{-}2}\text{ }{\mathrm{s}}^{\ensuremath{-}1}\text{ }{\mathrm{sr}}^{\ensuremath{-}1}$. This analysis includes four times the exposure of the previous ARA result and represents approximately $1/5$th the exposure expected from operating ARA until the end of 2022.