A search for the anomalous events detected by ANITA using the Pierre Auger Observatory

TL;DR

The paper reports a dedicated search for upward-going air showers using the Pierre Auger Observatory's Fluorescence Detector to test the ANITA anomalous events. It employs simulations of regular UHECRs and upward-going showers, along with a Global Fit–based reconstruction and a discriminant statistic to separate potential upward-going signals from background, yielding one event compatible with background and upper flux limits of $F^{95\%}=(7.2 \pm 0.2)\times 10^{-21}$ cm$^{-2}$ sr$^{-1}$ y$^{-1}$ for $dN/dE \propto E^{-1}$ and $F^{95\%}=(3.6 \pm 0.2)\times 10^{-20}$ cm$^{-2}$ sr$^{-1}$ y$^{-1}$ for $dN/dE \propto E^{-2}$. The study shows that Auger’s exposure to upward-going showers, especially at low altitudes, exceeds ANITA-III’s exposure by large factors, implying that normalizing ANITA’s anomalies to Auger would predict many events not observed, thus significantly constraining explanations based on upward-going showers within the Standard Model or simple beyond-Standard-Model scenarios. The results provide a stringent independent check on the ANITA anomalies and demonstrate the power of cross-observatory fluorescence-based searches for rare upward-going air showers.

Abstract

A dedicated search for upward-going air showers at zenith angles exceeding $110^\circ$ and energies $E>0.1$ EeV has been performed using the Fluorescence Detector of the Pierre Auger Observatory. The search is motivated by two "anomalous" radio pulses observed by the ANITA flights I and III which appear inconsistent with the Standard Model of particle physics. Using simulations of both regular cosmic ray showers and upward-going events, a selection procedure has been defined to separate potential upward-going candidate events and the corresponding exposure has been calculated in the energy range [0.1-33] EeV. One event has been found in the search period between 1 Jan 2004 and 31 Dec 2018, consistent with an expected background of $0.27 \pm 0.12$ events from mis-reconstructed cosmic ray showers. This translates to an upper bound on the integral flux of $(7.2 \pm 0.2) \times 10^{-21}$ cm$^{-2}$ sr$^{-1}$ y$^{-1}$ and $(3.6 \pm 0.2) \times 10^{-20}$ cm$^{-2}$ sr$^{-1}$ y$^{-1}$ for an $E^{-1}$ and $E^{-2}$ spectrum, respectively. An upward-going flux of showers normalized to the ANITA observations is shown to predict over 34 events for an $E^{-3}$ spectrum and over 8.1 events for a conservative $E^{-5}$ spectrum, in strong disagreement with the interpretation of the anomalous events as upward-going showers.

Figures (10)

• Figure 1: Distributions of the discriminating variable $l$, as defined in Eq. (\ref{['eq:discrimination-variable']}), for i) a simulated isotropic background weighted and normalized with the measured UHECR spectrum in the energy range $10^{17}$ eV to $10^{20}$ eV PierreAuger:2020qqz (red histogram with an exponential fit and its uncertainty band); ii) the signal simulation with energy $10^{16.6}$ eV to $10^{18.5}$ eV weighted with an $E^{-3}$ spectrum and arbitrarily normalized to one event (blue histogram); and iii) the data distributions, both for the 10% burn sample and the full data set (open and filled symbols). The cut value $l_c$ discriminating signal and background is indicated by the vertical dashed line.
• Figure 1: Example of a measured downward-going cosmic ray event with impact point behind a telescope building. The camera view is shown on top and a 3d view of the shower geometry is shown at the bottom. The colors indicate the time ordering of triggered pixels (purple first, red last). The light from $P_1$ at the low elevation angle $\alpha_1$ arrives at the FD-telescope earlier than that from $P_2$ at the higher elevation angle $\alpha_2$ so that the trace left by the pixels in the camera appears "upward-going" as would correspond to a genuine upward-going shower with its exit point in front of the telescope.
• Figure 2: Remaining event after the selection and search procedure. The top panel shows the triggered pixels of the camera, the earliest one in purple and the last one in red. The bottom plot shows the reconstructed profile that has been fit with the GF reconstruction in the upward-going mode. The time evolution of the signal across a pixel is divided into 50 ns bins to give information from different atmospheric depths.
• Figure 2: Simulated near horizontal UHECR shower ($\theta_{\rm sim}=89.6^\circ$) that is mis-reconstructed as upward-going and part of the background with a discrimination variable $l=1$ (c.f. Eq. 1 in main text). The top plot shows a camera view of the event. The colors indicate again the time ordering of triggered pixels (purple first, red last). Isolated pixels shown in gray color are noise pixels that are not used for the reconstruction. The bottom plot shows the mis-reconstructed profile of deposited energy for the upward-going geometry using the standard methods to convert the amount of light per time-bin into energy loss, $dE/dX$, per slant depth, $X$PierreAuger:2014sui.
• Figure 3: Exposure of the Auger Observatory (top) and the ANITA III flight (bottom) as a function of shower energy, $E$, and injection altitude, $h$, integrated over the zenith angle range $110^\circ \leq \theta \le 130^\circ$, for an isotropic distribution of arrival directions. The gray area indicates insufficient statistics. In the white cells on top of the exposure plots, we display the sums of the $h$-bins to facilitate comparison (see text).