Search for upward-going showers with the Fluorescence Detector of the Pierre Auger Observatory

Upward-going air showers are a possible interpretation of the recent events reported by the ANITA Collaboration, but the energies and the elevation angles of these events appear to be in tension with the predictions of the standard model of particle physics. Given its operation time and wide field of view, the Fluorescence Detector (FD) of the Pierre Auger Observatory is sufficiently sensitive to upward-going showers when used in monocular mode and thus can be exploited to test this interpretation. For this reason, the Pierre Auger Collaboration has performed a generic search for upward-going showers with the FD. The sensitivity of the FD to upward-going showers has been studied with dedicated simulations of upward-going events. To set up these simulations from a geometrical point of view, a Monte Carlo simulation for upward-going showers based on simple assumptions has been preliminarily developed. The background for this search, which consists of downward-going showers with specific geometric configurations erroneously reconstructed as upward-going events in a monocular reconstruction, has been estimated with extensive simulations. Also laser shots, which are continously fired to monitor the quality of the atmosphere and are genuine upward-going events, can produce background when they are not properly labelled. In order to reject laser events, a set of cuts has been developed by using a sample of 10% of the available FD data from 14 years of operation. Furthermore, a set of quality selection criteria has been defined to reduce the mis-identification of downward-going and upward-going events. After having set all the selection criteria, the detection efficiency of the FD to upward-going showers has been studied and the exposure has been calculated. A number of candidate events consistent with the number of events expected from background has been found after the unblinding of the full data set. An upper limit on the flux of upward-going showers has been then derived. To make the results of this analysis suitable to be applied to different physical scenarios, differential tables of exposure have been finally provided.