The Mystery of ANITA: Investigating Anomalous Radio Pulses with the Pierre Auger Observatory

* **Introduction**: The Antarctic Impulsive Transient Antenna (ANITA) has detected some unusual radio pulses that don't fit with the standard model of particle physics. These "anomalous" pulses, which appear to come from below the horizon, could potentially be caused by air showers developing in an upward direction. This podcast discusses a search using the Pierre Auger Observatory to either confirm or constrain the possibility of upward-going air showers.

* **The ANITA Anomalies**: ANITA, which flies on NASA balloons, has detected radio pulses consistent with ultra-high-energy cosmic ray air showers. Most of these pulses are reflected from the ice, but some anomalous ones have been observed with strong horizontal polarization but without the expected polarity inversion. These could be caused by upward-going air showers, possibly from tau lepton decays, but this interpretation faces significant challenges.

* **The Pierre Auger Observatory**: The Pierre Auger Observatory, a large cosmic ray detector, was used to search for these upward-going air showers. It combines a Surface Detector (SD) and a Fluorescence Detector (FD) which uses telescopes to collect the fluorescence light emitted by nitrogen as a shower develops. The search focused on data from the FD, as upward-going air showers are unlikely to trigger the SD.

* **The Search**: A dedicated search was conducted for upward-going air showers with zenith angles greater than 110 degrees and energies above 0.1 EeV. The search analyzed data collected between 2004 and 2018, using simulations of both regular cosmic ray showers and upward-going events to distinguish potential candidates from background. The analysis also employed a Global Fit (GF) reconstruction to eliminate misidentified events.

* **Background and Challenges**: A key challenge was distinguishing genuine upward-going showers from mis-reconstructed cosmic ray showers and other sources of background such as laser pulses. Several selection cuts were implemented to filter out background, including cuts based on time sequence of triggered pixels, the shower profile, and zenith angle. A discrimination variable, *l*, was defined to differentiate between upward and downward reconstructions based on the likelihood ratio.

* **Results**: After analyzing the data, only one event was found that passed all selection criteria. This was consistent with an expected background of 0.27 ± 0.12 events from mis-reconstructed cosmic ray showers. This result was used to calculate an upper bound on the integral flux of upward-going showers.

* **Implications**: The non-observation of upward-going air showers by the Pierre Auger Observatory puts constraints on the interpretation of the anomalous ANITA events as being produced by upward-going showers. The study found that the sensitivity of the Auger Observatory exceeds that of the ANITA-III flight, making the results particularly significant. The results suggest that if the ANITA events are caused by upward-going air showers, then those showers likely originated at very high altitudes, or have unusual shower profiles inconsistent with known particle decays or interactions.

* **Conclusion**: The search for upward-going air showers at the Pierre Auger Observatory did not find evidence to support the interpretation of the anomalous ANITA events as being caused by upward-going air showers. This implies either the ANITA events are caused by something else, or there is a need for new theoretical models beyond the Standard Model of particle physics to explain the data.

* **Reference**: A. Abdul Halim et al. (Pierre Auger Collaboration), "A search for the anomalous events detected by ANITA using the Pierre Auger Observatory", 2502.04513v1.pdf.

Acknowledements: Podcast prepared with Google/NotebookLM. Illustration credits: Pierre Auger Observatory