Search for the Anomalous Events Detected by ANITA Using the Pierre Auger Observatory.

Area of Science:

• Astroparticle Physics
• Cosmic Ray Physics
• High-Energy Astrophysics

Background:

• Two anomalous radio pulses observed by the ANITA (Advanced National Ice-cream Tracking Array) experiment suggest potential deviations from the standard model of particle physics.
• These anomalous events were interpreted as possibly originating from upward-going air showers.
• The Pierre Auger Observatory's Fluorescence Detector was used to search for such events.

Purpose of the Study:

• To conduct a dedicated search for upward-going air showers at ultra-high energies (E > 0.1 EeV) and large zenith angles (> 110°).
• To test the hypothesis that the ANITA anomalous events are upward-going cosmic ray showers.
• To establish upper limits on the flux of upward-going cosmic rays.

Main Methods:

• Utilized data from the Pierre Auger Observatory's Fluorescence Detector.
• Developed a selection procedure using simulations of both regular cosmic-ray showers and upward-going events to identify candidate events.
• Calculated the exposure for upward-going events in the energy range of 0.1 to 33 EeV.

Main Results:

• One candidate event was found between January 1, 2004, and December 31, 2018.
• This observed event is consistent with the expected background of 0.27 ± 0.12 events from misreconstructed cosmic-ray showers.
• Upper bounds on the integral flux of upward-going showers were set for different spectral indices (E⁻¹ and E⁻²).

Conclusions:

• The search found no significant excess of upward-going air showers beyond the expected background.
• The results strongly disagree with interpretations that the ANITA anomalous events are upward-going showers, especially for steeper spectra (E⁻³ and E⁻⁵).
• The study places stringent limits on the flux of potential upward-going cosmic rays, constraining new physics scenarios.