Cosmic rays, the highest energy particles of the universe, bombard Earth from all directions and yet, not much is understood about their origins. Charged particles, such as protons and heavier nuclei (in the class of hadrons), make up these cosmic rays, which can then collide with other particles in the atmosphere to produce secondary particles that cascade into so-called “air showers.”
In addition to hadrons, high-energy photons from cosmic rays can also produce air showers, providing insight into possible, exciting new physics.
In a new study submitted to Physical Review D, researchers at the Wisconsin IceCube Particle Astrophysics Center (WIPAC) at the University of Wisconsin–Madison present a new analysis technique to study photon-induced air showers using data collected from the Pierre Auger Observatory in western Argentina. The new detection method resulted in a higher search sensitivity and upper limit on the amount of photons in the cosmic flux.
The Pierre Auger Observatory is a unique hybrid detector that employs two independent methods to “see” the extremely rare high-energy cosmic rays, by observing 1) particle interactions with water in ground-based tanks and 2) air showers through ultraviolet light emitted high up in the Earth’s atmosphere. Measurements from both the surface detector (SD) and fluorescence detector (FD) are then used to detect primary photons with energies above 1 exaelectronvolt (EeV).
To search for photon-induced showers, WIPAC researchers looked at 12 years of data taken from the Pierre Auger Observatory and combined both FD- and SD-based measurements to separate photons from hadrons. They looked for signatures of photon-induced showers using the depth of the shower maximum (Xmax), which is typically larger for photon-induced showers than for hadron-induced showers. The search for high-energy photons was conducted in different energy ranges, particularly in the 1 EeV-10 EeV energy range.
“This method involved combining FD-based measurements of the depth at Xmax with SD signals through a parameter related to background or muon content, thereby exploiting the paradigm of the universality of air showers,” says Pierpaolo Savina, a former postdoctoral associate at WIPAC and corresponding author of the study. “The goal was to enhance photon/hadron separation to improve search sensitivity and achieve tighter upper limits.” Savina is now an assistant professor at the Gran Sasso Science Institute in Italy.
Through the integration of both FD and SD measurements, the study revealed a more effective separation of photons from the hadron-induced background. Compared to previous analyses, the study placed a more stringent upper limit on the amount of photons propagating through the cosmic space.
According to Savina, ongoing efforts are being made to correlate photon candidates measured by the Pierre Auger Observatory with ghostlike particles called neutrinos from the IceCube Neutrino Observatory, with the ultimate goal of understanding high-energy cosmic phenomena.
“The goal is to filter out charged cosmic rays and to rely on neutral particles to discover potentially transient sources of ultra-high-energy cosmic rays,” says Lu Lu, an assistant professor of physics at WIPAC and Savina’s advisor. “It’s truly an exciting opportunity to utilize these exceptional detectors and their invaluable data.”
+info “Search for photons above 1018 eV by simultaneously measuring the atmospheric depth and the muon content of air showers at the Pierre Auger Observatory,” The Pierre Auger Collaboration: A. Abdul Halim et al., Submitted to Physical Review D, arxiv.org/abs/2406.07439