Speaker
Description
Ultra-high-energy cosmic rays (E > $10^{18}$ eV) are the most energetic particles in the Universe and, as such, provide a unique opportunity to probe hadronic interactions at energies beyond those reached at the LHC. Due to their extremely low flux, these cosmic rays can only be studied indirectly through extensive air showers (EAS), cascades of secondary particles produced when a primary cosmic ray interacts with a nucleus in the atmosphere. The primary mass must therefore be inferred from EAS observables recorded by large ground-based detectors. The main mass-sensitive observables are the atmospheric depth at which the electromagnetic component reaches its maximum, $X_{\max}$, and the number of muons, $N_\mu$. More challenging to measure, the atmospheric depth at which muon production reaches its maximum, $X^{\mu}_{\max}$, also provides sensitivity to the primary mass. The mass composition is inferred by comparing measurements of these observables with air-shower simulations based on hadronic interaction models, which extrapolate collider data to ultra-high energies. Several experiments have reported a tension between the mass composition inferred from $X_{\max}$ and that derived from $N_\mu$ and $X^{\mu}_{\max}$. This discrepancy is commonly interpreted as a deficit of muons in simulations, indicating that the hadronic component of EAS -where muons are produced- is not accurately described by current hadronic interaction models. The Pierre Auger Observatory is the largest cosmic-ray observatory in the world. Its hybrid design enables the measurement of $X_{\max}$ with the Fluorescence Detector, while the Surface Detector provides sensitivity to $N_\mu$ and $X^{\mu}_{\max}$. In this contribution, we present a summary of hadronic interaction studies performed at the Pierre Auger Observatory over more than 20 years of operation.
