Abstract
The Pierre Auger Observatory is the largest detector of ultra-high energy cosmic rays (UHECR) in the world. These particles, presumably protons or heavier nuclei of energies up to 1020 eV, initiate extensive air showers which can be detected by sampling the particles that arrive at ground level or observing the fluorescence light generated during the passage of showers through the atmosphere – the Pierre Auger Observatory employs both these techniques. As the center-of-mass energies of the first interactions in the showers can be several orders of magnitude beyond the reach of the LHC, the UHECR provide an unique opportunity to study hadronic interactions. While the uncertainty in modeling these interactions is somewhat degenerate with the unknown composition of the primary beam, interaction models can be tested using data such as the depths of the maxima of the longitudinal development of the showers or their muon content. Particular sensitivity to interaction models is achieved when several observables are combined. Moreover, using careful data selection, proton-air cross section at the c.m.s. energy of 57 TeV per nucleon-nucleon pair can be obtained.
Highlights
The cosmic rays (CR), charged particles with extra-solar origin, are observed over more than ten decades in energies, between 109 eV and 1020 eV in the Earth’s laboratory system
The CR particles cannot be directly detected at the ground, because they interact in the upper levels of Earth’s atmosphere and, while the lower energy CR are readily observed directly at balloon and satellite experiments, at energies above 1014 eV the CR flux becomes so sparse that bringing a large enough instrument above the atmosphere for their detection becomes impractical
As often happens in physics, the hindrance of the CR interaction in the atmosphere can be turned into an advantage when the atmosphere itself is used as the detection volume and the detectors register phenomena caused by the interactions of the primary CR particles in it
Summary
The cosmic rays (CR), charged particles with extra-solar origin, are observed over more than ten decades in energies, between 109 eV and 1020 eV in the Earth’s laboratory system. A unique obstacle in identifying the sources of charged particles is that their trajectories are bent in the galactic and intergalactic magnetic fields so that most of the directional information is lost for all except the most energetic particles Being magnetic processes, both the acceleration and deflection of the particles depend strongly on their electric charge – as most of the UHECR are believed to be either protons or nuclei of heavier elements (up to iron), the term mass (or even chemical) composition is frequently used to describe the charge distribution of the primary UHECR beam. Where there is a complication, there is an opportunity – namely to study the hadronic interactions at energies well beyond the reach of any accelerator in the foreseeable future
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