Despite 100 years since the first discovery of cosmic rays, we still do not know what these particles are at the highest energy. The
Pierre Auger Observatory in Argentina is able to detect cosmic rays above 1017 eV with high accuracy due to its hybrid detection technique. Studying the distribution of Xmax, the position in the atmosphere where the air shower deposits the maximum number of electromagnetic particles, suggests that cosmic rays are getting heavier with increasing energy, but not heavy enough to be regarded as iron nuclei. This poses a challenge to cosmic ray propagation models to come up with the appropriate composition. Alternatively, if a pure proton or a proton-to-iron nuclei transition within the Auger energy range is to be favoured, the hadronic interaction models need modification. New experimental techniques such as those based on the detection of radio or microwave emitted by electrons in the air shower cascade, along with more precise measurement of the Xmax, are being considered for showers above 1019 eV.
– Eun-Joo Ahn