Hadronic physics from extensive air showers

Abstract

Although cosmic rays were discovered exactly one century ago, the most fundamental questions about them are still not answered, especially the origin and composition of ultra-high energy cosmic rays (UHECR). The Pierre Auger Observatory (PAO) is constructed with the goal of solving these mysteries. The PAO uses hybrid design and take advantage of both the air fluorescence and surface array technique. Since its debut in 2004, PAO has published several important scientific results. The most probable candidate for UHECR composition is proton or iron nucleus. The two candidates do show differences in both fluorescence detector (FD) signal and in the surface detector (SD) signal. PAO has utilized the statistical depth of shower maximum information of FD signal to study the composition and suggests that the nuclear mass is getting heavier from 1 EeV to 10 EeV and beyond. The result does not come without argument. The Telescope Array (TA)'s result consists with a pure proton primary. One other possible solution to check these results is to study the SD signal since SD has a lot more statistics. According to Matthews' Heitler model, the proton and iron primary air showers show significant differences in the muon production, thus muon number is very sensitive to the cosmic ray primary composition. "Leading particle" physics - where one of the many particles emerging from a collision carries a significant portion of the energy - is a well-known and studied concept in high energy physics. It gives a lot of information about the hadronic interaction and yet to be studied in highest energies levels. It has two observables, a "double bump" longitude profile in FD and a "double shell" geometrical structure in SD. In this dissertation, I will describe new methods to identify leading particles and to count muons in air showers. These observations are then compared to simulations using several hadronic physics modeling schemes.