Why We Measure Muon Arrival Time Distributions in Extensive Air Showers

Extensive Air Showers (EAS) are induced by interactions of high energy cosmic rays with the nuclei in the atmosphere. The high energy of the primary particle (proton, helium, also heavier nuclei) is distributed to millions of secondary particles, propagating as a wide disk through the atmosphere. There are three main components of charged particles in EAS: electromagnetic component, muons and hadrons (near shower axis). Muons form the penetrating component, since they get less absorbed and reach the ground, even if produced at high altitudes. Simple kinematics arguments show that the arrival time of the muon component carries some information about the height of production via time-of-flight effects. 'Global' times represent muon arrival times relative to the arrival time of the electromagnetic front of the shower core. 'Local' times are muon arrival times relative to the arrival time of the foremost muon reaching the detector. Since the muon arrival time distributions map the longitudinal EAS development, they can be used for the determination of elongation rate (Linsley's method) and for mass discrimination of the EAS primaries. The implications of measurements of muon arrival distributions are illustrated for the case of recent KASCADE results. (author)