Muons were discovered by Neddermeyer and Anderson [39] while studying cosmic ray particles at sea level in Pasadena and at 4300 m on Pike's Peak. Unlike the electrons, the muons did not create showers when passing through lead plates in their cloud chamber. With their long lifetime of 2.2μs and small cross section for interacting in matter they remain abundant at sea level. They are traditionally called the “penetrating component” of the cosmic radiation, yet because they are charged they are easy to detect. Thus muons give the dominant signal deep in the atmosphere and underground, and they are often used as a calibration source for cosmic ray detectors.

Neutrinos, the “little neutral ones”, were postulated in 1930 by Pauli in order to preserve conservation of energy and momentum in beta decays. In 1956, Cowan and Reines confirmed experimentally the existence of the (anti)neutrino using a nuclear reactor in Los Alamos as a source [52]. High-energy neutrinos are produced together with the muons, mainly in the two-body decays of charged pions and kaons wherever there are hadronic interactions. Neutrinos are also produced in the decay of muons, a process that is important in the atmosphere mainly at low energy. Because neutrinos are stable and interact only rarely, they are the most abundant component of the cosmic radiation at the ground. Neutrinos interact only by the weak interaction; hence they were the last component of the cosmic radiation to be measured.

We begin this chapter with a description of the production of both muons and neutrinos. We then go on to discuss measurements of muons in the atmosphere. In Chapter 7 we discuss the current understanding of neutrinos in light of oscillations, and the following Chapter 8 is devoted to both neutrinos and muons as observed underground.