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However, high energy particle physics is of course a much wider field, and there are numerous topics which, because of space limitations, cannot be covered in detail. In this section, a number of these topics are briefly touched upon. At the end of the chapter we include a few suggestions for further reading. 1 Neutrino Physics Neutrinos as elementary particles were first suggested by W. Pauli in the 1930s in order to explain the spectrum of nuclear β decay; the first experimental observation (of the electron neutrino) was in 1956 by Cowan and Reines; the muon and tau neutrinos were discovered in 1962 and 2000, respectively.

In recent years, heavy ion collisions using mainly gold and copper atoms have been investigated at the Brookhaven RHIC collider: here, four experiments have been or are still taking data: BRAHMS, PHOBOS, PHENIX, and STAR 9). The physics questions investigated centre around the specific features of the high temperature/high energy environment. One topic is the behaviour of the gluon density in the nucleon in the limit of extremely small momentum fractions, x, where presumably perturbative QCD is not applicable and saturation effects might set in.

All in all, neutrino physics is an extremely active field with many ongoing and planned facilities, and very promising prospects for the coming years. 2 Astroparticle Physics Astroparticle physics aims at measuring elementary particles of astronomical origin, and at understanding their origin, production, and acceleration mechanism. In doing so, astronomical and cosmological questions can be addressed. Various different elementary particles are used for astroparticle studies, and the relevant experiments are well adapted to the instrumental challenges (here only a small fraction of all experiments and results can be mentioned).