Neutrino beams: μ decay vs. π decay

We propose a preliminary comparison in terms of general features, neutrino yields and event rates, of neutrino factories based on muon decay and conventional neutrino beams based on pion decay. The comparison focuses on high-energy neutrinos, with average energy of 10 GeV or more. Emphasis is put on beams designed for investigations of long-baseline neutrino oscillations. Performance for conventional short-baseline neutrino experimentation is also considered. In both types of facilities, neutrino yields and event rates increase steeply with the average energy of the neutrino parents. At equal energy of the parent, νμ rates about 100 times larger and νe rates more than 10 000 times larger appear possible with neutrino factories. This large yield of high-energy νe that can be tagged by lepton number (charge) recognition in the neutrino detector, is considered the most important new feature of neutrino factories. A wide range of physics goals including a thorough study of the leptonic mixing-matrix and possibly of CP violation, can be addressed. The decay of a well-known number of muons provides a well-defined neutrino flux, free of the traditional uncertainties on the number and distribution of parent hadrons that plague conventional neutrino beams: one of the major advantages of the neutrino factory. In addition, the neutrino factory provides beams which are more flexible, tunable, serve simultaneously different baselines, and provide more neutrinos per unit of invested energy. Since only minor shielding is required in the neutrino factory, short-baseline neutrino detectors can profit from very intense and collimated beams. Sophisticated devices of small dimensions will be able to replace the traditional large coarse-grain detectors.