The aim of the CNGS project is to generate an intense neutrino beam in the direction of the Gran Sasso laboratory, making the greatest possible use of the existing CERN infrastructure. To prove the existence of neutrino oscillation, it is important for the beam produced at CERN to contain neutrinos of one type only, which in the case of the CNGS project is of the n_m type. The energy of the neutrinos produced is chosen in such a way that detection at Gran Sasso of neutrinos "transformed" by oscillation into n_t is optimised. The present plan is to provide n_m neutrinos with an energy between 5 and 30 GeV³.

The most usual method of producing n_m neutrino beams consists of six main steps (c.f. Figure 7 on page 14):

• produce high energy protons;

• transport these protons to a target;

• collide these protons with the atomic nuclei of the target, producing a secondary beam, which partly consists of pions and kaons;

• guide the pions and kaons through a system of magnetic horns towards the experiments (in the case of CNGS in the direction of Gran Sasso);

• let the pions and kaons decay in flight in an evacuated tunnel. In most cases, the decay products are a muon and a n_m neutrino. The direction of flight of the neutrinos is very close to that of the parent particles, pions or kaons;

• bring the beam to a stopper, which absorbs everything except the neutrinos and muons. The neutrinos will continue on towards Gran Sasso, while the remaining muons will have been completely absorbed by the earth’s crust within a kilometre.

All the components for producing the neutrino beam will be situated in the tunnels and service galleries described in Chapter 3 below. No new surface building will be needed for the CNGS project. A detailed technical presentation of the project is provided in "The CERN Neutrino Beam to Gran Sasso, Conceptual Technical Design", (ref. CERN 98-02 - INFN/ AE-98/05) and its addendum (ref. CERN-SL/99-034(DI) - INFN/AE-99/05).

2.2.1 The proton beam

All the existing proton accelerators at CERN are involved in producing the CNGS beam (c.f. Figure 8 on page 14): the Linac provides 50 MeV protons for the Booster, which accelerates them to 1.4 GeV before transferring them to the PS. In the PS, the protons reach an energy of 14 GeV, then they are ejected and transferred to the Super Proton Synchrotron (SPS)⁴.

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