The CERN Neutrinos to Gran Sasso (CNGS) project aims to unravel some of the mysteries surrounding neutrinos - light, neutral particles that hardly interact with matter. Three types or "flavours" of neutrino exist: the electron neutrino, the muon neutrino and the tau neutrino. But it seems that neutrinos are the chameleons of the particle world: they can change from one flavour into another. This phenomenon, called “oscillation”, occurs as neutrinos travel long distances through matter. The process is directly related to the neutrinos' tiny mass.
From July 2006 to December 2012, the CNGS project sent muon neutrinos from CERN to the Gran Sasso National Laboratory (LNGS), 732 kilometres away in Italy. Neutrinos interact so weakly with other particles that they pass easily through the intervening rock. At Gran Sasso, two experiments, OPERA and ICARUS, wait to find out if any of the muon neutrinos have transformed into tau neutrinos.
To create the neutrino beam, a beam of protons from the Super Proton Synchrotron at CERN is directed onto a graphite target. The collisions create particles called pions and kaons, which are fed into a system of two magnetic lenses that focus the particles into a parallel beam in the direction of Gran Sasso. The pions and kaons then decay into muons and muon neutrinos in a 1-kilometre tunnel. At the end of the tunnel, a block of graphite and metal 18 metres thick absorbs protons as well as pions and kaons that did not decay. Muons are stopped by the rock beyond, but the muon neutrinos remain to streak through the rock on their journey to Italy.