Neutrinos are amongst the most abundant particles in the universe and are mainly produced in the nuclear reactions that make stars shine. Every second billions of neutrinos pass through our bodies. Neutrinos are tiny subatomic particles that carry no electric charge and only very rarely interact with matter. A neutrino could pass through a light-year of lead without interacting, so to study their properties, physicists need to detect a large number of them and so need a big detector. There is now evidence that neutrinos have mass (albeit a minuscule one) and that they oscillate between flavours as they travel (the three neutrino flavours are the electron, muon, and tau neutrino).
MINOS is a long baseline neutrino oscillation experiment. An intense beam of neutrinos is made at the Main Injector particle accelerator at Fermilab (near Chicago), and travels 735 km through the earth to the Soudan mine in northern Minnesota. At its source the beam is mostly made of muon neutrinos with a small background of electron neutrinos. The energy spectrum of neutrinos of each flavour (electron, muon, tau) is measured in the "Near Detector" at Fermilab and the "Far Detector" in the Soudan Mine. MINOS then compares the spectra measured in the two detectors to understand if the flavor content of the beam changes as it propagates and, if so, how the changes depend on neutrino energy. The data is used to test theories, such as
MINOS began collecting beam data in March 2005 and is currently a running experiment which has already provided the world's best measurement of the neutrino oscillation parameter associated with the difference between the second and third neutrino mass state. The Far Detector has been recording atmospheric neutrino and cosmic ray data since construction was completed in the summer of 2003. The Near Detector's primary role is to determine the initial flavor content of the neutrino beam. However, the large quantity of data collected by the Near detector will also be used to measure neutrino cross sections and nucleon structure.