The Main Injector Neutrino Oscillation Search (MINOS) experiment was officially inaugurated in a ceremony at Fermilab on 4 March. MINOS is the latest weapon in the arsenal of neutrino-oscillation searches. Its main goal is to measure the largest difference in mass-squared between different neutrino species (Δm223) with an accuracy of 10% - more than a factor of two better than it is known today.

MINOS takes over from the KEK to Kamioka (K2K) experiment in Japan, which has finished taking data with a similar set-up. The unique feature of MINOS, however, is its 1.5 T magnetic field. This enables the experiment to distinguish positively and negatively charged tracks and hence discriminate between neutrinos and antineutrinos.

MINOS uses a neutrino beam produced by Fermilab's Neutrinos at the Main Injector (NuMI) facility, where 120 GeV protons from the Main Injector hit a graphite target, producing hadrons including pions. A "horn" focusing system selects positive pions, which then decay in a 700 m-long decay pipe. After passing through a beam absorber, the beam comprises mostly muon neutrinos. An important advantage of the system is that the energy of the neutrinos can be tuned by moving the horn focusing system.

The neutrino beam is aimed at the MINOS "far" detector, located in the Soudan Underground Laboratory in northeastern Minnesota, some 730 km away from Fermilab. The laboratory is 700 m underground in an old iron mine. To reduce errors by measuring directly the beam composition and neutrino energy spectrum, a "near" detector is also incorporated in the experiment 1 km from the target. It is essentially a miniature of the 6000 t far detector.

An important milestone was reached on 4 December 2004, when the first beam reached the target hall. The horns were powered in January and the near detector has already recorded its first events.

• MINOS is a collaboration of 200 scientists, engineers, technical specialists and students from 32 institutions in Brazil, France, Greece, Russia, the UK and the US.