An accelerator beam of muon neutrinos is manufactured at the Fermi Laboratory in Illinois, USA. The neutrino beam spectrum is sampled by two detectors: a 1kT Near Detector constructed 1km from the beam production point, and a 5.4kT Far Detector constructed 735km away at the Soudan …….

The MINOS Neutrino Oscillation ExperimentAndy Blake, Cambridge University

Main Injector Neutrino Oscillation Search

Manufacturing Neutrinos Detecting Neutrinos

protons

First Physics Results - Atmospheric Neutrino Oscillations at the MINOS Far Detector

ABOVEtravel length10~30 km

p, He

p, He

BELOWtravel length

up to 13,000 km

down-going neutrino

up-goinganti-neutrino

The Soudan Mine

The Fermi Laboratory

The Near Detector

The Far Detector

cosmicmuons

MINOS 418 days exposure

High energy cosmic ray interactions at the top of the atmosphere produce an intense flux of neutrinos. These can be studied using the MINOS Far Detector. Its deep location 700m underground provides shielding against the high flux of cosmic rays incident on the surface, and its large mass enables a high rate of neutrino interactions.

The MINOS neutrino beam is manufactured by the dedicated NuMI beam facility at the Fermi Laboratory. This extracts 120 GeV protons from the Tevatron Main Injector ring at a rate of ~1013 protons per second and directs them onto a fixed graphite target. Pions and kaons are selected from the resultant spray of secondary particles, and are focused into a 600m evacuated decay pipe where they decay to produce muons and muon neutrinos. At the end of the decay pipe, the muons are absorbed by 200m of rock, leaving a pure beam of muon neutrinos. The NUMI facility delivers ~1020 neutrinos per year.

Underground Laboratory in Minnesota, USA. The signature of neutrino oscillations is a periodic deficit in the neutrino beam spectrum at the Far Detector relative to the Near Detector. MINOS is a international collaboration of 175 particle physicists from 32 institutions in 6 countries.

Over the course of the past ten years, the phenomenon of neutrino oscillations has become firmly established, implying that neutrinos possess a non-zero mass.

The MINOS experiment has recently commenced operations and is conducting precise measurements of oscillations using an accelerator beam of neutrinos.

The MINOS detectors are designed to be functionally similar, enabling accurate comparisons of the neutrino spectrum in each detector. The detectors are both sampling calorimeters, composed of layers of steel and plastic scintillator. Muon neutrinos interact in the steel to produce muons which deposit ionization trails in the scintillator inducing photon emissions. These photons are detected by photo-multiplier tubes, and the signal is amplified and digitized by sensitive electronics. Particle tracks and showers are identified by analysing the timing and topology of hits recorded in the detector. The signature of a muon neutrino interaction is a muon track with a contained vertex, whose timing and direction is consistent with the Fermilab beam. The steel is magnetized by a current-carrying coil running through the centre of the detector. This enables measurements of the charge and momentum of particles to be carried out.

A clean sample of muon neutrinos is obtained by selecting muon tracks with contained interaction vertices. These events must be identified from a high background of cosmic muons. The signature of neutrino oscillations is a deficit of events at large zenith angles, corresponding to large neutrino path lengths and large oscillation probabilities. An analysis of 418 days data has been carried out. The atmospheric neutrino flux exhibits an up-down asymmetry.

The magnetic field of the Far Detector allows the charge of muons produced in neutrino interactions to be measured. This enables atmospheric neutrino and anti-neutrinos to be observed separately for the first time.