Neutrinos are pointlike, abundant particles with very little mass. They exist in three types or 'flavours' and recent experiments (including those at SNO - the Sudbury Neutrino Observatory) have demonstrated that neutrinos are capable of oscillating between these flavours (electron, tau and muon). This can only happen if one or more of the neutrino flavours does have mass, in contradiction to the Standard Model of particle physics.
The MINOS detector will start measurements of cosmic ray showers penetrating the Earth. It is situated in the Soudan Mine, Minnesota. The 30-metre-long detector consists of 486 massive octagonal planes, lined up like the slices of a loaf of bread. Each plane consists of a sheet of steel about 8 metres high and 2 ½ cm thick, covered on one side with a layer of scintillating plastic that emits light when struck by a charged particle.
"MINOS can separate neutrino interactions from their antimatter counterparts - the antineutrinos." explains UK MINOS spokesperson, Jenny Thomas from University College London. "The data taken now from neutrinos produced in cosmic ray cascades may provide new insight into why the Universe is made of more matter than antimatter. At least, for the first time we will be able to compare the characteristics of neutrinos and anti-neutrinos coming from the atmosphere."
However, MINOS has more ambitious plans in place for August 2004. Whilst most experiments like SNO measure neutrinos coming from the Sun, when complete, MINOS will instead study a beam of man-made neutrinos, all of the same type or 'flavour' - the muon neutrino flavour. This beam will be created at Fermi National Accelerator Laboratory (Fermilab) and sent straight through the Earth to Soudan - a distance of 735 kilometres. No tunnel is needed because neutrinos interact so rarely with matter. A detector is currently being built just outside Fermilab, known as the 'near' detector, similar but smaller than the now operational MINOS detector, known as the 'far' detector. The 'near' detector will act as a control, studying the beam as it leaves Fermilab, then the results will be compared with those from the 'far' detector to see if the neutrinos have oscillated into electron or tau neutrinos during their journey.
A million million neutrinos will be created at Fermilab each year, but only 1,500 will interact with the nucleus of an atom in the far detector and generate a signal; the others will pass straight through.
"The realisation that neutrinos oscillate, first demonstrated by the Super Kamiokande experiment in Japan, has been one of the biggest surprises to emerge in particle physics since the inception of the Standard Model more than 30 years ago." says Jenny Thomas. "The MINOS experiment will measure the oscillation parameters of these neutrinos to an unprecedented accuracy of a few percent; an amazing feat considering neutrinos can usually pass directly through the Earth without interacting at all and that their inferred masses are estimated to be less than 1eV. (The weight ratio of a neutrino to a 1kg bag of sugar is the same as the ratio of a grain of sand to the weight of the earth!). The parameter measurement will open up an entire new field of particle physics, to understand what effect on the universe this tiny neutrino mass has."
Within two years of turning on the neutrino beam, MINOS should produce an unequivocal measurement of the oscillation of muon neutrinos with none of the uncertainties associated with the atmospheric or solar neutrino source. If indeed the findings are positive, then a new era in particle physics will begin. Theorists will have to incorporate massive neutrinos into the Standard Model, which will have exciting implications. Furthermore cosmologists will have a strong candidate for the 'missing mass' of the Universe (which dynamical gravitational measurements show must exist). The experimental side will be just as exciting as we plan new experiments to measure precisely how the different neutrinos change their flavour.
Notes for Editors
A picture of UK spokesperson Jenny Thomas, and a selection of pictures of MINOS are available at http://www.
Additional MINOS images are at http://www.
The Standard Model of Particle Physics, the theory that we have been using for 30 years to describe the fundamental particles (quarks and leptons) and forces (bosons) works very well. It has successfully predicted and accounted for what's seen in experiments at LEP (the Large Electron Positron collider at CERN), the Tevatron at Fermilab in the US and other particle physics experiments.
The Standard Model is a quantum theory that describes the building blocks of matter in the Universe - the fundamental particles - and how they interact through the fundamental forces of electromagnetic, strong and weak. The fourth force of gravity, is not currently part of the model.
Matter particles are divided into three generations comprising six quarks of increasing mass (up, down, strange, charm, bottom and top) and six leptons (the electron, muon, and tau and their respective neutrino partners, predicted to have zero mass). Each particle also has an antiparticle of opposite charge.
The SNO results
The Sun emits electron neutrinos, created in vast numbers by the thermonuclear reactions that fuel our parent star. Since the early 1970s, several experiments have detected neutrinos arriving on Earth, but they have found only a fraction of the number expected from detailed theories of energy production in the Sun.
Data taken entirely from the Sudbury Neutrino Observatory [SNO] in Canada shows without doubt that the number of observed solar neutrinos is only a fraction of the total emitted from the Sun - clear evidence that they have chameleon type properties and change type en-route to Earth.
Funding for the MINOS experiment has come from the Office of Science of the U.S. Department of Energy, the UK's Particle Physics and Astronomy Research Council, the U.S. National Science Foundation, the State of Minnesota and the University of Minnesota. More than 200 scientists from Brazil, France, Greece, Russia, United Kingdom and the United States are involved in the project.
Fermilab is a national laboratory funded by the Office of Science of the U.S. Department of Energy, operated by Universities Research Association, Inc.
PPARC has funded the UK's involvement at £ 6 million, including £2 million worth of hardware. The total project cost is about $150 million, of which $60 million is on detectors and the balance on the beam line from Fermilab.
Institutions from the USA, UK, Brazil, France and Russia are part of the MINOS collaboration.
For a full list (with contact details), please see: http://www-numi.
UK institutions involved are: University College London, Rutherford Appleton Laboratory, Universities of Cambridge, Oxford and Sussex.
The UK groups have provided:
- Engineering deign
- Software for data analysis
- Data acquisition for both detectors - Near (not yet completed) and Far (completed)
- Far detector electronics
- Calibration detector at CERN (MINOS third detector) - used to calibrate the 'near' and 'far' detectors by using a beam of better-understood particles (electrons, pions, muons and protons) from CERN.
- Near detector readout (fibre optic cables, photo multiplier tubes and assemblies)
- Light-injection system to calibrate all 200,000 detector readout channels
Dr Jenny Thomas
Dr Mark Thomson
University of Cambridge
Dr Alfons Weber
University of Oxford
Rutherford Appleton Lab
University of Sussex
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The Particle Physics and Astronomy Research Council (PPARC) is the UK's strategic science investment agency. It funds research, education and public understanding in four areas of science - particle physics, astronomy, cosmology and space science.
PPARC is government funded and provides research grants and studentships to scientists in British universities, gives researchers access to world-class facilities and funds the UK membership of international bodies such as the European Laboratory for Particle Physics (CERN), and the European Space Agency. It also contributes money for the UK telescopes overseas on La Palma, Hawaii, Australia and in Chile, the UK Astronomy Technology Centre at the Royal Observatory, Edinburgh and the MERLIN/VLBI National Facility, which includes the Lovell Telescope at Jodrell Bank observatory.
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