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Huge underground detector may explain matter-antimatter riddle

DOE/Argonne National Laboratory : 22 August, 2006  (Technical Article)
Twenty years ago, Argonne scientists began tinkering around in a historic iron mine in Minnesota, looking for the theoretically predicted phenomenon of proton decay. They didn't find it, but they did find something only a very few other physicists had seen, atmospheric neutrino oscillations, the shifting of the tiny neutral particles from one type to another as they travel.
Atmospheric neutrinos are created when cosmic rays collide with air molecules in the Earth's upper atmosphere and have been observed only in deep underground particle detectors. The observation of neutrino oscillations proved that these elusive particles actually have tiny masses, instead of being massless as previously thought.

Over the years, the experiments at the Soudan mine have added knowledge to the fields of particle physics and cosmology, but the site is now on the threshold of a highly ambitious project to 'catch' neutrinos fired from a new facility, the NuMI neutrino beamline, 450 miles away from the mine, at Argonne's neighbor, Fermilab, in Batavia.

Argonne scientists, in collaboration with colleagues from institutions in the U.S., Great Britain, Greece and Russia designed and built the 5,400-ton 'MINOS' detector at Soudan, which is now complete.

The detector stands a half-mile below ground, with 486 planes of steel, each eight meters wide and one inch thick, in an octagonal shape with a small hole in the middle. The 486 planes are perfectly aligned, so that magnet coils can be threaded through the planes to magnetize the device. The last plane was mounted in June of this year and magnet coil was installed and turned on in July.

The detector's job at Soudan, once the beam line at Fermilab is completed next year, will be to catch the muon neutrinos that have traveled from Fermilab, to see if the caught neutrinos have become electron or tau neutrinos, proof of neutrino oscillation, said physicist David Ayres, who has been heavily involved with the construction of the device for the past five years.

The MINOS experiment will make precise measurements to reveal just how oscillations take place and also to search for the hitherto unseen muon- to electron-neutrino mode.

The giant project, called MINOS, is a testament to engineering, Ayres said. 'Because of the curvature of the Earth, the path of the beam from Fermilab to Soudan goes through rock 10 kilometers deep at its deepest point, traveling 735 kilometers across Illinois and Wisconsin to Minnesota, where it will be centered on the target (the new MINOS detector) within a few meters.'

Neutrinos don't interact much with other particles, in fact, a couple of billion neutrinos just passed through your body while you were reading this sentence, so the beam is very difficult to detect above normal background radioactivity and cosmic rays. This is why the MINOS detector is located deep underground and why it needs to be so massive.

The researchers have begun a study of atmospheric neutrinos, looking for differences in the oscillations of neutrinos and antineutrinos using the magnetized steel planes of the detector to tell the difference between the two types of particles. Other experimental devices for studying atmospheric neutrinos do not have that capability.

'Everyone expects that CPT symmetry will be preserved by neutrinos, as it has been for other elementary particles, but will it be?' Ayres said. 'If neutrinos do violate CPT this could be an important step toward understanding how the world we live in came to be made of matter instead of equal amounts of matter and antimatter. It would be very exciting to answer that question.'
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