Researchers hope studying the interactions and physics of neutrinos will provide a better understanding of the cosmic processes that led to the formation and early history of our universe. Two major international projects are under way: OPERA (Oscillation Project with Emulsion-tRacking Apparatus) at Gran Sasso National Laboratory (LNGS; Assergi, Italy), and MINOS (Main Injector Neutrino Oscillation Search), launched by Fermilab (Batavia, IL) in the United States in March.
The first of 486 planes of steel stands at one end of the framework, which will eventually support the complete MINOS detector, located 700 m below ground in the Soudan Mine, MN. Each plate of steel in the detector is only 2.5 cm thick but 8 m wide and 8 m tall. With a weight of 11.25 tons each, the sheets must be carefully supported to prevent them from crinkling like paper.
Neutrinos are among the fundamental building blocks of nature. Existing in three types, or flavors (electron, muon, and tau), neutrinos are believed to oscillate from one type to another. Knowledge of the properties of neutrinos, such as their stability, mass, and the existence of an electromagnetic moment, lags behind other elementary constituents severely. Neutrino oscillation investigation offers one of the most sensitive means to search for and measure neutrino mass. Lacking a charge and interacting with matter via "the weak force," neutrinos pass through the earth almost undetected, making oscillations extremely difficult to study until now.
Generating a beam of muon-type nutrinos at the CERN (Geneva, Switzerland) SPS accelerator, OPERA will detect neutrinos using both a near detector at CERN and a far detector at LNGS, located some 730 km away. The group developed a new technique based on emulsion Cloud Chamber (ECC) to achieve the required sensitivity. The OPERA detector is composed of two identical supermodules, each formed by a target section, followed by a muon spectrometer. The target section is made up of 31 brick walls interleaved with 31 target trackers, with each target tracker made up of four horizontal and four vertical modules to measure the x-y coordinates. Each brick wall contains 3328 automatically removable bricks, each module consists of 256 plastic scintillator strips, and each target brick contains 56 lead/emulsion layers. Neutrinos interacting with the 1-mm-thick lead target plates inside the bricks produce muons, which are tracked by the ECC with a spatial resolution of approximately 0.5 µm and an angular resolution of 5 mrad. OPERA researchers are concentrating their efforts on the oscillation between the muon and tau neutrino.
CERN's James Gillies says "both the OPERA and MINOS experiments are really very complementary." OPERA is due to start in 2006 and forms part of the larger international project CNGS (CERN Neutrinos to Gran Sasso).
The MINOS Experiment, funded by the U.S. Department of Energy, the U.S. National Science Foundation, and the UK Particle Physics and Astronomy Research Council (PPARC), generates muon neutrinos at Fermilab. The neutrinos travel 735 km through the earth to the Soudan Mine in northern Minnesota.
The muon neutrinos in MINOS are measured with both a near detector at Fermilab, which provides the control measurement, and a far detector in the Soudan Mine. A reduction in the expected number of muon neutrinos observed in the far detector, based on what was observed in the near detector, will be evidence for neutrino oscillations. The technologies used for detection are different, however, as the MINOS detectors are made from steel plates layered with plastic scintillators. "The use of magnetized iron-scintillator tracking-sampling calorimeters and associated detector design allows MINOS to classify neutrino interactions as muon- or electron-neutrino-charged current or neutral current interactions. The muon charge sign and energy are measured via curvature in the detector's magnetic field, and the energy of electromagnetic and hadronic showers is measured via calorimetry," explains Mike Kordosky of University College (London, UK), who will be reporting on recent experimental results from the near detector at the next International Conference on Non-Accelerator New Physics in Moscow this June.