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The NuMI Project

Using neutrinos to study the property of matter known as mass might seem strange, because neutrinos themselves have zero or very small mass. The Standard Model that serves as the conceptual framework for understanding all the observed particle phenomena includes only massless neutrinos. Nevertheless, for many years, certain tantalizing clues have led some scientists to wonder if neutrinos truly deserve their massless reputation.

Neutrinos at Fermilab's Main Injector

Astrophysicists, nuclear physicists and particle physicists have studied the electron neutrinos that occur naturally in the energy-producing reactions in the sun; and the muon and electron neutrinos from cosmic ray interactions in the upper atmosphere. These studies have turned up several clues that implicate the neutrino as a prime suspect in certain intriguing scientific mysteries.

For example, astrophysicists can calculate about how many electron neutrinos should arrive at the earth from the sun. But when they count the neutrinos that actually arrive, they find only about half the number they expect. Similarly, they come up short when they count muon neutrinos produced in the atmosphere as a result of cosmic ray interactions. Where are the missing neutrinos? What could explain these solar and atmospheric neutrino deficits? In the end, the neutrino may turn out to be a more complex particle than current theories account for.

Neutrinos come in three flavors: electron, muon, and tau. Although no experiment has directly observed the change from one neutrino flavor to another, such a change, called oscillation, is quite possible.

Fermilab physicist Dixon Bogert, project manager for NuMI, received a Ph.D. in physics from Yale University in 1969. After teaching at Yale, he joined the Fermilab staff in 1971. He has worked with the Physics Department in supporting the work of Fermilab experimenters and served in key managerial roles to upgrade the Laboratory's experimental facilities and build its newest accelerator, the Main Injector. He has collaborated on a number of previous neutrino experiments at Fermilab.

Scientists from Fermilab and many other U.S. and foreign institutions have designed a set of facilities, collectively called NuMI for "Neutrinos at the Main Injector," to search for neutrino mass by looking for neutrino oscillation. The MINOS (Main Injector Neutrino Oscillation Search) experiment will use these new facilities to take advantage of the unique capabilities of Fermilab's new particle accelerator, the Main Injector, which was commissioned in early 1999. The Laboratory is constructing a new particle beamline to direct a nearly pure beam of muon neutrinos from the Main Injector toward two detectors, "near" and "far," which are capable of counting all three types of neutrinos. If the far detector finds another flavor besides muon neutrinos, the experimenters will know that some of the muon neutrinos in the beam must have oscillated to the other flavor, and hence must have mass.

Scientists at Fermilab are planning an experiment, called MINOS for "Main Injector Neutrino Oscillation Search," to search for neutrino mass by looking for neutrino oscillation. The experiment will take advantage of the capabilities of Fermilab's new particle accelerator, the Main Injector, which began operating in 1999.

The MINOS long-baseline experiment, will detect the beam of neutrinos in a massive detector in a mine in northern Minnesota, 735 kilometers from Fermilab. View Illustration. The long baseline gives muon neutrinos more time to change flavor before they reach the detector, allowing experimenters to detect smaller neutrino masses than in a short-baseline experiment. The mine, located at Soudan, Minnesota, is presently the home of an underground laboratory that contains a one-thousand-ton detector built in the 1980s to study proton decay. MINOS will add a new five-thousand-ton detector, designed especially to observe the high-energy neutrinos produced at Fermilab. The near detector on the Fermilab site will calibrate the beam and the far detector.

How to Make a Neutrino Beam
Start with a beam of 120 GeV protons from the Fermilab Main Injector. Direct the beam to a target, creating a secondary beam of pions and kaons. Focus the positively charged particles produced in the target beam. Some of the positively charged pions and kaons will decay into muons and muon neutrinos. Send the beam through 750 feet of rock and steel to absorb the remaining pions and kaons and stop the muons. Result: a nearly pure beam of muon neutrinos.

One of the keys to the discovery of neutrino oscillations will be to exploit the fact that the Fermilab neutrino beam will have a high enough energy to produce the tau lepton, which would be produced by tau neutrinos created by the oscillation nm Æ nt. Actually seeing a tau lepton requires a very high resolution detector. The long-baseline detector for the MINOS experiment consists of five thousand tons of iron, studded with equally spaced particle scintillation counters. Although the MINOS detector cannot directly observe the tau lepton, it can recognize its signature in the particles of its decay.

Funding and flexibility

The U.S. Congress has committed funds to build both the beamline and the two detectors for the MINOS experiment, and experimenters hope to begin taking data in 2003. MINOS is expected to be the first high-energy long baseline neutrino experiment in the world.

Because of the long time required for construction, new physics results are likely to emerge in the next few years from other neutrino experiments already underway or soon to begin in Europe, Japan, Canada, and the United States. Knowledge gained from these experiments may change the detailed focus of Fermilab's MINOS experiment. The Fermilab program is designed to be flexible, able to evolve in response to new results and technologies. Experimenters are designing their apparatus to have the widest range of capability to explore the phenomena of neutrino physics.

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