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MINOS Experiment Sheds Light on Mystery of Neutrino Disappearance

Source: Fermilab Press Release, March 30, 2006

BATAVIA, Illinois—An international collaboration of scientists at the Department of Energy’s Fermi National Accelerator Laboratory announced today (March 30, 2006) the first results of a new neutrino experiment. Sending a high-intensity beam of muon neutrinos from the lab’s site in Batavia, Illinois, to a particle detector in Soudan, Minnesota, scientists observed the disappearance of a significant fraction of these neutrinos. The observation is consistent with an effect known as neutrino oscillation, in which neutrinos change from one kind to another. The Main Injector Neutrino Oscillation Search (MINOS) experiment found a value of delta m2 = 0.0031 eV2, a quantity that plays a crucial role in neutrino oscillations and hence the role of neutrinos in the evolution of the universe.

“Only a year ago we launched the MINOS experiment,” said Fermilab Director Pier Oddone. “It is great to see that the experiment is already producing important results, shedding new light on the mysteries of the neutrino.”

Nature provides for three types of neutrinos, yet scientists know very little about these “ghost particles,” which can traverse the entire Earth without interacting with matter. But the abundance of neutrinos in the universe, produced by stars and nuclear processes, may explain how galaxies formed and why antimatter has disappeared. Ultimately, these elusive particles may explain the origin of the neutrons, protons and electrons that make up all the matter in the world around us.

“Using a man-made beam of neutrinos, MINOS is a great tool to study the properties of neutrinos in a laboratory-controlled environment,” said Stanford University professor Stan Wojcicki, spokesperson of the experiment. “Our first result corroborates earlier observations of muon neutrino disappearance, made by the Japanese Super-Kamiokande and K2K experiments. Over the next few years, we will collect about fifteen times more data, yielding more results with higher precision, paving the way to better understanding this phenomenon. Our current results already rival the Super-Kamiokande and K2K results in precision.”

Neutrinos are hard to detect, and most of the neutrinos traveling the 450 miles from Fermilab to Soudan–straight through the earth, no tunnel needed–leave no signal in the MINOS detector. If neutrinos had no mass, the particles would not change as they traverse the Earth and the MINOS detector in Soudan would have recorded 177 +/- 11 muon neutrinos. Instead, the MINOS collaboration found only 92 muon neutrino events–a clear observation of muon neutrino disappearance and hence neutrino mass. The deficit as a function of energy is consistent with the hypothesis of neutrino oscillations and yields a value of delta m2, the square of the mass difference between two different types of neutrinos, equal to 0.0031 eV2 +/- 0.0006 eV2 (statistical uncertainty) +/- 0.0001 eV2 (systematic uncertainty). In this scenario, muon neutrinos can transform into electron neutrinos or tau neutrinos, but alternative models–such as neutrino decay and extra dimensions–are not yet excluded. It will take the recording of much more data by the MINOS collaboration to test more precisely the exact nature of the disappearance process.

Details of the current MINOS results will be presented by David Petyt of the University of Minnesota at a special seminar at Fermilab on March 30, 2006, at 4:00 p.m. A day later, the MINOS collaboration will commemorate MINOS co-spokesperson Doug Michael at a memorial service at Fermilab. Michael, senior research associate at Caltech, died at age 45 on December 25, 2005, after a year-long battle with cancer.

The MINOS experiment includes about 150 scientists, engineers, technical specialists and students from 32 institutions in 6 countries, including Brazil, France, Greece, Russia, the United Kingdom and the United States. The institutions include universities as well as national laboratories. The U.S. Department of Energy provides the major share of the funding, with additional funding from the U.S. National Science Foundation and from the United Kingdom’s Particle Physics and Astronomy Research Council.

“The MINOS experiment is a hugely important step in our quest to understand neutrinos–we have created neutrinos in the controlled environment of an accelerator and watched how they behave over very long distances,” said Professor Keith Mason, CEO of PPARC. “This has told us that they are not totally massless as was once thought, and opens the way for a detailed study of their properties. UK scientists have taken key roles in developing the experiment and in exploiting the data from it, the results of which will shape the future of this branch of physics.”

The Fermilab side of the MINOS experiment consists of a beam line in a 4,000-foot-long tunnel pointing from Fermilab to Soudan. The tunnel holds the carbon target and beam focusing elements that generate the neutrinos from protons accelerated by Fermilab’s Main Injector accelerator. A neutrino detector, the MINOS “near detector” located 350 feet below the surface of the Fermilab site, measures the composition and intensity of the neutrino beam as it leaves the lab. The Soudan side of the experiment features a huge 6,000-ton particle detector that measures the properties of the neutrinos after their 450-mile trip to northern Minnesota. The cavern housing the detector is located half a mile underground in a former iron mine.

The MINOS neutrino experiment follows up on the K2K long-baseline neutrino experiment in Japan. From 1999-2001 and 2003-2004, the K2K experiment in Japan sent neutrinos from an accelerator at the KEK laboratory to a particle detector in Kamioka, a distance of about 150 miles. Compared to K2K, the MINOS experiment uses a three times longer distance, and the intensity and the energy of the MINOS neutrino beam are higher than the K2K beam. These advantages have enabled the MINOS experiment to observe in less than one year about three times more neutrinos than the K2K experiment did in about four years.

“It is a great gift for me to hear this news,” said Yoji Totsuka, former spokesperson of the Super-Kamiokande experiment and now Director General of KEK. “Neutrino oscillation was first established in 1998, with cosmic-ray data taken by Super-Kamiokande. The phenomenon was then corroborated by the K2K experiment with a neutrino beam from KEK. Now MINOS gives firm results in a totally independent experiment. I really congratulate their great effort to obtain the first result in such a short time scale.”

Fermi National Accelerator Laboratory, founded in 1967, is a Department of Energy National Laboratory in Batavia, Illinois, about 40 miles west of Chicago. Fermilab operates the world’s highest-energy particle accelerator, the Tevatron, on its 6,800-acre campus. About 2,300 physicists from universities and laboratories around the world conduct physics experiments using Fermilab’s accelerators to discover what the universe is made of and how it works. Discoveries at Fermilab have resulted in remarkable new insights into the nature of the world around us. Fermilab is operated by Universities Research Association, Inc., a consortium of 90 research universities, for the United States Department of Energy, which owns the laboratory.

More information on the MINOS experiment is at www-numi.fnal.gov

A 12-minute streaming video on the MINOS experiment is at http://vmsstreamer1.fnal.gov/VMS_Site_02/VMS/MINOS/MINOS.htm


MINOS Institutions:

Argonne National Laboratory

University of Athens (Greece)

Benedictine University

Brookhaven National Laboratory

Cal Tech

University of Cambridge (U.K.)

Fermilab

College de France

Harvard University

Illinois Institute of Technology

Indiana University

ITEP-Moscow

Lebedev Physical Institute

Lawrence Livermore National Laboratory

University College, London (U.K.)

University of Minnesota

University of Minnesota-Duluth

Oxford University (U.K.)

University of Pittsburgh

IHEP-Protvino

Rutherford Appleton Lab (U.K.)

University of Sao Paulo (Brazil)

Soudan Underground Laboratory

University of South Carolina

Stanford University

University of Sussex (U.K.)

Texas A&M University

University of Texas at Austin

Tufts University

UNICAMP (Brazil)

Western Washington University

University of Wisconsin

College of William and Mary

Click image to see full size photo
Neutrinos, ghost-like particles that rarely interact with matter, travel 450 miles straight through the earth from Fermilab to Soudan — no tunnel needed. The Main Injector Neutrino Oscillation Search (MINOS) experiment studies the neutrino beam using two detectors. The MINOS near detector, located at Fermilab, records the composition of the neutrino beam as it leaves the Fermilab site. The MINOS far detector, located in Minnesota, half a mile underground, again analyzes the neutrino beam. This allows scientists to directly study the oscillation of muon neutrinos into electron neutrinos or tau neutrinos under laboratory conditions. (1 of 9)
Fermilab completed the construction and testing of the Neutrino at the Main Injector (NuMI) beam line in early 2005. Protons from Fermilab’s Main Injector accelerator (left) travel 1,000 feet down the beam line, smash into a graphite target and create muon neutrinos. The neutrinos traverse the MINOS near detector, located at the far end of the NuMI complex, and travel straight through the earth to a former iron mine in Soudan, Minnesota, where they cross the MINOS far detector. Some of the neutrinos arrive as electron neutrinos or tau neutrinos. (2 of 9)
When operating at highest intensity, the NuMI beam line transports a package of 35,000 billion protons every two seconds to a graphite target. The target converts the protons into bursts of particles with exotic names such as kaons and pions. Like a beam of light emerging from a flashlight, the particles form a wide cone when leaving the target. A set of two special lenses, called horns (photo), is the key instrument to focus the beam and send it in the right direction. The beam particles decay and produce muon neutrinos, which travel in the same direction. Photo: Peter Ginter. (3 of 9)
The 1,000-ton MINOS near detector sits 350 feet underground at Fermilab. The detector consists of 282 octagonal-shaped detector planes, each weighing more than a pickup truck. Scientists use the near detector to verify the intensity and purity of the muon neutrino beam leaving the Fermilab site. Photo: Peter Ginter. (4 of 9)
The MINOS far detector is located in a cavern half a mile underground in the Soudan Underground Laboratory, Minnesota. The 100-foot-long MINOS far 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 25 feet high and one inch thick, with the last one visible in the photo. The whole detector weighs 6,000 tons. Since March 2005, the far detector has recorded neutrinos from a beam produced at Fermilab. The MINOS collaboration records about 1,000 neutrinos per year. (5 of 9)
Far view The University of Minnesota Foundation commissioned a mural for the MINOS cavern at the Soudan Underground Laboratory, painted onto the rock wall, 59 feet wide by 25 feet high. The mural contains images of scientists such as Enrico Fermi and Wolfgang Pauli, Wilson Hall at Fermilab, George Shultz, a key figure in the history of Minnesota mining, and some surprises. (6 of 9)
More than 140 scientists, engineers, technical specialists and students from Brazil, Greece, Poland, the United Kingdom and the United States are involved in the MINOS experiment. This photo shows some of them posing for a group photo at Fermilab, with the 16-story Wilson Hall and the spiral-shaped MINOS service building in the background. (7 of 9)
Neutrino oscillations depend on two parameters: the square of the neutrino mass difference delta m2 and the mixing angle sin22θ. Experiments conducted since the 1990s are constraining the possible values of these two values. The MINOS result yields a value of delta m2 that already rivals the Super-Kamiokande and K2K results in precision: delta m2 = 0.0031 +/- 0.0006 (statistical) +/- 0.0001 (systematic) eV2. Taking more data over the coming years, the MINOS collaboration will further reduce the statistical uncertainty. (8 of 9)
A magnified plot of the MINOS results, using a different scale for the vertical axis (delta m2) of the plot. (9 of 9)