New Evidence on Disappearing Neutrinos
Posted on: Apr 9, 2006
Physicists from the California Institute of Technology and an international collaboration of scientists at the Department of Energy's Fermi National Accelerator Laboratory have observed the disappearance of muon neutrinos traveling from the lab's site in Illinois to a particle detector in Minnesota. 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 at Fermilab's site in Batavia, Illinois, revealed a value of delta m^2 = 0.0031 eV^2, a quantity that plays a crucial role in neutrino oscillations and hence the role of neutrinos in the evolution of the universe.
The MINOS detector concept and design was originated by Caltech physicist Doug Michael. Caltech physicists also built half of the massive set of scintillator planes for the five-kiloton far detector. Michael led the formulation and pushed forward the program to increase the intensity of the proton beams that are the source of the neutrinos used by MINOS, leading to the present results.
'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 15 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, Minnesota-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 m^2, the square of the mass difference between two different types of neutrinos, equal to 0.0031 eV^2 +/- 0.0006 eV^2 (statistical uncertainty) +/- 0.0001 eV^2 (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 were presented by David Petyt of the University of Minnesota at a special seminar at Fermilab on March 30. On Friday, March 31, the MINOS collaboration commemorated Michael, who was the MINOS co-spokesperson, at a memorial service at Fermilab. Michael died on December 25, 2005, at the age of 45 after a yearlong battle with cancer.
The MINOS experiment includes about 150 scientists, engineers, technical specialists, and students from 32 institutions in six 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 (PPARC).
'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. U.K. 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 a long tradition of studying neutrino properties originated at Caltech by physics professor (and former LIGO laboratory director) Barry Barish. Earlier measurements by the Monopole Astrophysics and Cosmic Ray Observatory (MACRO) experiment at the Gran Sasso laboratory in Italy, led by Barish, also showed evidence for the oscillation of neutrinos produced by the interactions of cosmic rays in the atmosphere.
The MINOS result also complemets results from the K2K long-baseline neutrino experiment in Japan. In 1999-2001 and 2003-2004, the K2K experiment in Japan sent neutrinos from an accelerator at the KEK laboratory in Tsukuba 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 those of 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 timescale.'
According to Harvey Newman, a professor of physics at Caltech who now leads the MINOS group, the campus group has also had a key role in the research and development of the MINOS scintillators and optical fibers.
'Our Caltech group, then led by Michael, also had a key role in the research and development of the scintillators and optical fibers that led to MINOS having enough light to measure the muons that signal neutrino events.
'We are also working on the analysis of electron-neutrino events that could lead to a determination of the subdominant mixing between the first and third neutrino flavors, which is one of the next major steps in understanding the mysterious nature of neutrinos and their flavor-mixings. We are also leading the analysis of antineutrinos in the data, and the prospects for MINOS to determine the mixing of antineutrinos, where comparison of neutrinos and antineutrinos will test one of the most fundamental symmetries of nature (known as CPT).
'We are leading much of the R&D for the next generation, 25-kiloton detector called NOvA. Building on our experience in MINOS, we have designed the basic 50-foot-long liquid scintillator cell which contains a single 0.8 mm optical fiber to collect the light (there will be approximately 600,000 cells). We will measure and optimize the design this year in Lauritsen [a physics building on the Caltech campus], in time for the start of NOvA construction that will be completed by approximately 2010. We've also started prototype work on a future generation of megaton-scale detectors for neutrino and ultrahigh-energy cosmic rays. This has generated a lot of interest among Caltech undergraduates, who are now starting to contribute to these developments in the lab."