Last modified: Thursday, March 30, 2006
MINOS experiment sheds light on mysterious neutrinos
FOR IMMEDIATE RELEASE
March 30, 2006
BLOOMINGTON, Ind. -- An international collaboration of scientists at Fermi National Accelerator Laboratory announced today (March 30) the first results of a new experiment on very light particles called neutrinos. The results are consistent with an effect known as neutrino oscillation, in which neutrinos change from one kind to another. A team of Indiana University scientists participated in the experiment, which is called MINOS (Main Injector Neutrino Oscillation Search).
"Neutrinos are the least understood of all the fundamental particles, despite being among the most abundant particles in the universe. The total mass of all neutrinos may equal the mass of all visible matter -- such as the stars and planets -- in the universe today, affecting the evolution of the largest structures of the universe," said Mark Messier, one of the IU physicists involved in the MINOS experiment.
When the scientists sent a high-intensity beam of muon neutrinos from the Fermilab site in Batavia, Ill., straight through the Earth to a particle detector in Soudan, Minn., the small number of neutrinos detected was fewer than expected. In effect, the scientists had observed the disappearance of a significant fraction of these neutrinos. Most neutrinos pass through the entire planet without interacting with matter, so the MINOS result is consistent with an effect known as neutrino oscillation, in which neutrinos change from one kind to another as they move. The experiment confirmed that the neutrino has a tiny mass -- not zero mass, as was thought for many years.
"At the moment a property of neutrinos as basic as their mass is unknown," Messier said. "Neutrinos were believed to be exactly massless for many years until 1998, when there were the first signs that they may in fact have a tiny amount of mass. The smallness of the neutrino mass raises the question of why it is so much smaller than its companion particles such as the electron. This may indicate that the mechanism that leads to neutrino mass is fundamentally different than the mechanism that leads to the masses of the other particles."
Nature provides for three types of neutrinos, yet scientists know very little about these "ghost particles." 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.
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 pass through the Earth, and the MINOS detector in Soudan would have recorded about 177 muon neutrinos. Instead, the MINOS collaboration found only 92 muon neutrino events -- a clear observation of muon neutrino disappearance and hence neutrino mass. 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.
IU professors Mark Messier, Stuart Mufson, James Musser and Jon Urheim are heavily involved in MINOS. The main contributions of the IU group have been to the detector design, construction and operation as well as to the analysis of the data. A postdoctoral research associate in the group, Masaki Ishitsuka, carried out one of four independent analyses of the data for the new result. Former IU astronomy graduate student Brian Rebel (Ph.D. 2004), now at Fermilab, made key contributions to the analysis, and he has also been responsible, with Mufson, for studies of MINOS data on cosmic rays, soon to be published, Urheim said. Messier, IU postdoctoral researcher Jon Paley and IU graduate student Nick Graf have contributed to the understanding of the Fermilab neutrino beam, an essential component of MINOS, and they carried out a separate experiment at Fermilab for this purpose, Urheim added.
"We are very happy to be seeing such a large signature in our initial data set, consistent with expectations for neutrino oscillations. This result bodes well for future experimental efforts in this area of research, and we are working to improve the sensitivity of the MINOS analysis as we collect more data," Urheim said.
The IU group is also involved in preparations for the NOvA experiment, which will have the capability to detect other oscillation modes and may provide additional information on neutrino masses. NOvA is expected to begin collecting data by the end of the decade. Messier is currently serving as co-spokesperson for the NOvA collaboration.
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.
Mark Messier can be reached for comment at 812-855-0236 and messier@indiana.edu. Jon Urheim can be reached at 812-855-4178 and urheim@indiana.edu.
Fermilab's media contact for this experiment is Kurt Riesselmann, Fermilab Public Affairs, 630-840-3351, kurtr@fnal.gov.
More information on the MINOS experiment is at https://www-numi.fnal.gov/.
A list of institutions collaborating on MINOS can be found at https://www-numi.fnal.gov/collab/institut.html.
Photos are available at https://www.fnal.gov/pub/presspass/press_releases/NuMI_photos.
A 12-minute streaming video on the MINOS experiment is at
https://vmsstreamer1.fnal.gov/VMS_Site_02/VMS/MINOS/MINOS.htm.
Two scientific graphics summarizing the result are at
https://www.fnal.gov/pub/presspass/press_releases/MinosPictures.html.
A magazine article with more information about neutrino oscillation is at
https://www.symmetrymag.org/cms/?pid=1000112.
