New Results from Neutrino Oscillations Experiment

  In a seminars at KEK, the Japanese national particle physics laboratory located in Tsukuba Science City, Ibaraki, Japan, and at several universities in the the USA and Korea, members of the K2K Collaboration described new results from a joint experiment designed to test whether neutrinos have mass.
  K2K is a Japan-Korea-US collaboration which directs a beam of neutrino particles, produced by a particle accelerator at KEK, through the earth a distance of 250 km, to the Super-Kamiokande underground neutrino detector located near Kamioka, Gifu, Japan. Host institutes for K2K (KEK to Kamioka), which is the world's first long-baseline neutrino experiment, are KEK, and the Institute for Cosmic Ray Research of the University of Tokyo.
  Neutrinos are subatomic particles associated with radioactive decay processes, and other phenomena related to the "weak interaction", one of the fundamental forces of nature. Neutrinos have no electrical charge, and until relatively recently were assumed to also be massless.

  Three years ago, the Super-Kamiokande Experiment, a separate collaboration of physicists in Japan and the USA (although some participants overlap), presented the first data clearly indicating the existence of "neutrino oscillations", a quantum mechanical phenomenon in which neutrinos, which are known to come in three distinct varieties, or "flavors", apparently change from one flavor to another in flight. The existence of such "oscillations" indicates that at least one flavor of neutrino must have mass, whereas the Standard Model of particle physics assumes massless neutrinos. Thus neutrino oscillations indicate the existence of phenomena beyond the Standard Model, an exciting prospect for physicists.

  The new results from K2K represent the first significant data on neutrino oscillations using a man-made neutrino beam, whose flavor purity has been sampled by a "near detector" array at KEK, over a distance measuring hundreds of kilometers. Previously, experiments have been confined to short distances, within the boundaries of an accelerator laboratory. The 250 km baseline between KEK and the "far detector" at Kamioka allows finer resolution of oscillation effects.

  Results presented today represent data accumulated by K2K from June 1999 up to April 26, 2001. The KEK Proton Synchrotron, which generates the K2K neutrino beam, is operated approximately 6 months per year. A total of 44 neutrinos from KEK have been identified in the Super-Kamiokande detector.
  Based on a wide variety of measurements made at KEK, the number of events expected in the absence of neutrino oscillations would be 64, with error margins conservatively estimated as approximately 10%. Thus the K2K results are statistically inconsistent with the no-oscillations hypothesis (i.e., Standard Model assumption of massless neutrinos) at about the 97% confidence level.
  In addition to the overall number of beam-induced neutrinos observed, the new results presented include a preliminary energy spectrum of the K2K events in Super-Kamiokande. Because the oscillation phenomenon has energy dependence which is in turn dependent upon the difference in mass between neutrino flavors, any differences between the neutrino spectrum measured by the near detectors and the spectrum observed at Super-Kamiokande will provide a sensitive indicator of the mass differences involved. However, at present the total number of observed events available (44) is too low to allow definitive conclusions to be claimed.

  The data published by Super-Kamiokande in 1998 were obtained from observations of neutrinos produced in the earth's atmosphere by cosmic ray particles. Additional observations by Super-Kamiokande, on neutrinos produced in the Sun, were recently confirmed and extended by the SNO Collaboration, which operates another underground neutrino detector in Sudbury, Ontario, Canada. These experiments depend upon detailed models of neutrino production in the atmosphere and in the Sun, to analyze their data. The K2K experiment provides an independent test, using an artificial neutrino beam whose characteristics are first calibrated by independent detector systems. Other long-baseline neutrino experiments are planned but will not begin taking data until after 2005.

  K2K has been operating since 1999, and is expected to continue accumulating data through early 2004. By that time, the number of neutrinos observed in the far detector should be three times larger than at present, and a statistically significant, detailed analysis of theneutrino energy spectrum will be possible.