K2K - KEK to Kamioka- Long Baseline Neutrino Oscillation Experiment
Recent Results From K2K
(KEK to Kamioka Long Baseline Neutrino Oscillation Experiment)
The K2K Collaboration
Spokesperson: Koichiro Nishikawa (Kyoto-University), nishikawa@neutrino.kek.jp
Japan KEK / ICRR / Kobe Univ. / Kyoto Univ. / Niigata Univ. / Okayama Univ. /Tokyo Univ. of Science / Tohoku Univ. / Hiroshima Univ. / Osaka Univ.
U.S.A. Boston University / Duke University / Massachusetts Institute of Technology / SUNY at Stony Brook / University of California, Irvine / University of Hawaii at Manoa / University of Washington, Seattle
Korea Chonnam National University / Dongshin University / Korea University / Seoul National University
Italy University of Rome "La Sapienza"
Canada TRIUMF / University of British Columbia
France Saclay
Switzerland University of Geneva
Spain UAB/IFAE, Barcelona / University of Valencia
Poland Warsaw University
Russia Inst. for Nuclear Research, Moscow

June 11, 2004, 11:00 am EDT(USA) = June 12, 2004, 00:00 am JST(Japan)
KEK, High Energy Accerelator Research Organization
ICRR, Institute for Cosmic Ray Research, University of Tokyo


K2K is an international collaboration of physicists, organized to study the properties of the subatomic particles called neutrinos. A neutrino beam is generated at KEK, the Japanese National High Energy Accelerator Laboratory in Tsukuba, Japan, and directed through the Earth to the Super-Kamiokande underground neutrino detector, located about 250km away. Using data collected through February, 2004, K2K has observed 108 beam-induced neutrino interactions in Super-Kamiokande. In the absence of the phenomenon called neutrino oscillations, which implies that neutrinos have mass, the expected number of such events would be 150.9 (+11.6, -10.0) showing appearant deficit of the observed data. However, the K2K data are consistent with the oscillation effects previously reported by Super-Kamiokande, using data from naturally-produced (atmospheric) neutrinos. K2K also reported the first significant evidence for the energy dependence of the oscillation effect, which is an expected consequence of the oscillation phenomenon. Taking into account measurements of the beam obtained from "near" detectors on the KEK site, the probability that the observed data are consistent with the hypothesis of no oscillations (hence, massless neutrinos) is negligible (10-4).

K2K expects to increase the number of observed events by about 30% during the next year, before the anticipated shutdown of the KEK proton accelerator in 2005. These additional data will include special studies needed to refine plans for the next-generation experiment at the new J-PARC accelerator facility under construction in Tokaimura, Japan.


Goals of the experiment
In 1998, clear evidence for neutrino oscillations was first reported in studies of atmospheric neutrinos by Super-Kamiokande. Neutrino oscillation occurs among the three known “flavors” of neutrinos when neutrinos have non-zero mass and mixing. Under these circumstances, flavor states are mixtures of mass states, and a neutrino may change its apparent flavor with time, for example during the time of flight from KEK to Kamioka. Neutrino masses were assumed to be zero in the current standard model of elementary particle physics. The discovery of neutrino oscillations and hence non-zero neutrino masses requires a theory beyond the standard model, and the apparent smallness of the neutrino masses indicates physics with a large energy scale. The aim of the K2K experiment is to confirm the neutrino oscillation phenomena observed with atmospheric neutrinos, and more precisely determine the neutrino oscillation parameters (mass differences and mixing angles).
The experimental method

The K2K neutrino beam is produced by the 12 GeV proton accelerator at KEK. These artificially created neutrinos are almost entirely muon-neutrinos, as confirmed by data from the near detectors at KEK, which measure the neutrino flux and energy spectrum immediately after production, and before neutrino oscillation effects become visible. The beam was directed towards the Super-Kamiokande detector located 250km away from KEK. The energy spectrum of the neutrino beam is similar to that of atmospheric neutrinos, and the long baseline of the experiment allows us to test neutrino oscillations effects observed in atmospheric neutrinos. The K2K experiment is the first truly long baseline neutrino oscillation experiment, in which a neutrino beam was directed to a distant off-site detector.

Neutrino beam pulses, of duration a few millionths of a second, were sent from KEK to Super-Kamiokande every 2.2 sec. The pulsed time structure of the beam made it possible to distinguish the interactions of beam neutrinos from those of cosmic ray (atmospheric) neutrinos, which arrive randomly in time.

Data Analysis and results
The K2K experiment measures the intensity and the energy spectrum of the neutrino beam using the near detectors at KEK, and then measures any differences in far detector, Super-Kamiokande. A deficit in the flux or distortion of the energy spectrum provides evidence of neutrino oscillations. We present here results from all data analysed so far, through May, 2004. Since data-taking began in 1998, we have continuously improved our data analysis and understanding of detector systematics. We have obtained the following results:

1) We observe 108 events in Super-Kamiokande, while the expected number is 150.9 (+11.6, -10.0) for the no-oscillation hypothesis.
2) The observed neutrino spectrum reveals exactly the type of distortion expected from neutrino oscillation effects. (Fig.3)。
     Based on these results, we have concluded that
1) If there is no oscillation, the probability to observe only 108 events and the observed energy spectrum distortion due to statistical fluctuation is negligible (10-4), and
2) K2K results are consistent with those expected from the atmospheric neutrino oscillation results published previously by Super-Kamiokande. We now have sufficient data to observe significant deviations between the far- and near-detector energy spectra, which can only be attributed to the effects of neutrino oscillations.
     K2K will have an additional beam run starting in October, 2004. The ending date may be in March, 2005. While the new data will not significantly increase the overall statistical weight of the experiment, it will be very important for several investigations of detector response and systematics.
     The KEK 12 GeV proton synchrotron will be shut down in 2005, in order to make equipment and human resources available for construction of the new J-PARC 50 GeV proton accelerator, currently under construction at Tokaimura. One of the main experiments planned for J-PARC is T2K (Tokai to Kamioka), the next-generation long baseline neutrino experiment. The new accelerator will provide an effective factor of 100 increase in event rates, permitting much more detailed investigations of neutrino oscillations and related phenomena.
     A new Scintillating Bar (SciBar) detector system was installed in the K2K near detector, and commissioned in September, 2003. Scibar was intended in part as an in-service prototype for a critical detector element in the T2K experiment. The 2004-5 K2K run will significantly increase the available SciBar data. We will also perform studies of beam targeting and control, in preparation for T2K. Finally, we will use the next run to complete a variety of background and calibration measurements on the K2K near detectors.
     K2K and T2K explore fundamental questions in particle physics and astrophysics. However there are still many unresolved issues in these fields, for example the discovery of CP violation, which is closely related to the origin of the asymmetry of matter and anti-matter in the universe. We will continue to challenge those puzzles in near future.
  Fig.1   Fig.2
Fig.1  Schematic overview of the K2K experiment. A high-purity muon neutrino beam, with a broad energy spectrum centered around 1 GeV,  is generated at KEK, in Tsukuba, and directed through the earth to Super-Kamiokande, near Kamioka, 250 km away. Fig.2  Timing diagram, illustrating the method for identifying KEK beam-induced events in Super-Kamiokande. Histograms show the difference between event time and expected beam arrival time, from GPS observations at Super-K and KEK respectively, in microseconds. There is negligible probability for an atmospheric neutrino background event to occur in the timing window shown (the nearest such event is over 120 microseconds out of synchronization).
  Fig.3   Fig.4
Fig.3 Distortion of neutrino energy spectrum observed at Super-Kamiokande (data points), compared to scaled KEK beam spectrum (black histogram), which represents expectation in the absence of neutrino oscillations. Also shown is the expected spectrum taking into account neutrino oscillation effects (red histogram), applying the K2K best-fit oscillation parameters. Fig.4  Contours enclosing the 90% confidence regions for two-flavor (mu-tau) neutrino oscillation parameters (mass difference squared, and mixing parameter) for the new K2K results (green), compared to results from Super-Kamiokande atmospheric neutrino(black).
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