The purpose of the K2K
(KEK-to-Kamioka) Long Baseline Neutrino Oscillation
Experiment is to confirm neutrino oscillation.
The University of Tokyo's 50,000-ton water-Cherenkov
detector, Super-Kamiokande, located at Kamioka
in Gifu Prefecture, 250 km away from KEK,
detects artificially produced muon neutrinos
fired from KEK 12 GeV Proton Synchrotron.
The phenomenon of
neutrino oscillation means neutrinos to have
non-zero mass. In June 1998, the Super-Kamiokande
collaboration reported strong evidence for
neutrino oscillation (muon neutrino to tau
neutrino) in the atmospheric neutrino data
taken with the Super-Kamiokande detector.
If muon neutrinos oscillate into tau neutrinos
on their way to Kamioka from KEK, the number
of muon neutrinos observed in the Super-Kamiokande
detector will be much smaller than the number
expected without oscillation.
On June 19, 1999,
the K2K Experiment observed its first neutrino
event due to the KEK neutrino beam in the
Super-Kamiokande detector. The data were
taken in June and November, 1999, and in
January, February, March, May, and June,
2000. Until the end of March, 2000, 17 neutrinos
fired from KEK were detected inside the central
22,500 tons of the 50,000-ton water Cherenkov
detector (*). If these neutrinos came from
KEK, the time of occurrence of each event
in Super-Kamiokande had to coincide with
the expected time, which is calculated from
the time at which the neutrino beam was shot
from KEK and its time-of-flight between KEK
and Super-Kamiokande. The agreement between
the observed and expected times were found
to be better than a millionth of second,
verifying that all the 17 events are really
due to neutrinos fired form KEK.
On the other hand,
29.2+3.5-3.3 neutrino events were expected
to be observed in the Super-Kamiokande detector
if neutrinos do not oscillate. This number
can be estimated from the number of neutrino
events observed in the near neutrino detector
located at KEK. The fact that the observed
number of events in the Super-Kamiokande
is smaller than expected means that null
neutrino oscillation is disfavored at 2 sigma
level (in other words, the hypothesis of
neutrino oscillation is favored at 2 sigma
level or at about 95% probability), where
the statistical and systematic errors of
the data are taken into account. This result
was reported for the first time on June 17,
2000, at the Neutrino 2000 Conference held
in Sudbury, Canada. Scientifically, neutrino
oscillation can be concluded when the probability
of supporting this hypothesis becomes higher
than 99%. We aim at this goal by running
the experiment for some more years.
(*) By using the central 22,500 tons of the
Super-Kamiokande detector, we can make reliable
observations with the smallest systematic
errors. All the Super-Kamiokande observations
including atmospheric neutrinos and solar
neutrinos use the central 22,500 tons. |
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Click for the large image(35KB) |
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Fig.1. |
One of the Super-Kamiokande events caused
by neutrinos fired from KEK. |
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Click for the large image(7.5KB) |
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Fig. 2. |
The time at which a neutrino is detected
in Super-Kamiokande and the time at which
the neutrino beam pulse is shot from KEK
are both measured with use of GPS. The difference
of these times, after further subtracting
time of flight of the neutrinos between KEK
and Super-Kamiokande, distribute between
-0.2 and +1.3 microsecond. This is consistent
with the pulse width (1.1 microsecond) of
the neutrino beam from KEK, considering the
resolution of time measurements. While 17
events were observed within the central 22,500
tons, 9 events were observed between 22,500
tons and 32,000 tons. Corresponding to the
latter, 12.4+-2.5 events were expected if
neutrinos do not oscillate. |
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