Institute of Particle and Nuclear Studies (IPNS)

4.3 K2K Experiment
As reported in the KEK Annual Report 2001,
Super-Kamiokande (SK), the far detector of the K2K
experiment, suffered from an accident on November
12, 2001, in which about 60% of the 20-inch photomultiplier tubes (PMTs) for the inner detector and 8inch PMTs for the outer detector were destroyed.
After extensive investigation of the cause of theaccident and subsequent tests of protection measuresagainst the shock waves produced by the PMT implosion, the Super-Kamiokande and K2K collaborationsstarted rebuilding the SK detector in April 2002. The
work took about half a year to complete. The innerdetector was, however, only partially rebuilt. The 20inch PMTs then available, most of which were those
PMTs that fortunately survived the accident with theremainder from the stock at the Institute of Cosmic Ray
Research (ICRR), were only 47% of the 11,146 PMTs
necessary to cover the whole inner wall of the large
water tank of the SK detector. In the restoration, allPMTs were encased in plastic covers as shown in Fig.
4-3-1. The front panel of the PMT case is made of 12mm-thick UV-transparent acrylic so that, even if one
PMT might implode due to water pressure, the shockwaves should be safely confined in its plastic case.
Although the inner detector still suffers from the accident, the outer detector has been completely restored
thanks to the support of the U.S. Department of Energy.
The Super-Kamiokande experiments before andafter the accident are now called Super-Kamiokande-I(SK-I) and Super-Kamiokande-II (SK-II), respectively.
Similarly, we refer to the K2K experiment with the SKII detector as K2K-II and the one before the accident asK2K-I. While K2K-I was essentially a Japan-USKorea collaboration with a few members from Poland,
K2K-II enjoys a larger collaboration including
members from France, Italy, Spain, Switzerland,
Russia, and more recently from Canada.
Filling of the SK-II detector with water started at
the beginning of October 2002, and K2K-II started datataking at the end of December 2002. After the New
Year holidays in 2003, data taking resumed on January
18, 2003. Figure 4-3-2 shows the status of the K2K-II
experiment around the end of April 2003. The total
number of protons delivered onto the pion production
target (protons on target: POT) for the K2K-II experiment has reached to 1.7 × 1019. The corresponding SKII live time was about 1.5 × 1019 POT.
SuperKrebuilding

K2K-I

K2K-II

80

Accumulated POT (1018)

70
60
50
4030
2010

Accumulated POT

Protons/Pulse (1012)

0-10
654
32
10-1
Jan/99Jan/00

Jan/01

Jan/02

Jan/03

DateFig. 4-3-2 Status of the K2K experiment. The integrated number of protons delivered onto thetarget (POT) and beam intensity per pulse
are shown.

Fig. 4-3-1 An encased 20-inch PMT for the SuperKamiokande-II detector.

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Institute of Particle and Nuclear Studies (IPNS)
interactions with proper detector response. The box
histogram is the expected spectrum without oscillations, where the height of boxes indicates the systematic error. The solid line is the best-fit to the data withsin22θ = 1.0 and ∆m2 = 2.8 × 10−3 eV2. The histograms
are normalized to 29 observed events. The dashed line
shows the expected spectrum with no oscillations,which is normalized to the expected number of events(44). The probability that the present observation isdue to statistical fluctuation rather than neutrino oscillation is less than 1%.
Figure 4-3-4 shows the allowed regions of theoscillation parameters. The dashed, solid, and dotdashed lines are the 68.4%, 90%, and 99% confidence
level contours, respectively. The best-fit is marked by
the star. The measured oscillation parameters areconsistent with those suggested by the atmospheric
neutrino observations in SK-I.

EventsThe K2K-I results for neutrino oscillations werepublished in January 2003. As already described in the2001 Annual Report, 56 neutrinos from the KEK 12
GeV Proton Synchrotron (PS) were observed inside theSK-I fiducial volume of 22,500 tons of water. Assuming
no oscillation, we expect 80.1+6.2−5.4 events based on theneutrino intensity measured in the 1,000-ton water
Cerenkov detector located at the KEK site.
Figure 4-3-3 shows the Eν spectrum for 29 fully
contained 1-ring µ-like events observed in the SK-I
fiducial volume. In the event reconstruction, the kinematics of a two-body quasi-elastic (QE) reaction wereassumed. The observed energy spectrum, therefore,
mostly consists of that from real muon neutrinos or thatof non-QE background. The non-QE background wasestimated by a Monte Carlo simulation for the neutrino

2∆m (eV )

122

108-2

1064-3102
0012345Eνrec-4

10Fig. 4-3-3 The neutrino spectrum observed at the SK-1
detector compared with the spectr um
expected for no oscillation and that for oscillation of best-fit.

00.20.4

0.60.812sin 2θFig. 4-3-4 Allowed regions of the oscillation parameters.

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