K2K Experiment
The KEK-to-Kamioka long baseline neutrino oscillation experiment (K2K) is therst accelerator-based experiment with hundreds of km neutrino path length. Theintense, nearly pure neutrino beam (98.2 % , 1.3 % e, and 0.5 %  ) has an average
L=E  200(L = 250km; < E > 1:3 GeV). K2K focuses on the study of the existenceof neutrino oscillation in  disappearance that is observed in atmospheric neutrinos,and on the search for  ! e oscillation with well understood ux and neutrinocomposition in the 1m2  2 2 1003 eV2 region.
In scal year 2001, K2K took data from April to July. The total number ofprotons delivered onto the pion production target (POT) since the start of the K2K
in 1999 reached 5:6 2 1019. The net POT, successfully used for the analysis of theSuper-Kamiokande data, is 4:8 2 1019. The balance is due to various reasons: tuningof the beam channel, testing of the horns, Super-Kamiokande calibration, failure ofSuper-Kamiokande due to power failure, etc., and failure in the online data acquisitionof the beam-monitoring data. As of July, 2001, K2K is midway to the goal which isrepresented by 1020 POT.
With all the available data analyzed, 56 neutrinos coming from KEK were observed inside the central 22,500 tons of the Super-Kamiokande ducial volume. On theother hand, from the +7observationin the 1000-ton water Cherenkov detector locatedat the KEK site, 80:608::30 neutrino events were expected to be observed in the SuperKamiokande detector if neutrinos do not oscillate. The expected number of events fully
contained in the Super-Kamiokande ducial volume is also calculated for the hypothesis of neutrino oscillation with the maximal mixing. Table 1 shows the results. Thistable also shows numbers for 1-ring -like and e-like events and multi-ring events.
Figure 1 shows overall arrival time of events through all runs. The horizontal
axis is POT rather than the elapsed time. The uniformity of the arrival times is testedby the KS (Kolmogoro -Smirnov) probability, which is 43.1 %, indicating satisfactorily
uniform distribution.
For the oscillation analysis, the goal is to compare the observed spectrum at thefar detector with the non-oscillated spectrum extrapolated from the spectrum measurement at the near detector. Figure 2 shows the reconstructed E spectrum with 30
fully contained 1-ring -like events in the Super-Kamiokande ducial volume. Here,kinematics of two-body quasi-elastic (QE) reaction is assumed. Therefore the observed
spectrum mainly consists of the real muon neutrino energy spectrum and non-QE background. The histogram shows a preliminary spectrum extrapolated from the measurement at the near detector, assuming no neutrino oscillation. The non-QE backgroundis taken into account by the Monte Carlo simulation for the neutrino interactions andthe detector response. The systematic errors associated with this spectrum have yet tobe estimated. Note, however, that neutrino oscillation with 1m2 = 3 2 1003 eV2 , whichis consistent with the atmospheric neutrino oscillations, gives an oscillation minimumin the observed spectrum at E  600 MeV even with the existence of the non-QE
background. Although preliminary, the data seem to indicate this minimum.
On November 12, 2001, Super-Kamiokande, the far detector of the K2K experiment and operated by Institute for Cosmic Ray Research, the University of Tokyo,
su ered from a sad accident in which about 60 % of both 20-inch PMTs for the in-Table 1: The observed numbers of events fully contained in SK with the vertices locatedinside the 22500-ton ducial volume are compared to the expected numbers calculated
from the observation in the 1000-ton water Cherenkov detector at the KEK site for thehypotheses of no oscillation and neutrino oscillation with sin2 2 = 1.
1m2 (eV2 )observed
NSK
Null Oscillation 3 2 1003 5 2 1003 7 2 1003
:3
FC in 22500 ton
56
80:6+7
52:4
34:6
29:2
08:0
1-ring32
48:4 6 6:7
28:1
17:8
16:0
-like30
44:0 6 6:8
24:4
14:6
13:5
e-like2
4:4 6 1:7
3: 7
3: 2
3:0
Multi-ring
24
32:2 6 5:3
24:3
16:8
12:6
ner detector and 8-inch PMTs for the outer detector were destroyed. After 5 years ofoperation, Super-Kamiokande had been emptied and dead PMTs were replaced withnew ones since July, 2001. The accident occurred during water ll, after completionof the replacement work: the water level was about 30 m at that time. The cause ofthe accident was implosion of one of the PMTs at the inner bottom PMT plane. This
implosion generated intense shock waves which destroyed neighboring PMTs, resulting
in chain reactions and mass destruction of the PMTs. The rst PMT most probably
imploded due to a light damage of the PMT glass during the repair or due to strains
given at the bottom replacement work.
As a result, the K2K run, originally scheduled to resume in January 2002, had tobe deferred. As the spokesperson of the Super-Kamiokande Collaboration, Professor
Totsuka, announced immediately after the accident, the Super-Kamiokande detector
will be rebuilt as quickly as possible with the PMT density reduced by about a half,in order to restart the K2K experiment. The goal may be within a year. It should benoticed that for the detection of GeV neutrinos which are shot from KEK, half PMT
density will not cause any serious deterioration of the detector performance. The K2KCollaboration will fully support the recovery work of Super-Kamiokande.

Figure Captions
Fig. 1. POT (number of protons delivered onto the target) versus accumulated numberof events at Super-Kamiokande.
Fig. 2. The neutrino spectrum observed at Super-Kamiokande. The histogram shows
the spectrum extrapolated from the observation at the near detector, assuming noneutrino oscillation (preliminary).

56 events @ 48×1018 POT

Events/0.5 GeV

Note: ∆m2=3×10-3 eV2corresponds to600 MeV Eν

ilePryranimEν (GeV)