Institute of Particle and Nuclear Studies (IPNS)

4.3 K2K Experiment
expanded to include members from Canada, France,Italy, Spain, Switzerland, and Russia in addition to theoriginal members from Japan, Korea, Poland and theU.S. (see Fig. 4-3-1).
In FY2003, K2K took data from April to June andOctober to February. The total number of protonsdelivered onto the pion production target (protons ontarget: POT) since the start of K2K in 1999 reached1.01 × 1020 (see Fig. 4-3-2). The net POT, successfully

The KEK-to-Kamioka long-baseline neutrino oscillation experiment (K2K) is the first accelerator-based
experiment with a neutrino path length extending
hundreds of kilometers. The intense, nearly pureneutrino beam (98.2% ν µ , 1.3% ν e , and 0.5% ν µ ),having an average L/Eν ∼ 200 (L = 250 km, < Eν > ∼ 1.3
GeV), is directed toward the Super-Kamiokande
detector at Kamioka. K2K focuses on the study of theexistence of neutrino oscillation in νµ disappearance
that is observed in atmospheric neutrinos, and on thesearch for νµ → νe oscillation, with well-understoodflux and neutrino composition in the ∆m2 ≥ 2 × 10−3 eV2region.
The K2K experiment started data taking in June
1999. On November 12, 2001, however, the SuperKamiokande detector suffered an accident. Over a
period of one year, it was rebuilt with about half of theoriginal photomultiplier density. The rebuilt detector iscalled Super-Kamiokande-II, as compared to SuperKamiokande-I before the accident. Consequently, theK2K experiment is called K2K-I and K2K-II corresponding to Super-Kamiokande-I and -II, respectively.
The K2K-I experiment was formally a Japan-U.S.Korea collaboration, with a few people joining fromPoland. In the K2K-II experiment, the collaboration

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Accumulated POT (1018)

10080
60
4020

Protons/Pulse (1012)

0-20
7
65
432
10-1
Jan/99Jan/00

Jan/01

Jan/02Date

Jan/03

Jan/04

Fig. 4-3-2 History of the K2K experiment, showing accumulated POT and beam intensity per pulse.

Fig. 4-3-1 K2K-II collaboration.

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Institute of Particle and Nuclear Studies (IPNS)-110-2∆m2 (eV 2 )10K2K 99% C.L.
K2K 90% C.L.
-310CHOOZ 90% C.L.
-4Fig. 4-3-4 Extruded scintillator strips and WLS fibers.

1000.20.40.6sin2 2θ µe

0.8

1Fig. 4-3-3 Exclusion plots of νµ → νe oscillations.

used for analysis of the Super-Kamiokande data, is 8.9
× 10 1 9 . The difference is attributable to various
reasons: tuning of the beam channel, testing of thehorns, Super-Kamiokande calibration, failure of SuperKamiokande due to power failure, etc., and failure inthe online data acquisition of the beam-monitoringdata. As of February 2004, K2K was approaching thegoal of 1020 net POT.
Although the main purpose of the K2K experimentis the study of νµ disappearance, νµ → νe appearance
can also be studied. Using the K2K-I data taken before
the Super-Kamiokande accident, corresponding to 4.8
× 10 19 POT, one candidate event was found. The
expected background in the absence of neutrino oscillations is estimated to be 2.4 ± 0.6 events and is dominated by misidentification of events from neutral
current π0 production. The resulting confidence intervals for the νµ → νe oscillations are shown in Fig. 4-33. This is the first accelerator long-baseline result forthis oscillation mode. Assuming 3-flavor neutrino
oscillations and CPT invariance, this result can becompared to the result from the CHOOZ reactor experiment. The limit on sin2θ13 by CHOOZ is converted byassuming sin2θµe = 12 sin2θ13, and is also shown in Fig.
4-3-3.
A brand-new fine-grained near detector called theSciBar (Scintillator Bar) detector was installed during
the summer shutdown in 2003. It is a fully activeFig. 4-3-5 This photograph shows how 64 WLS fibers
are assembled in order to couple them with a64-channel multianode PMT. A front-end
electronics board is attached to the multianode PMT.

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Institute of Particle and Nuclear Studies (IPNS)
tracking detector consisting of 14,848 extruded scintillator strips, each 1.3 cm thick, 2.5 cm wide, and 300
cm long. A group of 116 strips are glued together toform a plane, and a combination of two planes, one
horizontally and the other vertically segmented, forms
a unit layer. Altogether there are 64 layers and the totalscintillator volume weighs 15 tons. Each strip of scintillator is read out by a wavelength shifting (WLS)
fiber, and 64 WLS fibers are coupled to a 64-channel
multianode photomultiplier tube (PMT) (see Figs. 4-34 and 4-3-5). Figure 4-3-6 schematically illustrates theSciBar detector and the near detector complex. Figure
4-3-7 shows a sample of an interesting event observedin SciBar.
SciBar is a powerful detector to improve measurements of the neutrino energy spectrum and to study
neutrino interactions around 1 GeV. As a result, SciBar
will significantly contribute to improved oscillation
analysis by reducing systematic errors due to uncertainties in the neutrino energy spectrum and neutrino interactions.

Fig. 4-3-6 Schematic illustration of the SciBar detector.
Its location in the K2K-II near detector complex
is also shown. The detector behind SciBar isa spaghetti calorimeter recycled from theCHORUS experiment at CERN.

K2K fine-grained detector (top view)

K2K fine-grained detector (side view)Fig. 4-3-7 A neutral-current single π0 production event observed in the SciBar detector. The left panel shows a top view andthe right panel shows a side view.

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