1 K2K Experiment

The neutrino flux and spectrum at KEK are measured by near neutrino detectors: a one kiloton
Water Cherenkov detector (1KT), a scintillatingfiber/water-target tracker (SciFi), a full-active finesegmented scintillator tracker (SciBar), and a muon
range detector (MRD). The number of interacted
neutrinos corresponding to the flux is measured by
1KT with the similarly high efficiency as SK. The
number of neutrino events in SK without neutrino
oscillation is estimated to be   based on themeasured neutrino events by 1KT. The number ofobserved neutrino events in SK is  events summarized in Table 1. Thus, we observe a significant

K2K (KEK PS-E362) is the first accelerator-based
long-baseline neutrino oscillation experiment from
KEK to Kamioka. The main physics goal of K2K
is to confirm
 oscillation discovered inatmospheric neutrinos. In addition, K2K has rich
physics to study the other oscillation channel, suchas
 appearance, and neutrino-nucleus interactions in the few GeV region. In 2004, the evidenceof neutrino oscillation: the energy-dependent disappearance of muon neutrinos is published. The result
is one of the most important physics goals in K2K,
and is described in in this report.
An intense muon neutrino beam is produced
at KEK-PS, and the beam is detected at Super- Table 1: The SK event summary. MC is a prediction
Kamiokande (SK) after traveling 250 km. K2K by the MC simulation without neutrino oscillation.
Data MC
started data taking in June 1999 and finished in
1-ring -like
57
85.5
November 2004 with    protons on target
1-ringe-like108.7(POT) accumulated for analysis. The record of demulti-ring4056.7livered POT to K2K is summarized in Figure 1.
total107   120

Accumulated POT (10^18)10080

deficit of neutrino events in SK, as one of the evidences of neutrino oscillation.
The neutrino energy spectrum at KEK is measured
by 1KT, SciFi and SciBar complementarily. The
measured neutrino spectrum, which is a product offlux times cross section, is shown in Figure 2. The
measured spectrum agrees with the prediction of thebeam Monte Carlo simulation. The neutrino spectrum at SK is estimated based on the measurement atKEK. In SK, neutrino energy is reconstructed withan assumption of the charged-current quasi elastic(CCQE) interaction. In order to enhance the fraction of CCQE events, the single-ring -like events
are selected in SK. Fifty-seven events are selected
to measured neutrino energy. The distribution of thereconstructed neutrino energy in SK is shown in Fig-60
40
20

Protons/Pulse (10^12)

0-20
7654
3
2
1
0-1
Jan/99Jan/00

Jan/01

Jan/02Date

Jan/03

Jan/04

Jan/05Figure 1: Delivered POT and beam intensity perpulse to the K2K experiment from June 1999 toNovember 2004.
In the analysis, we use a part of data before  POT.
February 2004, corresponding to 

1Events / 0.2 (GeV)

arbitrary unit

Neutrino Energy Spectrum at KEKMeasurement
Beam MC

integrated

0123Eν

416
128400

12

34Eν

5GeV

rec5(GeV)∆m2 (eV2)

Figure 3: The reconstructed neutrino energy distriFigure 2: The neutrino energy spectrum measured at bution of single-ring -like events. Points with error
KEK. The spectrum is a product of flux times cross bars are data, the solid line is the best fit spectrumwith neutrino oscillation, and the dashed line is thesection.
expected spectrum without oscillation. These histograms are normalized by the number of observed
events (57).
ure 3. The observed spectrum is not matched withthe expectation without neutrino oscillation, but one
with neutrino oscillation.
10-1
A two flavor neutrino oscillation analysis is per68%
formed by the maximum-likelihood method. In the90%analysis, both the number of neutrino events and99%
the energy spectrum shape are used. The best fit
point in the physical region is found at (  ,10-2 )=   
. Allowed regions ofoscillation parameters are shown in Figure 4, where
the likelihood ratio of each point to the best fit pointis evaluated. With neutrino oscillation of the best fit
10-3
0 0.2 0.4 0.6 0.8 1
parameters, the expected number of events is 103.8,
sin22θ
which agrees with the observation of 107 events. The
neutrino energy distribution with the expected distributions of the best fit parameters is also shown Figure 4: Allowed regions of oscillation parameters.
in Figure 3. The no-oscillation probability is calcu- Dashed, solid and dot-dashed lines are 68.4%, 90%
lated to be 0.0050% ( ). When only normaliza- and 99% C.L. contours, respectively.
tion (spectrum) information is used, the probability

2

is 0.26% (0.74%). The 90% C.L. contour crosses the    axis at    and    eV ,which is consistent with the results from atmosphericneutrinos.
In 2004, K2K achieved the original goal and confirms neutrino oscillation. For the oscillation analysis, knowledge of neutrino-nucleus cross section isimproved by near neutrino detectors: 1KT, SciFi andSciBar. As an improvement, the analysis of SciBardata gives the much smaller cross section of chargedcurrent coherent pion production than the prediction
by several models, which resolves the long-standing
problem of the forward muon deficit in K2K. Moreresults are expected to come in year 2005.

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