Institute of Particle and Nuclear Studies (IPNS)

4.2 Experiments at the 12 GeV Proton Synchrotron(E521). A promising enhancement in the neutron spectrum of E471 found in the FY2002 data will be locatedby increasing the statistics. E521 continued with thebeam momentum at 1.2 GeV/c to establish the 10Λ Li
production and accumulated data for another 700 × 109
π−-injections. The search for H-dibaryon resonance via
12C(K−, K+ΛΛ) (E522) was performed to compensatefor the loss of beam time in FY2002. The invariantmass spectrum of ΛΛ near the threshold (Fig. 4-2-2)
shows the enhancement, which could be attributed tothe attractive final-state interaction of ΛΛ. In E546, theelectronic X-ray emission correlated with each pionic
X-ray was measured to understand electronic rearrangement in the pionic atom during the pionic cascade. To
examine the correlation between πX-rays and eX-rays,
the photon events, Mo, Sn, Dy, Ho, Ta, and Pb metals,and Mo, Sn, Ba, Nd, Gd, Ho, Yb, and Hg oxides wereused as the target. For these four experiments, datataking has been completed.
Test experiments were carried out in FY2003 forcalibrations and R&D of detectors, not only for PSexperiments but also for experiments at other laboratories and future experiments. The T526/542 group forthe international Muon Ionization Cooling Experiment(MICE) at RAL has tested the basic performances ofscintillating fiber trackers to minimize the effects ofIn FY2003, the KEK 12 GeV Proton Synchrotron(PS) was operated mainly in fast beam-extraction mode
to compensate for the suspension due to rebuilding
work of photomultiplier tubes (PMTs) at a SuperKamiokande in FY2002. The slow beam-extraction
mode was operated for two weeks before the summer
shutdown and for one and a half months at the end ofFY2003. During the period in slow beam-extractionmode, six experiments and 12 tests were performed.
E391a is the first experiment dedicated to a flavorchanging neutral current (FCNC) process, the KL →π0 νν decay. Almost all of the detectors are installed ina vacuum (Fig. 4-2-1). The detector system consists of576 CsI calorimeters to measure the two gammas fromKL → π0 νν, and sampling calorimeters surrounding thedecay fiducial region to veto the other decays and beam
backgrounds. The vacuum level reaches less than 1 ×
10−1 Pa in the detector region and less than 2 × 10−5 Paaround the beam region. Data taking was started onschedule in February 2004, and will be continued up to
the end of June 2004. The experiment will reach thegoal sensitivity of O(–10) for the KL → π0 νν decay.
The extension of the beam time was approved fortwo experiments: the search for a deeply bound K −ppn
state (E471), and production of neutron-rich Λ hypernuclei by the (π−, K+) double-charge-exchange reaction

Fig. 4-2-1 E391a detector system.

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Institute of Particle and Nuclear Studies (IPNS)

Y pixelFig. 4-2-3 Director Totsuka discussing with Dr.
Evtoukhovitch (JINR), Prof. Mjavia (JINR),
Dr. Garutti (DESY), and Prof. Takeshita(Shinshu Univ.).

Fig. 4-2-2 Invariant mass spectrum of ΛΛ near thethreshold in E522. The statistical significanceof the enhancement below the 2261 MeV/c2region was found to be 2.7 standard deviations with respect to the combinatorial background.

250500200400150300
100200background X rays from RF cavities and multiple scattering. The purpose of T538 was R&D of the Time
Projection Chamber with CF4 gas for heavy ion collisions at BNL. Tracking resolution, double track separation, and particle identification capability were tested
and a 100 µm position resolution with a 2.5 mm square
pad was achieved. The T539 group has been developing beam monitoring detectors in the intermediate
beam intensity from 108 to 1010 particles/spill in thesecondary beam line at J-PARC. Stable operation ofthe fiber detectors (glass and scintillating fibers) in thesecondary beam line was confirmed, at least up toabout 5M particles/spill/fiber. The T540 group verified
the performances of the prototype particle detectors forthe newly developed BESS-Polar superconducting
spectrometer. A timing resolution of the scintillation
counter hodoscope of below 250 ps for each pad wasobtained for 0.5 GeV/c protons, which is better than thedesigned value. The objective of T543 is to provide
sample emulsion bricks to conduct a rehearsal of theevent locations procedure for the OPERA experimentat CERN. A scintillating fiber tracker is used to determine the true location by tagging an incoming parent

50100000100200300

400500-50

X pixelFig. 4-2-4 Image of fibers measured with EBCCD.

particle and outgoing daughter tracks. An extremely
low intensity beam of about 1– 10 particles/spill, whichis required for this rehearsal, was found to be possibleat the π2 beam line. The T545 experiment for very
high granularity calorimetry for precise determinationof the reaction final state in linear collider experiments
was carried out as a collaboration with a group fromJINR and a group from DESY. Figure 4-2-3 shows ascene in front of the JINR EMC module during thebeam test in which Director Totsuka was discussing thetile fabricated by the JINR group using the injectionmold method. Figure 4-2-4 shows an example of areconstructed image of EBCCD data with five-fiber
readout for plastic-scintillator-based fine-granularity
calorimeters. The T547 group has been studying a24

Institute of Particle and Nuclear Studies (IPNS)

Fig. 4-2-5 Measured range (left) and dE/dx (right) of protons in scintillator as a function of momentum. Black (red) points
represent data (MC simulation).

very high resolution time-of-flight counter that uses
Cerenkov radiation from a quartz radiator. The test
counter consists of the quartz radiator and a microchannel-plate PMT. The purpose of experiment T551
is to measure the basic parameters of the scintillator
used for the K2K SciBar detector, which wasconstructed in the summer of 2003 to upgrade the K2K
near detector system. Figure 4-2-5 shows the measured
range and dE/dx of protons as a function of momentum.
The measured range shows excellent agreement withthe Monte Carlo (MC) expectation, whereas a discrepancy is seen in the low-momentum region for dE/dx.
This is presumably due to saturation and nonlinearityof the readout electronics for a large amount of charge,
which is not included in the MC at this time. These
data will give important information about the responseof scintillators in the SciBar detector.
Several results have been obtained from executed
experiments in the analysis stage. E246 has searched
for a T-violating transverse muon polarization (PT) inthe K+ → π0µ+ν decay. In 2003, the combination of two
independent analyses was completed by sorting all
good events into three categories: common events andtwo types of uncommon events. The null asymmetry
A0, the sensitivity AN, and other systematics such as the
decay plane angular distribution of each data set were
carefully checked and it was confirmed that all the data
were of high quality. For the polarimeter analysis, a
new method has been adopted to explicitly take into

account the muon stopping distribution in the stopper.
In this analysis, PT is extracted differentially along thepolarimeter axis, and its averaging gives the result of PT= –0.0017 ± 0.0023(stat) ± 0.0011(syst) corresponding
to Imξ = –0.0053 ± 0.0071(stat) ± 0.0036(syst), whichshows no evidence of T violation within the experimental accuracy. This result improved the previous
limit from BNL-AGS by a factor of 3 as shown in Fig.
4-2-6, and can constrain the model parameters ofseveral non-standard CP violation models.
0.2
0.15

Bevatron
BNL
Argonne

0.1

Im

0.05

BNL

KEKKEKKLKL

0-0.05KLK+++K

K-0.1-0.15-0.2
1960

1980

2000

YearFig. 4-2-6 History of PT including E246 results.

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Institute of Particle and Nuclear Studies (IPNS)
The major scientific interest of E325 is to investigate the properties of hadrons (and then of quarks) indense matter, naturally existing as nuclei. The basic
experimental concept is to measure vector mesons,which are produced and decay in a nucleus, throughinvariant mass spectroscopy in the electron-positronpair channel. Figure 4-2-7 shows the electron pair
spectrum for a copper target, after subtracting thecombinatorial background. Free decay peaks of ω andφ are clearly observed and significant excess can beseen on the low mass side of the ω peaks.
In the analysis of the hybrid emulsion experimentE373, a clear event, “NAGARA”, was detected asshown in Fig. 4-2-8. A Ξ− hyperon was captured andthree charged particles were emitted. Since each of thethree charged tracks at two decay points was coplanar
within angle errors, it was natural that no neutral particles were emitted. Kinematic analysis showed that66
He + 4He + t, and ΛΛHeonly the modes of Ξ− + 12C → ΛΛ
5−→ ΛHe + p + π , satisfy the conditions for the production and decay of a double-Λ hypernucleus without any
ambiguities by the excited state for the double- andsingle-Λ hypernucleus. BΛΛ and ∆BΛΛ for the 6ΛΛHenucleus were obtained by kinematic fitting as follows:BΛΛ = 7.25 ± 0.19 + 0.18 – 0.11 MeV, and ∆BΛΛ = 1.01 ±0.19 + 0.18 – 0.11 MeV. This means that the ΛΛ interaction is weakly attractive. Taking into account theresult of the NAGARA event, the lower limit of themass of the H-dibaryon was determined to be 2223.7
MeV/c2 at the 90% confidence level.

φFig. 4-2-7 Observed invariant mass spectra of e + e −data, after subtracting the combinatorial
background, for the copper target in E325.
Lines are the model calculations.

Fig. 4-2-8 NAGARA event in E373.

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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 > ∼ 1.3GeV), 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

120

Accumulated POT (1018)

10080604020Protons/Pulse (1012)

0-20
76543210-1
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)

-1

10-2

∆m2 (eV 2 )10

K2K 99% C.L.
K2K 90% C.L.
-3

10CHOOZ 90% C.L.
-4

Fig. 4-3-4 Extruded scintillator strips and WLS fibers.

1000.20.40.6sin2 2θ µe0.8

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 in
the 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 total
scintillator 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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