Institute of Particle and Nuclear Studies (IPNS)

4.2 Experiments at the 12 GeV Proton Synchrotron
In FY2002, the 12 GeV Proton Synchrotron (PS) wasoperated in the slow beam-extraction mode from April
to late December. Seven physics experiments and 19
test experiments were performed in the period. Thenthe accelerator was switched to the fast extraction
mode for the KEK-to-Kamioka (K2K) long-baseline
neutrino oscillation experiment and sent neutrinos
until the end of FY2002 (March 2003) to the SuperKamiokande detector that was finally revived after itsaccident in November 2001.
Experiment E391a was a physics experiment tosearch for the rare kaon decay KL0 → π 0νν. This flavorchanging neutral current (FCNC) process is sensitive to
new physics beyond the Standard Model, and is thus
one of the most interesting processes in the study of CP
violation in K-meson system. The experiment is now
at the stage of detector construction, in preparation for
physics runs starting in February 2004. The E391a
detector consists of an electromagnetic calorimeter
made of pure CsI crystals, barrel veto counters made oflead-scintillator sandwich detectors, and five stages ofbeam counters. In FY2002, the downstream section of
the detector including a part of the electromagnetic
calorimeter was fabricated and tested with a neutralbeam at the PS. Using the same trigger scheme andelectronics, a very clean KL mass was reconstructed(Fig. 4-2-1). Another important step in FY2002 was

the fabrication of 16 front-barrel veto counter modules(Fig. 4-2-2).
The other six physics experiments are studing
various effects of strange particles in nuclei: a search
for the deeply bound Kppn state (E471), a coincidence
measurement of the weak decay of 12Λ C (E508), γ -rayspectroscopy of hyper fragments with stopped kaons(E509) and by the (π +, K +) reaction (E518), a study ofthe production of neutron-rich Λ hypernuclei by the(π +, K +) double-charge exchange reaction (E521), and asearch for H-dibaryon resonance (E522). In threeexperiments, E508, E509, and E518, data taking wascompleted in FY2002.
Many of the 19 test experiments carried out inFY2002 were actually detector calibrations anddetector R&D for experiments at other laboratories
rather than KEK. Experiment T500/530 tested detector
packages for (e, e′ K +) Λ-hypernuclei spectroscopy at
the Thomas Jefferson National Accelerator Facility(Jefferson Lab). The packages were composed ofwater Cerenkov counters, aerogel Cerenkov counters,
and a time-of-flight (TOF) counter. The waterCerenkov counters were read out by the amino-G-salt
wavelength shifter and have achieved 98% detection
efficiency for K + with 2% contamination of protons.
The T512/523 group has been developing an emulsion
cloud chamber for π /µ identification to reduce backgrounds in the OPERA experiment at CERN. The
purpose of experiment T513 was to evaluate the performance of a neutron-insensitive photon detector that has
been developed for the future KOPIO experiment at theBrookhaven National Laboratory (BNL). The T514/531

Fig. 4-2-1 The invariant-mass plot for 6 γ rays shows avery clean peak of K 0L → π 0π 0π 0{Kπ 3} decays(E391A experiment).

Fig. 4-2-2 Front-barrel modules fabricated for experiment E391a.

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Institute of Particle and Nuclear Studies (IPNS)
conducted in two independent ways. One of theanalyses was completed in FY2002. Combining thenew result with the previous results for 1996-1998 data,
PT was found to be consistent with zero: PT = (–1.12 ±2.17(stat) ± 0.90(syst)) × 10−3. The final result will bepresented by combining this result with that obtainedby the second method. The group also studied K+ →µ +νγ (Kµ 2γ ) decay. For this decay process, the groupobtained the result P T = (–0.57 ± 1.85(stat) ±0.10(syst)) × 10−2. Experiment E470, which recycled
the detector of experiment E246 with some minor
modifications, presented a new result on the direct
photon emission branching ratio extracted from thedecay process K + → π +π − γ (Kπ 2γ ). The branching ratio
based on the 2001 data is (6.1 ± 2.5(stat) ± 1.9(syst)) ×10−6, which is consistent with the results from a stopped
K + experiment.
Experiment E325 was carried out to study nuclear
media effects appearing in the invariant mass spectra ofρ, ω, and φ mesons decaying into e+e− or K +K −. The
experiemnt took data for about 3,200 hours over fiveyears from 1998 to 2002. Figure 4-2-5 shows the newgroup tested aerogel threshold-Cerenkov counters toenhance particle identification capability for thePHENIX experiment at the relativistic heavy ion collider(RHIC) at BNL. One of the prototype counters achieved
99.5% detection efficiency for 2 GeV/c π with 0.5%
proton contamination. The T517 group is now developing an electromagnetic calorimeter with scintillator
strips for the future worldwide linear collider (Fig. 4-23). The group obtained the angle resolution of 54 mradfor 4 GeV showers, a good result for dense (nongaseous) calorimeters. The T525 experiment was set up
to test two types of ring imaging Cerenkov detector for
the upgraded B-factory: (1) Time-Of-Propagation (TOP)counter, and (2) proximity-focusing ring-image
Cerenkov counters using an aerogel radiator (Fig. 4-2-4).
The T527 group measured the characteristics of thePhoswich-type NaI(Tl) counter surrounded by plastic
scintillators to detect mono energetic protons up to 500
MeV. To study the pion capture process in molecules,
the T529 group measured the correlation spectra of electronic and pionic X rays for various metals (Zn, Mo, Ag,Sn, and Ta) and metal oxides (Zn, Mo, and Sn). The
T532 group has been developing a highresolution TOF
counter by detecting Cerenkov lights from a crystal radiator using MCP-PMT. For the measured number ofphoto-electrons of 150–200, the time resolution of σ t =10.6 ps was obtained.
Some of previous experiments at the PS presented
new results in FY2002. Experiment E246, which issearching for T-violating muon polarization (PT) in K +→ π 0µ +ν (Kµ 3) decays, has been analyzing the datafrom its 1999 and 2000 runs. The analysis has beenFig. 4-2-4 The upper picture shows the test setup of anaerogel RICH detector. This is a detector
study for the upgrade of the KEKB now underdiscussion. In the picture at lower right, 11
Cerenkov lights (black squares) observed fora particle (the red square at the center)
nicely form the expected ring image (redcircle).

Fig. 4-2-3 R&D prototype of the electromagnetic
calorimeter made of scintillator strips forexperiments at the future worldwide linear
colliders.

25Institute of Particle and Nuclear Studies (IPNS)
preliminary results for the data taken in 2002. The e+e−
invariant mass spectra after the subtraction of combinatorial backgrounds, which show clear peaks of ω and φmesons, are well reproduced by vector meson decays.
However, significant excess is found in the low-massside of the ω peak and some hint of possible excess
below the φ peak may be also seen.
Hybrid emulsion experiment E373 has been carriedout to search for the hadronic states with double
strangeness: double-Λ hypernuclei and the H di-baryon.
About 60% of the data have been analyzed and a third
candidate event was found. One possible explanation
for the event topology is the sequential decay of thedouble-Λ hypernuclei. For all data samples, more than
10 3 Ξ − stopping events should be found. To study
hyperon-nucleon spin-orbit interactions, the E452 groupmeasured the asymmetries of Λ and Σ+ elastic scattering
off protons. Λ and Σ+ particles were produced through( π + , K + ) interactions in an active liquid-scintillator
target. To define the polarization axis, the Λ polarization and the left-right asymmetry of the Λp scattering
were defined in the neutron rest frame of the Λ production taking into account the Fermi motion of neutrons incarbon nuclei of the target. The same definition will beapplied to the Σ+ case so as to increase statistics byadding Σ + hyperon events by Σ + production on thebound protons in the carbon nuclei to those from freeprotons.
Experiment E462 is studing baryon-baryon weakinteraction in nuclei by measuring the ratio Γn/Γp = Γ(Λn → nn)/ Γ (Λp → np) in non-mesonic weak decay(NMWD) of Λ5 He, where two-nucleon pairs, np and nn,
with back-to-back angular correlation were selected.
The ratio found is 0.42 ± 0.10, which is close to a
recent theoretical calculation that takes into accountheavier mesons. Although the result is still preliminary, the experiment has established the picture of thedominant Λp → np decay process with a significant
contribution from the Λn → nn process in baryonbaryon weak interaction in nuclei.

Fig. 4-2-5 The e+e− invariant mass spectra for the 2002
data of experiment E325A. Combinatorial
backgrounds have been subtracted. Clear
peaks of ω and φ mesons are seen together
with significant excess in the low-energy sideof the ω peak.

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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 collaborations
started 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 thosePMTs 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 as
K2K-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-IIexperiment 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
20
10

Accumulated POTProtons/Pulse (1012)

0-10
6543210-1
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 bythe star. The measured oscillation parameters are
consistent 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

10
8-2

10

64-3

10

200

1

2

34

5Eνrec-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.

00.2

0.4

0.6

0.8
12
sin 2θ

Fig. 4-3-4 Allowed regions of the oscillation parameters.

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