Institute of Particle and Nuclear Studies (IPNS)

4.2 Experiments at the 12 GeV Proton Synchrotron
experiment has also measured the form factors of K + →π 0e+ν decay. The result shows no evidence for tensor
or scalar couplings in contradiction to current world
averages.
The partial restoration of chiral symmetry in dense
matter predicts the in-medium modification of vector
mesons. At the EPA-B beam line of the KEK PS,
Experiment E325 aims to detect the effect by measuring the invariant masses of the vector mesons producedin 12 GeV proton-nucleus interactions. Both the e+ e−
decays and the K+ K− decays of the vector mesons aremeasured using the same apparatus. The e+ e− mass
spectrum taken in 1998 with a Cu target shows abroader ω peak as is seen in Fig. 4-2-1. Although thetheoretical interpretation of the data is not yet conclusive, it is consistent with the prediction. This was thefirst observation of leptonic decay of vector mesons innormal nuclear matter. Data from the 1999 and 2000
runs are now being analyzed and the next data taking isscheduled in 2001.
The study of the double strangeness system isimportant for the unified description of baryon-baryon
interactions in SU(3) f symmetry. The E373 Hybrid

In FY2000, the KEK 12 GeV Proton Synchrotron(KEK PS) was in operation altogether for 7.5 months:
3.5 months in the slow beam-extraction mode and 4
months in the fast beam-extraction mode, providing aneutrino beam for the K2K (E362) long-baseline neutrino oscillation experiment. During the periods inslow beam-extraction mode, seven experiments wereperformed. Data taking is now complete for four ofthem: E246, E373, E443 and E462.
At the K5 beam line of the KEK PS, Experiment
E246 searched for a time-reversal violation (T-violation) effect in transverse muon polarization of thecharged kaon decay K + → π 0µ+ν. The experiment useda superconducting toroidal spectrometer. Final datataking for the experiment was completed in FY2000,increasing the total statistics by 25%. Analysis of thedata taken in 1998 was continued while adopting two
independent methods. New results obtained for combined 1996-1997 and 1998 data are again consistent
with no T-violation with a statistical accuracy of ∆Imξ
= 0.0175. The systematic error is estimated to be about
one-sixth of the statistical error. The 1999-2000 data is
now being analyzed. As a physics byproduct, the[events / 50MeV/c 2 ]100806030a)from lighttargets

from heavy
target25
20

fitresultω, ρ, φ → e+e−
ω → π0 e+e−
combinatorial

fitresultω, ρ, φ → e+e−
ω → π0 e+e−
combinatorial

154010200b)

500 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2[GeV/c2]e+e− invariant mass

0 0.2 0.4 0.6 0.8

1 1.2 1.4 1.6 1.8 2[GeV/c2]e+e− invariant mass

Fig. 4-2-1 Invariant mass spectra of e+ e− pairs: a) for the light nuclear targets (C and CH2) and b) for the heavy nuclear target(Cu). The solid lines show the best fits obtained as a cocktail of known hadronic sources with a combinatorial
background. The dotted lines indicate the contributions from ρ, ω, and φ decays and the dashed lines the contributions from ω → π 0e+e− decays. The combinatorial background is given as the dot-dashed lines. A clear excess ofevents in the mass region below the ω meson peak is seen.

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Institute of Particle and Nuclear Studies (IPNS)
data, the group has found one event with twin single-Λ
hypernuclei and two events with a double-Λ hypernucleus. The twin single-Λ hypernuclei event is uniquely
identified as Ξ−+14N → Λ5He+Λ5He+4He+n. The firstdouble-Λ hypernucleus event is interpreted as Ξ−+12C→ ΛΛ10Be (or ΛΛ10Be*) + t, assuming that the two-bodyreaction occurs in the Ξ−+12C system. The second double-Λ hypernucleus event, shown in Fig. 4-2-2, has aclean topology where the hypernuclei and their decay
have been uniquely identified as Ξ−+12C → ΛΛ6He+4He+t ;−655
ΛΛHe → Λ He+π +p; ΛHe → p+d+2n. Preliminary analy6
sis of the ΛΛHe nuclei by the group has established theattractive Λ-Λ interaction with ∆BΛΛ ~ 1 MeV, wherethe error is estimated to be less than 0.5 MeV. This
∆BΛΛ value is significantly smaller than in previous estimates, and gives a new lower dibaryon mass limit of ~2224 MeV/c2 (90% confidence level).
The E391 searches for KL0 → π 0ν ν rare decay. Oneof the key issues in the success of the experiment is theconstruction of a new pencil K 0 beam. The beam line
was completed in March 2000 at the EP2-C beam
channel of the KEK PS. It consists of six stages of collimators, a pair of bending magnets and a vacuum system, as shown in Fig. 4-2-3. In April and December
2000, beam surveys were performed to measure thebackground neutrons and photons (Fig. 4-2-4). The
beam collimation system was found to be very effective in providing a beam tail at a level of 10−4, the best
achieved collimation of GeV neutral beams so far. The
experiment plans to use a lead/scintillator sampling
detector as the barrel γ veto detector. This year, a 1 mlong prototype detector (Fig. 4-2-5) consisting of 86
lead/scintillator (1 mm/5 mm) layers was tested in Test
Experiment T466. The energy resolution measured forEmulsions Experiment searches for S=−2 nuclei bycollecting Ξ− - hyperon stopping events with a probability of ten times higher than that seen in the previous
experiment, E176. Having analyzed about 10% of theFig. 4-2-2 Emulsion image of the double-Λ hypernucleus6Λ Λ He event measured by the experimentE373. A Ξ− hyperon was captured by 12C inthe emulsion at Point A. Particle track #1,
produced together with the stable nuclei #3
and #5 in the capture process, decays into #2,#5 and #6 tracks at Point B. Particle #2 thendecays into two tracks #7 and #8 and one ormore neutral particles. Tracks #5 and #7 stopin the emulsion. Track #6 is identified as a πmeson by the topology of the track. Although
Track #7 escapes from the emulsion, its energy is measured in the scintillation fiber blockthat follows the emulsion. The ranges of thedouble-Λ hypernucleus (track #1) and the single-Λ hypernucleus (track #2) are 8.3 and 9.2µm, respectively.

Fig. 4-2-3 New K0 beam line for Experiment E391a.

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Institute of Particle and Nuclear Studies (IPNS)
electrons was σ/E = 5.3%/E(GeV) including the beam
momentum spread, and the photoelectron yield is Npe =5.7/MeV. The uniformity of the detector response andthe light attenuation in the Wave-Length-Shifting(WLS) fibers were also studied with hadron beams.
Extrapolating these results, the number of photoelectrons in the 5.5 m module designed for the E391 experiment is estimated to be N p e = 7−8/MeV. This
photoelectron yield is large enough to veto photon
backgrounds effectively. A prototype CsI calorimeterfor the experiment was also tested in Experiment T473.
Experiment E393 explored the thermodynamic
nature of highly excited nuclear matter produced inGeV-energy nuclear reactions. Looking for possible
density effects in the Target Multi-Fragmentation(TMF) reaction, the group is measuring the inclusive
energy spectra of fragments in 8 and 12 GeV protoninteractions with heavy nuclear targets at KEK PS andcomparing with them those acquired from 8 and 12
GeV light-ion beams. The ion beam data are taken at
the National Institute of Radiological Sciences (NIRS)
in Chiba. The fragments with masses of 3 ~ 30 produced by the 8 GeV 16 O and 20 N ion beams show
Maxwell-Boltzmann-type distributions, as in the caseof the TMF reactions with 8 GeV protons, but theircross sections are 4−5 times larger and their Coulomb
peaks are much broader.
The E438 group studies the Σ -nucleus opticalpotential by measuring the (π −, K +) -inclusive spectrawith different nuclear targets: C, Si, Ni, In, and Bi.
Figure 4-2-6 shows the spectra measured for the Si target, together with calculations in the DWIA frameworkwith three different Σ-nucleus potentials. A significant
number of events seen in the bound region (B∑ > 0)
imply that a finite strength of the Σ-nucleus potentialexists. Neither the "shallow" potential nor the "atomDD" potential reproduce this measurement. The latter
potential was derived from the analysis of Σ −-atom Xrays and has a strong repulsive core. The data, however, seems to indicate a much stronger repulsive core.
The present result provides important information forthe study of hyperon-nucleon interactions as well as forunderstanding the role of hyperons in dense mattersuch as neutron stars.
The E443 experiment measures the neutron spallation induced by protons in order to improve the calculation codes based on the intra-nuclear-cascade
evaporation model. These codes, such as NMTC andHETC, are often used in designing spallation neutron
sources and accelerator-driven subcritical reactors fornuclear waste transmutation and energy production.
The experiment was carried out at the π2 beam line at
the KEK-PS. It measured neutrons produced at 0˚ and5˚ in 0.8 and 1.5 GeV proton interactions with Fe andPb targets. Neutrons were detected in two detectors
filled with an NE213 liquid organic scintillator. Theirenergy was measured by the time-of-flight technique.
In this fiscal year, the hyperon-nucleon scattering
Experiment E452 has started at the K2 beam line. The
experiment is the followup to the previous two experiments (E251, E289). The purpose of the present experiment is to study the spin-dependent hyperon-nucleon

Fig. 4-2-4 Profiles of neutrons and photons measured at
the new K0 beam line.

Fig. 4-2-5 Side view of a 1 m-long prototype of the barrel
γ veto detector for Experiment E391a.

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Institute of Particle and Nuclear Studies (IPNS)

Fig. 4-2-6 (π−, K−) inclusive spectra measured for an Si target in Experiment E438. Shown together are predictions in theDWIA framework with three different ∑-nucleus potentials (left). The real (upper) and imaginary (lower) parts ofthese potentials are also shown (right).

Charged Particle
IITLens

CCD

Scintillator
Fig. 4-2-7 New SCITIC (SCIntillating Track Image Camera) track detector developed by the E452 group.

were selected for large Σ + polarization. Approximately
60,000 pictures were taken during the test runs, ofwhich about 500 pictures are expected to record Σ +hyperon scatterings. Data analysis is currently under
way with a new computer-assisted scanning algorithm.
The next data taking is scheduled for fall 2001 with a
new lens system for SCITIC.
The E462 group studies the Non-Mesonic Weak
Decays (NMWD) of Λ hypernuclei to gain an understanding of the weak decay process in nuclear matter.
One of the immediate goals of the experiment is todetermine the ratio of the two decay widths Γ(Λp →interaction; in particular, to determine the spin-orbit
coupling of the Σ +p interaction by measuring the leftright asymmetry of the polarized-Σ + scatterings. Thepolarized hyperons were produced via (π+, K+) reactions in a liquid scintillator active target. The group
has developed a new-type of tracking detector, theSCIntillating Track Image Camera (SCITIC) to record
pictorial data of the hyperon scatterings with a fast trigger signal (Fig. 4-2-7). Test data were taken to checkthe performance of SCITIC together with a fast pictorial-data acquisition system. Kaons in the production
angles from 200 to 400 in 1.6 GeV/c pion interactions

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Institute of Particle and Nuclear Studies (IPNS)

Energy / eff(E) / Ω / incnp)/(Γ(Λ n → nn) by measuring the nucleon pairs, bothn + p or n + n, from Λ5He, the lightest "spin-isospin saturated" Λ hypernucleus. The large ratios reported byrecent experiments have been one of the puzzles in theNMWD study. If the 5He NWMD is dominated by thetwo-body decay process Λ + N → N + N, which is possible only in nuclear matter, the two decay nucleons
have a strong back-to-back correlation and the sum of
their energies peaks at a Q-value of approximately 150
MeV. By detecting such nucleon pairs, it should bepossible to determine precisely the ratio of the decaywidths. So far this year, about 20,000 5He events havebeen accumulated. The measured spectrum of thedecay neutrons, although preliminary, does not show
any peaks around 76 MeV, half of the Q-value, as seenin Fig. 4-2-8. This result suggests a significant contribution from the two-nucleon induced decay Λ + N + N→ N + N + N. The experiment will be continued nextyear to improve the statistics by a factor three or more.
In FY2000, twenty test experiments were carriedout for detector calibrations and detector R&D. The
purpose of Experiment T467 was to check the performance of the thin gap chambers for the ATLAS endcap muon trigger system. Mass production of thechambers is now starting at KEK. For the design of thecentral tracking detector at the Japan Linear Collider(JLC), the T469 experiment performed dE/dx measure-ments with a gas mixture of CO 2 (90%)/isobutane(10%). The measurements were made using a small jetchamber with 10 sampling wires. The T472 groupbrought a new position sensor, a β -selection typeCerenkov counter (POSCH) for η - and ω -mesoic
nuclear search (S214) at GSI. The background rejection by POSCH was confirmed to be better than 10−3
over the wide incident angles of ± 60 mrad, with 3He
detection efficiencies of more than 98%. The position
resolution for 3He was estimated to be σ ~ 2 mm. TheT475 group, which proposes an experiment to study theGerasimov & Drell-Hearn (GDH) sum rule at Spring-8,
tested a prototype gas Cerenkov counter with atmospheric CO2 gas (n = 1.00041). The T476 experiment
was to study basic performance of a fine-segmented
liquid scintillator detector with wave-length-shifting(WLS) fiber readout (Fig. 4-2-9). The detector is oneof the candidates for neutrino detection in the upgraded
K2K experiment. Experiments T465 and T480 focus
on the study of the pionic hydrogen atoms by measuring the pion capture process in the liquid and gas
phases. The chemists doing the experiment are now
designing an experiment to measure the Auger electrons emitted during the initial step of pion capture andthe electronic X-rays (eX-rays) in coincidence with thepionic X-rays (πX-rays).

0.25

0.2

Efficiency corrected
0.15

0.1

0.05

00204060

80100

Single neutron enegy spectrum [MeV]Fig. 4-2-8 Energy spectrum of neutrons from Λ5He decays.
No peak at Q-value/2 ~ 76 MeV is seen, suggesting a significant contribution from the twonucleon induced non-mesonic decays Λ + N +N → N + N + N.

Fig. 4-2-9 Schematic view of the fine-segmented liquid
scintillator detector with wave-length-shifting(WLS) fiber readout; one of the candidates forthe neutrino detector in upgrading the neardetector system of the K2K experiment.

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Institute of Particle and Nuclear Studies (IPNS)

4.3 K2K Experimentfirst neutrino event originating from the KEK neutrino
beam. In fiscal year 1999, the experiment observed 17
neutrino events in its 22,500-ton volume, while approximately 29 events were expected to occur when theneutrino does not oscillate. Considering both the statistical and systematic errors in the measurements, it wasthen concluded that the null oscillation result was disfavored at the 95% confidence level.
The experiment continued to run during the year
2000 and had observed 28 events by the end of June
2000. All the events coincided with the expectedarrival time of the neutrino beam as calculated from thebeam spill time (1.1 µsec) and the time of light over
250 km, thus are safely identified as neutrino events
originated by the KEK neutrino beam. When no oscillation occurs, the expected number of events is 37.8 +3.5
−3.8 ,
which is derived from the number of neutrino interac-

The K2K (KEK-to-Kamioka) experiment is thefirst long-baseline neutrino oscillation experiment to beperformed by an international collaboration between
physicists from Japan, Korea and the USA. An almostpure muon neutrino beam (98.2% ν µ , 1.3% ν e and0.5%νµ ) is produced at the KEK 12 GeV PS and sent
over 250 km to the Super-Kamiokande (SK) detector at
Kamioka, Gifu prefecture, operated by the Universityof Tokyo. The SK 50,000-ton water Cerenkov detector
observes the neutrino interactions generated by theKEK neutrino beam. The experiment focuses on theexistence of neutrino oscillation in the νµ disappearance
mode, which was first reported by the SK collaboration
after a careful study of the deficit in the atmosphericmuon neutrinos. The experiment also searches νµ → νeoscillations in the ∆m2 ≥ 2 × 10−3eV 2 region.
On June 19, 1999, the SK detector recorded theFig. 4-3-1 Horizontal and vertical profiles of the νµ beam measured by the muon range detector in November 1999. The
curves are predictions computed from simulations. The arrow shows the direction of Super-Kamiokande.

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Institute of Particle and Nuclear Studies (IPNS)
tions measured in the 1000-ton water Cerenkov detector of the near detector system. The near detector system is located 300 m downstream of the production
target. The background from atmospheric neutrinos isestimated to be around 10−3 events for the current data
sample.
The characteristics and stability of a neutrino beam
are vital for the K2K experiment. In particular, theneutrino beam must be pointed precisely at the SKdetector. The profile of the neutrino beam is monitored
by a muon range detector at the near-detector systemmeasuring the muon neutrino interactions with iron.
Figure 4-3-1 shows the measured profile together withthe simulation result. As seen in the Figure, the beam

is exactly centered on the SK detector. In Fig. 4-3-2,
the center of the beam profile is plotted as a function oftime. It can be seen that the neutrino direction was stable within ± 1 mrad throughout this running period.
Because the calculated neutrino spectrum at 250 km isalmost constant over nearly 1 km, the required precision of the beam pointing is 3 mrad. The measured
pointing accuracy, therefore, assures the stable deliveryof the KEK neutrino beam to the SK detector.
From January to March, 2001, more data weretaken without any serious problems. The experimentwill continue to run until July, 2001. It is expected thatthe statistical measurements will be doubled by thistime.

Fig. 4-3-2 The pointing stability of the KEK neutrino beam. The fitted centers of the horizontal and the vertical profile are plotted over five-day periods. The dashed lines show directions ± 1 mrad to the Super-Kamiokande detector.

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