Part II
Brief summary of each experimentNeutrino Physics
E362
Long Baseline Neutrino Oscillation Experiment1

Status and results of E362 (K2K)
K2K collaborationaaHigh Energy Accelerator Research Organization(KEK), ICRR U. of Tokyo, Kobe U., Kyoto U.,
Niigata U., Okayama U., Tokyo U. of Science, Tohoku U., Chonnam National U., Dongshin U., Seoul
National U., Boston U., U. of California (Irvine), U. of Hawaii (Manoa), State U. of New York at Stony
Brook, U. of Washington at Seattle, Warsaw U., Solton Institute,
New members since 2002
Hiroshima U., Osaka U., TRIUMF, U. of British Columbia, U. of Barcelona, U. of Geneva, U. of Rome,
Saclay, U. of Valencia, Institute of Nuclear Study-Moscow

1. Status of neutrino oscillation studies
E362 (K2K) was proposed in 1995 to establish
neutrino oscillation which was indicated in atmospheric neutrino observation in old Kamiokande
experiment. Since then, rapid progresses of neutrino physics have been made.
The large mixing angle solution (LMA) has
been shown to be the right solution of solar neutrino deﬁciency by Super Kamiokande (SK) and
SNO experiment. Also it has been shown thatνµ and ντ components exist in solar neutrinos, aclear evidence of neutrino oscillation. Also Kamland results on reactor anti-neutrinos show the∆m2 and mixing angle are consistent with solar
neutrino observations with a hint of spectrum distortion.
As for the atmospheric neutrino deﬁciency, adeﬁnite evidence that the deﬁcit is in deed due toνµ → ντ oscillation is not ﬁrmly established yet.
2. Aim of the K2K experiment
It is still important to show deﬁnite evidenceof neutrino oscillation at about ∆m2 ∼ 10−3 eV 2 .
The advantages of accelerator neutrino experiments, compared to cosmic ray experiments canbe summarized as follows.
(1)Neutrino beam can be measured just after
its production to be compared with beam at far
detector(2)Since the distance is ﬁxed, the eﬀects of oscillation is a well deﬁned function of neutrino en-ergy.
(3)The recent progress in neutrino experiments
has made it clear that further precision measurement of neutrino oscillations will be one of themost important and fruitful experimental challenge for a decades or two. The precision requiredin those measurements need well controlled environment which most likely need accelerator measurements. It is important to gain experiences
and basic data, such as knowledge of neutrino interactions at low energy, for future high precisionexperiments.
We report present status, results obtained sofar, and future prospects.

3. Status of K2K
K2K was approved as a KEK PS experimentin 1996 and ﬁnish construction 1999. Data taking started in November 1999. K2K has accumulated data corresponding to 4.8 × 1019 protons on
target (POT) by July 2001. We call this period
K2K-1. After a long shutdown due to the accident of SK in November 2001, the conﬁgurationof PMTs in SK has been changed. K2K started
again in December 2002 and accumulated data
till February 2004. We call this period K2K-2.
In K2K-2, although analysis is still in progress,
we have accumulated about 4 × 1019 POT. The
analysis during this period shows no degrading of
the SK performance for analyzing neutrino eventswith relevant energy region.

2
events

FC
10

3

10

2

10
1
-500

-250

0

250

500
∆(T) µs

events20
15
10
5

Figure 1. A event display of QE event candidate
in SciBar detector.

In summer 2003, Lead Glass counters was replaced by a new ﬁne grain detector (SciBar). The
SciBar detector is a ’fully active’ tracking detector with ﬁne segmentation consisting of plastic
scintillator bars. Figure 1 show a candidate ofcharged current quasi elastic (QE) event in the
SciBar detector. Clear signature of stopping proton and muon can be seen. An improvementof the measurement of neutrino energy spectrum
and of knowledge of neutrino interactions are expected.

0
-5

0

5
∆(T) µs

Figure 2. The time diﬀerence between the beam
extraction time and the observed time in SuperKamiokande. The time of ﬂight between Tsukuba
and Kamioka is already subtracted. In the upper
ﬁgure, dark hatched histogram is for FC eventsfor ± 500 µsec is shown. The lower ﬁgure is for± 5 µsec. One background event is observed in
the ±500µ second interval. This one event is consistent with an atmospheric neutrino event. The
width of the time distribution is consistent withthe accelerator spill duration of 1.2 µsec.

4. The method of long baseline experimentK2K has been successful in controlling neutrino
beam. Neutrino beam is monitored its ﬂux anddirection by two methods. One is pulse-by-pulse
measurements of ﬂux and proﬁle of high energymuons which accompany neutrinos in π → µ + ν
decay. This monitor has been indispensable also
for tuning the primary proton beam.
The other is measuring neutrino interactions
in near detectors (ND), which is located 300 m
from the proton target. Measurements made at
the ND are used to verify the stability and the
direction of the beam, and to determine the ﬂux
normalization and the energy spectrum before theneutrinos travel the 250 km to SK.
The results show that both high energy and low
energy parts of the secondary (mainly pions) canbe directed to SK with a precision better than
1 mrad. for entire running period, by adjusting

center of the high energy muon proﬁle. The widthof the neutrino beam proﬁle is well reproduced by
the Monte-Carlo calculation.
Also spectrum and angular distribution ofmuons produced by neutrino interactions in neardetectors show no change during entire running
period of K2K-1 and K2K-2, indicating the stability of neutrino beam.
K2K has also shown that the neutrino events
associated with accelerator beam can be identiﬁed by GPS timing with negligible background in
SK, once events are selected by requiring no entering particle from out-side (fully contained events,
called FC events) and about 30MeV electronequivalent energy deposit in the inner part of SK
as shown in Figure 2. The ﬂux at SK is estimatedfrom the measured one at ND by multiplying the

3

5. Physics results in K2K-1
5.1. νµ disappearance
We observed 56 FC events at SK in K2K-1.
The expected number of FC events at SK without oscillation is estimated to be 80.1 +6.2
−5.4 by extrapolating the observed number of events in the
one kilo-ton water Cherenkov detector (1KT) at
KEK. The correlations between energy bins fromthe spectrum measurement at the near detector
and the F/N ratio are taken into account in estimating the systematic errors. The major contributions to the errors on the expected number of
events come from the uncertainties of the F/N ratio (+4.9%−4.0% ) and the normalization (±5.0%). The
latter errors are mainly due to the vertex reconstruction both in the 1KT and in SK.
The neutrino energy in SK is reconstructed using events which have only visible muon in theﬁnal state (1Rµ events). Assuming quasi-elastic(QE) interactions νµ + n → µ− + p, and neglecting Fermi momentum, energy of the neutrino canbe calculated. Inelastic interactions with additional pion(s) associated (non-QE interaction) are
possible background for the energy reconstruction when the pions are not detected due to, eg.,
Cherenkov threshold. We observed 29 1Rµ eventsin K2K-1. An oscillation scenario with νµ → ντis tested by the maximum-likelihood method assuming two ﬂavor oscillation. In the analysis,both the number of FC events and the energy
spectrum shape for 1Rµ events are used. Figure 3 shows a lego plot of ln-likelihood in ∆m2
and sin2 2θ parameter space. The observed Eνrec
distribution of the 1Rµ sample is shown in Fig 4
together with the expected distributions for thebest ﬁt oscillation parameters, and the expectation without oscillations. Fig. 5 is the log likelihoods as a function of ∆m2 at maximum mixing

22.5
20
17.5
15
12.5
10
7.5
5
2.5
0

-∆lnL

Far/Near (F/N) ratio, the ratio of ﬂuxes between
the far detector(SK) to that of the ND. The ratio is estimated by the beam Monte Carlo (MC)simulation which is conﬁrmed by measurements
in-situ by gas Cherenkov detector. The error ofthis measurements reﬂect into the error of F/Nratio, which is one of the large source of systematic errors in predicting neutrino beam at SK

0
s0.2
in 2
2θ0.4
0.6
0.8
1
-4

-3.5

-3

-2.5

-2

-1.5

-1

2
2 eV )(
log∆m

Figure 3. A lego plot of ln-likelihood in the oscillation parameter space. Likelihood has a sharp
peak in ∆m2 and sin2 2θ.

sin2 2θ = 1 for normalization and shape terms
separately. Both the suppression of number of
events and the distortion of the spectrum indicate the same ∆m2region.
Results can be summarized as follows.
(1)The no-oscillation probabilities are calculated to be 0.7% from the likelihood ratio between
the best ﬁt point to no-oscillation case.
(2)The spetrum distortion is indicated at 84%
level in K2K-1.
(3)The best ﬁt point in the physical regionof oscillation parameter space is found to be at(sin2 2θ, ∆m2)=(1.0, 2.8×10−3 eV2 ). This is consistent with atmospheric neutrino observation by
SK.
Figure 6 show the comparison of the allowed
parameter region with that obtained in SK results
on atmospheric neutrinos. For sin2 2θ = 1, ∆m2 is
1.5 and 3.9 × 10−3 eV2 at 90% C.L..
5.2. νe appearance limit
In K2K-1 data, νµ → νe appearance wassearched. K2K-1 observed one electron candidate event in SK, which is consistent with theEvents-ln(L)

4

12

8
Shape+Norm
Shape Only
Norm Only

7
6

10

5

8

4
6

3
4

2

2
0

1
0

1

2

3

4

5
Eνrec

Figure 4. The reconstructed Eν distribution for
1Rµ sample. Points with error bars are data.
Box histogram is expected spectrum without oscillations. The solid line is the best ﬁt spectrum.
These histograms are normalized by observed 29
events.. In addition, the dashed line shows the expectation with no oscillations normalized to the
expected 44 events.

expected background (mainly due to neutral current π 0 production) of 2.4 events in the case ofno oscillation. In the present analysis, we assume θµµ = π4 based on the nearly full mixing observed by atmospheric neutrino experiments, and∆m2µµ = ∆m2µe .
A probability density function (PDF) for Nexp
is constructed from the Poisson distribution convoluted with the systematic uncertainty. The
conﬁdence interval is calculated using the methodsuggested by Feldman and Cousin in 1998. In thecalculation, the best-ﬁt parameters are searched
for in the 2-dimensional parameter space withsin2 2θµe bounded in [0,1].
Figure 7 shows the limit on sin2 2θµe as a function of ∆m2µe . Neutrino oscillations from νµ to νe
are excluded at 90% C.L. for sin2 2θµe > 0.15 at∆m2µe = 2.8 × 10−3 eV 2 .
Assuming 3-ﬂavor neutrino oscillations and
CPT invariance, our appearance results can be

00

1

2

3

4
5
6
2
-3 2
∆m (10 eV )Figure 5. Log likelihood functions as a functionof ∆m2 at sin2 2θ = 1. Solid, dashed and dottedlines are − ln Ltotal , − ln Lshape and − ln Lnorm ,respectively.

compared to CHOOZ reactor experiments, although it is a disappearance experiment. The
CHOOZ limit on sin2 2θ13 is converted by assuming sin2 2θµe = 12 sin2 2θ13 (θ23 = π/4). The results corresponds to a limit of sin2 2θµe < 0.05 at∆m2µe ∼ 3 × 10−3 eV2 .
6. Summary and prospects
E362 (K2K) has observed indications of neutrino oscillation. Indications consists of the reduction of neutrino rate and the distortion of theenergy spectrum at SK. Analysis of data which
was accumulated from year 1999 to 2001 observed
56 events in SK where 80.1+6.2−5.4 events are expected. Out of 56 events, 29 single ring muon likeevents are used to obtain the energy spectrum at
SK.
The probability of no neutrino oscillation is lessthan 1% and the reconstructed neutrino energy
distribution matches with the one with oscillation.
For the ﬁrst time, electron appearance by νµ →
νe oscillation with ∆m2 ∼ 3×10−3eV 2 region was
searched. The result is consistent with negative22∆m (eV )

5

-2

10

-3

10

-4

10

68%
90%
99%

-1

10

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
2
sin 2θ
-2
K2K 99% C.L.

2∆m (eV )10
K2K 90% C.L.

2

Figure 6. The allowed regions of oscillation parameters obtained in this experiment are compared with atmospheric neutrino results. Threelines corresponds 99%, 90%, and 68% C.L.

-3

10

CHOOZ 90% C.L.

-4

results from reactor disappearance search.
After long shutdown, K2K restarted in December 2002 and has accumulated similar statistics as
K2K-1. New detector is installed in near site toimprove the sensitivity of the experiment and to
perform detailed studies of neutrino interaction
at about 1 GeV.
New results with all the data which accumulated will be reported in summer 2004.

10

0

0.2

0.4 2 0.6
sin 2θµe

0.8

1Figure 7. The conﬁdence interval for νµ → νeoscillations as a function of the eﬀective ∆m2µe
at 90% C.L. (solid line) and 99 % C.L. (dashedline). The area to the right of each curve is excluded. Dotted line shows the limit at 90% C.L.
by CHOOZ assuming sin2 2θµe = 12 sin2 2θ13 .

K Decay Experiments
E246/E470
Search for T violation in the K+ → π0µ+ν decay / Measurement of direct photon
emission in K+ → π+π0γ
E391a_

Measurement of the KL → !0νν Decay

E246 “Search for T violation in the K+ ➛π0µ+ν decay”
E470 “Measurement of direct photon emission in K+ ➛π+π0γ”
Experiment E246 has searched for a T-violating transverse muon polarization in the K+➛π0µ+ν decay. This was one of the PS experiments which ran for a long time over many
years. It was approved in 1991 and the detector [1] was constructed and tested
between 1992 and 1995. After the beam tuning of the K5 channel and the magnet field
mapping of the superconducting toroidal spectrometer in 1995, data acquisition was
performed from 1996 to 2000 using 650 shifts of PS beam time; 200 shifts of whichwere extended shifts in 1999 and 2000. Although the intensity of the slow extracted
proton beam was limited by the bad extraction efficiency and very bad spill structure at
EP1 which was never been improved until the last few shifts of data taking, we could
accumulate Kµ3 good events as was initially expected reaching the design sensitivity.
The data analysis was carefully continued after the run, and the final result has recently
been obtained.
We employed a two-analysis method, namely two completely independent analyses
were performed at KEK and INR in Russia. This method provided the means to do across-checking of results. Furthermore we combined two results by sorting all good
events into three categories:-- 1) common events, 2) KEK-uncommon events and 3)
INR-uncommon events. The null asymmetry A0, the sensitivity AN and othersystematics such as the decay plane angular distribution of each data set were carefully
checked and confirmed that all the data were of high quality. For the polarimeter
analysis we employed the “integral method” for the analysis of 1996-1997 using 1/4 ofdata [2]. However, because this method was found to be easily affected by potential
systematics, we changed to the “differential method” after spending considerable time
for the analysis formulation. All data sets were then subjected to this differential
analysis in order to obtain the final result. In this analysis, PT is extracted differentiallyalong the polarimeter axis y as shown in Fig.1 using a y-dependent analyzing powerfunction α(y) ~ AN(y) which is the asymmetry associated with the normal in-plane
polarization. The integration of PT(y) gives the result of
PT = -0.0017
0.0023 (stat)0.0011 (syst) [3],corresponding toImξ = -0.0053
0.0071 (stat)0.0036 (syst) [3]which shows no evidence for T violation within the experimental accuracy. This result
improved the previous limit from BNL-AGS by a factor 3 as is shown in Fig.2, and canconstrain model parameters of several non-standard CP violation models. In the actual
analysis all the data were divided into three different periods of the experiment, eachhaving similar experimental conditions and data size. The distribution of Imξ wasplotted as an ideogram to show the good stability of the results. (Since π0 in K+➛π0µ+ν was detected not only as two γ but also as one γ with high energy, data wereseparately accumulated for 2γ and 1γ events.)

In E246 several byproducts could be analyzed using special-trigger runs forcalibration and background studies. The transverse muon polarization in K+➛µ+νγ wasanalyzed and the world’s first data was published [4] . Further accumulation of statistics
gives PT(Kµνγ) = -0.0067 0.0143 (stat)0.00014 (syst) [5]. The following results
have also been published:1) Ke3: “Test of exotic tensor and scalar couplings in K+➛ π0e+ν decay” [6],2) Ke3/Kµ3: “Measurement of Γ(Kµ3)/Γ(Ke3) ratio using stopped positive kaons” [7], and
3) Kπ3: “Kinematically complete measurement of K+➛π+π0π0” [8].
Experiment E470 was performed in 2001 using the same setup as E246 with a slight
modification of the detector and trigger mode, subsequent to the E246 data taking. This
experiment aimed at the determination of the branching ratio for the direct photon
emission (DE) in K+➛ π+π0γ (Kπ2γ) in the dominant bremsstrahlung (IB). This process isa chiral-anomalous magnetic transition and its branching ratio is one of the importantdata for testing chiral perturbation theory (ChPT). We successfully lowered the Eγ
threshold to enhance the sensitivity to DE. Using 120 shifts of PS beam we observed a
total of 4400 good Kπ2γ events and could extract from the spectral analysis the DEbranching ratio of
BR(DE) = [ 6.1 ± 2.5 (stat) ± 1.3 (syst) ] x 10-6 [9]for the whole energy region. This results is less than all the previous in-flight decay
results and supports the one other stopped K+ result from BNL-AGS [10].
Also in E470 some other physics results could be obtained:
1) Ke4 : “ Measurement of K+➛π0π0e+ν (Ke400) decay using stopped positive kaons” [11]2) Ke3γ: “ Search for T violation”.
Now we are designing a next generation T-violation experiment at J-PARC aiming
for the sensitivity of PT~10-4 range. A letter of intent was submitted last year and a full
proposal will be worked out soon. It will explore PT in both Kµ3 and Kµνγ as its merit
was discussed in [12].

References[1] J. Macdonald et al., Nuclear Instruments and Method A506 (2003), 60.
[2] M. Abe et al., Phys. Rev. Letters 83 (1999), 4253.
[3] M. Abe et al., to be published[4] V.V. Anisimovsky et al., Phys. Letters B562 (2003), 166.
[5] V.V. Anisimovsky et al., to be published.
[6] S. Shimizu et al., Phys. Letters B495 (2000), 33.
[7] K. Horie et al., Phys. Letters B513 (2001), 311.
[8] Y.H. Shin et al., Euro Phys.J. C12 (2000), 627.

[9] M. Aliev et al., Phys. Letters B562 (2003), 166.
[10] S. Adlelr et al., Phys. Rev. Letters 85 (2000), 4856.
[11] S. Shimizu et al., to be published in Phys. Rev. D[12] M. Kobayashi, T.-T. Lin and Y. Okada, Prog. Theor. Physics 95 (1995), 361.

Fig. 1 PT distribution as a function of
the polarimeter axis y. (• are 2γ ando are 1γ events.)

Fig. 3

Ideogram of Imξ for the 18 data sets.
(• are data sets from 1996~ 1997,o are from 1998, and stars are from1999 ~ 2000.)

Fig. 2 History of PT and our E246 result.

Status of E391a
KL→π0ννdecay is one of the best processes to check critically the Standard Model (SM) and to lookfor Beyond-Standard-Model (BSM) effects. Through the measurement of this decay rate a fundamental
parameter of the flavor physics can be obtained with an ideally small theoretical ambiguity.

Once anexperiment finds a deviation from the SM prediction, it clearly indicates a discovery of a BSM effect.
Nevertheless, no dedicated experiment has been performed.

This is because KL→π0νν decay ishighly suppressed as a small branching ratio (O(-11)), and moreover, because measurement has been thought
to be difficult due to few kinematical constraints in the KL→π0νν decay.
E391a is the first experiment dedicated to the decay KL →π 0 νν.
experiment for a future experiment using a high intensity beam at JPARC.

It was planned as a pilot

In addition to that, it will uncovera new region of the branching ratio with an improved sensitivity by three orders of magnitude from the current
experimental limit. In that region several theories predict an appearance of the decay based on new physics ofBSM.
E391a was approved in 2001. The collaboration has been gradually expanded and now 60 people areworking from ten institutes of five countries.

Among collaborators there are several people who define

E391a as a pilot experiment for KOPIO at BNL and KLOD at IHEP.
Characteristic features of the E391a detection system are a well-collimated pencil beam and hermetic
calorimeters covering the fiducial decay region, which are operated in vacuum.
A new beam line K0 was constructed in Fy-2000 and its performance had been examined through threeruns of beam surveys for two years.

A very sharp collimation with a reduction of halo by more thanfour orders of magnitude was proved. The yields and spectra of neutrons and gammas were well
reproduced by a GEANT simulation within a factor of two.
Fig. 1
E391a Apparatus

Fig.1 shows an experimental setup. The 575 CsI calorimeters, which were installed in thedownstream end cap and measure the two gammas from a KL→π0νν decay, are recycles of theprevious E162 experiment and rentals from the KTeV group.

The voltage dividers for their readout PMTs

were revised to give low power consumption in order to fit an operation in vacuum.

For thesampling calorimeters surrounding the CsI crystals we developed an extruded MS-resin plastic
scintillator, several techniques for fabrication and a new type of PMT having high quantum efficiency.
All signals are processed by amp-discriminator (AD) circuits of 16-channel inputs near the detectors.
Three outputs are extracted from the AD circuit, through-signal for individual ADC, discriminatorsignal for individual TDC and summed signal of eight inputs. The ADC is sensitive as 50 fC / ch. All
signals were adjusted as 10 ADC-channel / MeV-deposit and the discriminator threshold can belowered to 0.5 mV, which enabled to get both values of ADC and TDC even for very low energy
deposit of 1 MeV.

The summed signal is used for trigger.

The vacuum vessel was designed as it can be separated into three sections.

The

downstream section was fabricated in Fy-2001 and an engineering run was performed in 2002 after
installing detectors. All CsI crystals and other detectors in the downstream were calibrated by using
cosmic ray μ, punch-through μ, single π0 produced off a thin aluminum target placed on the
beam axis and 3π0 from KL→π0π0π0 decay. The upstream and middle sections, in which two large
sampling calorimeters made of lead-scintillator sandwich, front and main barrels, were installed,were constructed in Fy-2002 and Fy-2003, respectively.

The fabrication, assembling andinstallation of the calorimeters are one of highlights of detector construction in Fy-2003. After
connecting the three section we started evacuation from 1st February 2004.
On 17 February 2004 we started a running for data taking on schedule.
the end of April reaches less that 1-1

The vacuum level at

10 Pa in the detector region and less than 2-510 Pa aroundthe beam region. During the data taking we monitored the line shape peaks of KL→π0π0 and KL→π0π0π0 decays as shown in Fig 2. Now, we have almost spent half of the allocated beam-time of
300 shifts. Judging from the yields of these peaks we will reach the sensitivity of O(-10) for the KL→π0νν decay.

Mass of π0π0π0 (GeV)
Fig. 2Mass of π0π0 (GeV)

Effective mass distributions ofπ0π0π0 and π0π0 samples.

Hadron Physics and Neutron Physics(Except Hypernuclear Physics)
E325
Study of Chial Property of Dense Nuclear Matter through Measurementof Meson Mass Modification in Medium
E443
Neutron-Production Differential Cross Sections in the (p,nx) Reaction at Forward
Angle

Experiment E325 at KEK 12-GeV PS
Study of Chiral Property of Dense Nuclear Matter throughMeasurements of Meson Mass Modification in Medium.
In the North Hall of KEK 12 GeV-PS, there is a primary beam line called EP1B. Experiment E325 has
been running there from1996 to 2002, spending about 3200 hours of beam time. This manuscript briefly
describes the physics of the experiment and the status of the analysis.
We know that 99% of hadron mass is originated from the spontaneous breaking of the chiral symmetry.
Such effective mass of hadron is determined in vacuum, and it is quite natural to expect that those masses
will be modified when their circumstance is changed. The major scientific interest of E325 is to investigate theproperty of hadron (and then of quark) in a dense matter, naturally existing as nucleus.

The basic

experimental concept is to measure vector mesons, which areproduced and decay in a nucleus, through the invariant mass
spectroscopy

in

the

electron-positronpairchannel.

Experimentally, to measure electrons, targets cannot be thick,but the branching ratios to the electron pair are as small as
91/1000. Consequently the experiment requires 10 proton/sec,bombarding the target as thin as 0.1% interaction length. The
spectrometer, having a large acceptance as can be seen in
the photo (Fig.1), was newly built at EP1B.
Fig1: Top view of the experimental setup
H2 target:
Decay invacuum.

Cu target:
Containing
in-medium
decays.

ω

ω

ρ

ρ
GeV

GeV

Fig.2: Theoretical prediction of mass modification of vector
mesons by Hatsuda et al. (left). Expected mass spectra of ρ andω mesons, for a proton target (above left) and a copper target(above right).
A theoretical guideline given by the QCD sum rule [1] is shownin Fig.2. Linear decrease of the meson masses as functions of density are predicted as a consequence ofpartial restoration of the chiral symmetry. Experimentally, those mesons are produced in a nucleus, and somefraction of them decay eventually in a nucleus and should show the in-medium property of the mesons asshown as the mass spectra in Fig.2.

From the earlier data we have reported the signature of in-medium modification of ρ and ω mesons [2]. This
was the first observation in the leptonic in-medium decay of vector meson at a normal nuclear matter density.
Here, we describe our preliminary results from the data taken in 2002. Figure 3 shows the electron pair
spectrum for carbon (left) and copper (right) targets. Free decay peaks of ω and φ are clearly observed. We
have reproduced the mass shape with combinatorial background and the known hadronic sources as shownin the figure. The combinatorial backCarbon

Copperground was evaluated by the eventmixing method. In the lower panels of
Fig.3, we show the spectra after

excessexcesssubtracting the combinatorial background. Significant excess can beseen on the low mass side of the ωpeak, as consistent to our previous
publication [2].

CarbonCopper
Fig.3: Observed invariant mass+
spectra of 2002 e e- data. The left
panels are for the carbon and theright panels are for the copper target.
The upper and lower panels are
before and after the combinatorial
background subtraction, respectively.
Note that these tails are far beyond
the typical background like a radiative
tail.

With these improved statistics, weare able to determine the free decay ρ/ω ratio from the high mass tails of the spectra, as well as theexcess/ω ratio from the lower mass tails. Surprisingly, the free decay ρ/ω ratios are statistically consistent withzero for the both targets. Because most of ρ will be decaying inside a nucleus due to their short lifetime, it isnatural to consider the excess is mainly dominated by the ρ meson modification. Figure 4 shows theexcess/ω ratio as functions of βγ of the parents. It can be seen that the excess/ω ratio is smaller for the
carbon target, and the ratio decrease as βγ increase.
The analysis is still in progress to provide the momentum dependence of the excess, i.e., a dispersion
relation of the meson mass, and also on another key measurement, the spectral-shape modification for φmeson. The physics behind those phenomena will be obtained when we finalize the analysis of all the data
we have.

Excess/ω ratio(Preliminary)Fig.4 (Right): Excess/ω ratio as functions of βγ of the parents. Notethat the observed ratio for the copper target at the lower βγ isalmost one, which is close to the known ρ/ω ratio, unity, at thisenergy region in p+p interactions.

Reference:[1] T. Hatsuda and S. H. Lee, Phys. Rev. C46 (1992) R34.
[2] K. Ozawa et al. Phys. Rev. Lett. Vol.86 (2001) p5019.

βγ

Experiment 443
Neutron-Production Differential Cross Sections in the (p, xn) Reaction at
Forward Angle(Kyushu-JAERI-KEK)
In recent years, studies on spallation reaction have been made for various applications,such as spallation neutron sources and accelerator-driven subcritical reactors for nuclear wastetransmutation and energy production. For the lack of experimental data, Calculation codes
based on the intranuclear-cascade-evaporation model such as NMTC and HETC are often
used in the design of facilities. However, calculation codes are not capable of describing theparticle production in the most-forward directions satisfactorily. Then it is important tomeasure the cross section and obtain information for improvement of calculation codes.
The neutron-production differential cross sections at forward angle were measured for a
spallation reaction induced by 0.8 and 1.5 GeV protons on Fe and Pb targets in E443
experiment. The experiment was carried out at the π2 beam line of the KEK-PS. The
experimental arrangement is shown in Fig. 1. Time-of-flight method was adopted because it
has the higher detection efficiency and simpler data analysis than the recoil proton method.
Neutrons were measured by time-of-flight technique with two different flight path lengths, i.e.
3.5 and 5.0 m at 0.8 and 1.5 GeV, respectively. NE213 liquid organic scintillators 12.7 cm indiameter and 12.7 cm in thick were set at 0˚ as an neutron detector. It is important to improvethe energy resolution because of shorter flight path. For the improvement, the scintillator at 0˚
was connected with three Hamamatsu H2431 photomultipliers 5.1 cm in diameter. The
photomultiplier has a faster rise time. The incident beam was bent at the downstream of thetarget by an electric magnet in order to prevent irradiation of the 0˚ detector.
In order to obtain the detection efficiency for high energy neutron, the SCINFUL code
were modified, because the upper limit of application of the SCINFUL is 80 MeV. The QMD
calculation was adopted to obtain the reaction cross sections above 150 MeV. The modified
SCINFUL was called SCINFUL-QMD code. The detection efficiency of NE213 was obtainedby the code. Figures 2 and 3 show neutron production doble-differential cross sections of aniron and a lead targets. Experimental data were compared with calcluation results by the
QMD and the NMTC/JAM codes in these figures.

Fig. 1 Experimental setup

Fig. 2 Neutron-production double-differential crosssections of iron for 0.8 GeV protons Figure (a) shows
original values (solid lines) and broadened re-sults(dashed lines) adjusted to fit the present experimental
energyresolution.
Comparison
betweenourexperimental data and the broadened results is shown in(b). Open circles indicate the possibility of thecontaining effect of scattering at bending magnet
polepieces.

Fig. 3 Same as for Fig. 2, except for use of a lead target

Hypernuclear Physics
E373
Study of Double Strange Nuclei by Emulsion-Scintllating fiber Hybrid Methods
E419
Measurement of B(E2) Transition Rate in 7ΛLi Hypernucleus
E438
Study of Σ-Nucleus Potential by the (π-, K+) Reaction on Heavy Nuclei
E452
Spin-dependent Interactions in Σ+-p Scattering
E462/E508
Exclusive Measurement of the Non-Mesonic Weak Decay of 5ΛHe / Coincidence
Measurement of the Weak Decay of 12ΛC
E471
Search for Strongly Bound Kaonic Nuclear State
E509
γ-ray Spectroscopy of Hyperfragmentas using stopped K- method
E518
High-Precision γ Spectroscopy of 11ΛB
E521
Production of Neutron-rich Λ Hypernuclei by the the (π-, K+) Double-charge
Exchange Reaction
E522
Search for H-dibaryon Resonance via 12C(K-, K+Λ Λ) and Study of Ξ-N Interactions

Study of Doubly Strange Nuclei by Emulsion-Scintillating fiber
Hybrid method (E373)
Aichi, BNL, Chonnam, CMU, Gifu, Gyeongsang, Higashi-Nippon, KEK, Kobe, Konkuk,
Korea, Kyoto, Kyoto-Sangyo, Nagoya, New Mexico, NIRS, Osaka-City.U, Osaka-EC,
Osaka-Pref., Toho, Tokyo, Tsuru, UCL, Wonkwang.
The study of hyperon-hyperon interaction will provide us of an opportunity to verify various ideas on baryon-baryon interaction such as the SU(3)flavor symmetry. The information
about Λ-Λ force is also useful to investigate multi-strangeness system such as ”strange matter”. Experimentally, the interaction energies, ∆BΛΛ , between two Λ hyperons are obtainedby the mass measurement of double-Λ hypernuclei. The double-Λ hypernucleus is strongly
related to the existence of the H-dibaryon predicted by R.L.Jaﬀe [1]. The study of hyperonhyperon interaction will also provide us the solution for the problem of non-existence of the
H particle [2]. However, the experimental information has been quite limited.
Therefore, we have carried out the E373 experiment, where we would expect ten times
higher statistics of Ξ− hyperon capture at rest in nuclear emulsion than that of the pastexperiments. The K− beam with the momentum of 1.66 GeV/c was exposed to the target
diamond in the size of 2×2×3(beam direction) cm3 . Total number of exposed K− mesons
was 1×1010 with the K− /beam ratio ∼ 1/4, typically, at K2 beam line. To tag the quasi-free(K− , K+ ) reaction, the KURAMA spectrometer system was used for the detection of the highmomentum K+ mesons. We set the Scintillating-ﬁber bundle tracker [3, 4] to reconstruct
the tracks of Ξ− hyperons. Some of Ξ− hyperons were injected into nuclear emulsion andcaptured at rest by emulsion nucleus. We have followed Ξ− hyperon candidate tracks in 90%
of all ones and succeeded to locate ∼103 Ξ− stopping events, so far.
Among them, three events of the sequential weak decay of double-Λ hypernucleus andtwo events of twin single-Λ hypernuclei [5] have been detected.
The ﬁrst event of the double-Λ hypernucleus by E373 is so-called ”Demachi-Yanagi”
event [6], where the photograph and schematic drawing are shown in Fig.1. At point A,a Ξ− hyperon was captured by a light emulsion nucleus and a double-Λ hypernucleus wasproduced. Its double-Λ hypernucleus decayed to a single-Λ hypernucleus with some neutral
particle(s) at point B, and the single-Λ decayed to three charged particles at point C. By the1010
kinematical ﬁtting, there remained two possibilities such as Ξ− + 12 C → ΛΛBe + t, or ΛΛBe∗+ t, in the case of the emission of two charged particles at point A. For the ground state case,∆BΛΛ and BΛΛ (binding energy of two Λ hyperoms) were obtained as -1.14±0.19 MeV and
15.29±0.17 MeV, respectively. In the second interpretation with the excited state, the values
were +1.86±0.19 MeV and 12.29±0.17 MeV for the ∆BΛΛ and the BΛΛ , respectively. For
the case of three body fragmentation, such as the association with another neutral particle,
so many possibilities could not be excluded. If we would take into account the lowest ∆BΛΛ−
14
13
of +1.47+2.4
N → ΛΛ
B + p + n.
−0.7 MeV, the interpretation was Ξ +The photograph of the second event of NAGARA [7] is shown in Fig.2 with a schematic
drawing. A Ξ− hyperon was captured at point A and three charged particles were emitted.
Among them, the track #1 decayed at point B and its daughter (#2) also decayed at point
B. Since each three charged tracks at point A and B were coplanar within angle errors, it
was natural that no neutral particles were emitted from point A and B. The energies forcharged particles, except for pions, were calculated by SRIM2000 [8] after the energy-range
calibration with use of ranges of α particles from the decay of 222 Po and 228 Th. For pions,
its energy was calibrated by the range of µ+ mesons originated by the decay at rest of π +mesons.

6
6
By the kinematical analysis, only the mode of; Ξ− + 12 C → ΛΛHe + 4 He + t, and ΛΛHe →5−
Λ He + p + π , satisﬁes the condition of the production and decay of a double-Λ hypernucleus
without any ambiguities by the excited state for double and single-Λ hypernucleus.
The absorption of Ξ− hyperons would likely be occurred in atomic 3D state by a theoretical calculation [9]. Its BΞ− is determined almost by the Coulomb interaction. We estimatedthe value of 0.13+0.21−0.13 MeV as the most probable energy corresponding to the experimental
6
He nucleus were obtained by the
result of BΞ− hyperon. The BΛΛ and the ∆BΛΛ for ΛΛkinematical ﬁtting, as follows;BΛΛ = 7.25 ± 0.19 +0.18
MeV, and ∆BΛΛ = 1.01 ± 0.19 +0.18
MeV.
−0.11
−0.11
It will be very interesting to compare all of the past data to the ΛΛ interaction energy
given by NAGARA event. In Fig.3, the values of the ∆BΛΛ are plotted. If the excitedstates are possible for the interpretation, such cases are also plotted. The most probable∆BΛΛ will be easily understood to be 0 ∼ 2 MeV. This means that the ΛΛ interaction is
weakly attractive. The result of the NAGARA event has given great impact to theoretical
calculations [10, 11]. Taking into account the result by NAGARA event, the lower limit of
the mass of the H-dibaryon was obtained 2223.7 MeV/c2 at the 90% conﬁdence level.

#5
#6
B #1
#7

A

#3
#2

#4
Ξ-

C
#8

Figure 1: Photograph and schematic drawing of adouble-Λ hypernucleus; ”Demachi-Yanagi” event.

Figure 2: Photograph and schematic
drawing of NAGARA event. See text
for detail explanation.

Figure 3: Comparison with the past results of the ∆BΛΛ measurement.

References[1] R.L. Jaﬀe, Phys. Rev. Lett. 38, (1977) 195.
[2] B. Bassalleck, in: Proceedings of the 6th Conference on the Intersections of Particle andNuclear Physics, Big Sky, Montana, May/June (1997).
[3] A. Ichikawa et al., Nucl. Instrum. Meth. A417 (1998) 220.
[4] H. Takahashi et al., Nucl. Instrum. Meth. A483 (2002) 689.
[5] A. Ichikawa et al., Phys. Lett. B500 (2001) 37.
[6] A. Ichikawa, Ph.D. thesis, Kyoto University, 2001 (unpublished); J.K. Ahn et al.,Hadrons and Nuclei : F irst International Symposium, edited by Il-T. Cheon etal., AIP CP594 (2001) 180.
[7] H. Takahashi, Ph.D. thesis, Kyoto University, 2003 (unpublished); H. Takahashi et al.,Phys. Rev. Lett 87, (2001) 212502.
[8] J.F. Ziegler and J.P. Biersack, IBM Co. Ltd., http : //www.srim.org/.
[9] C.J. Batty et al, Phys. Rev. C59 (1999) 295.
[10] E. Hiyama et al., Phys. Rev. C66 (2002) 024007.
[11] K.S. Myint et al., Eur. Phys. J. A16, (2003) 21.

E419: High-Precision γ Spectroscopy of 7ΛLi
D. Abe, K. Araki, T. Endo, Y. Fujii, O. Hashimoto, T. Miyoshi, J. Sasao, K. Ozawa,T. Saito, Y. Sato, S. Satoh, T. Takahashi, H. Tamura (Tohoku Univ.);
T. Fukuda, Y. Kakiguchi, T. Nagae, H. Noumi, H. Outa, M. Sekimoto (KEK);
H. Akikawa, K. Imai, T. Murakami (Kyoto Univ.);
H.H. Xia, S.H. Zhou, L.H. Zhu (China Inst. Atomic Energy);
H. Bhang, J.H. Kim, Y.D. Kim (Seoul National Univ.); H. Hotchi, K. Tanida (Univ. Tokyo);
R.I. Sawafta (North Carolina A&T State Univ.); L. Tang (Hampton Univ.)
High-precision γ spectroscopy with germanium
detectors is one of the most powerful methods tostudy nuclear structure, but it was not able to be
applied to hypernuclei because of sever backgroundof high-energy particles from scattered beam andbeam halo. In 1998, we solved this problem andconstructed Hyperball (Fig. 1), a large-acceptancegermanium (Ge) detector array dedicated to hypernuclear γ spectroscopy. It consists of 14 sets of Ge
detectors for γ ray detection and BGO counters forbackground suppression. Each Ge detector isequipped with special readout electronics, whichallows γ ray detection under extremely high counting
rate and energy-deposit rate conditions.
E419 is the first experiment with Hyperball. In the
setup shown in Fig. 2, we produced 7ΛLi hypernuclei

Fig.1. Hyperball installed at KEK-PS K6 beam line.

in the 7Li (π+,K+) reaction employing the K6 beam line and the SKS
spectrometer, and detected γ rays
with Hyperball. As shown in Fig. 3(bottom), we observed four γtransitions in 7ΛLi and determined its
level scheme and energies as shownin Fig. 4 [1]. Compared with Fig. 3(top) showing the 7ΛLi energy
spectrum measured by the conventional method of 7Li (π+,K+) 7ΛLi
reaction with SKS only (E336),
the Hyperball spectrum (bottom)
demonstrates drastic improvement
Fig.2. Experimental setup of E419.
of the energy resolution; theresolution has been improved by three orders of magnitude, from 1.5-2 MeV (FWHM) with magnetic
spectrometers only into a few keV (FWHM) with Hyperball.
Figure 4 illustrates physics output from E419. From the energy spacing of the ground-state doublet(3/2+,1/2+) of 7ΛLi, we extracted the strength of the spin-spin interaction between a Λ and a nucleon (Δ = 0.5
MeV in the Millener’s parameterization)[1]. From the partly-Doppler-broadened peak shape of the E2 (5/2+

Fig.3. Top: energy spectrum of 7Λ Li measured in E336 withthe 7Li(π+,K+) reaction employing SKS with 2 MeV(FWHM) resolution. 7Λ Li bound states are not resolved.
Bottom: γ -ray spectra of 7Λ Li measured with Hyperball in
E419 (without and with Doppler-shift correction). Four γ transitions of 7Λ Li (a, b, c, d) are observed. They are
assigned as shown in the middle.

→1/2+) transition (b in Fig. 3, see the inset),
we obtained a lifetime of the 5/2+ state using
the Doppler shift attenuation method andthen extracted the transition probability ofB(E2)= 3.6 0.5+0.5-0.4 e2fm4. This B(E2)value is only 1/3 of the B(E2) value for thecorresponding transition of the core nucleus
6
Li(3+ →1+). It indicates that a Λ particle
makes a 6Li nucleus shrunk by 19 4% [3].
This shrinking effect occurs because a Λ isfree from Pauli effect and occupies the 0s
orbit to attract surrounding nucleons. Thisphenomenon, which was predicted in 1983
by Motoba et al.[4], has been experimentallyconfirmed for the first time in thisexperiment.
In addition, we observed a 429 keV γ linefrom 7Be produced by the weak decay of7
77
ΛLi, ΛLi→ Be (1/2 , 429 keV) + π . Fromthe yield of this γ ray, we derived thebranching ratio of this decay to be 6.0+1.3-1.6
10-2. Because of the dominant spin- nonflip amplitude in the mesonic decay of
Λ, this large branching ratio indicates the7
ΛLi ground-state spin to be 1/2 [5].
After E419, we moved Hyperball to BNL
and investigated precise structure of several
hypernuclei with intense K- beam available
at AGS. This experiment (E930) enabled us
to experimentally determine the strengths ofthe ΛN spin-orbit and tensor interactions [6].
Then, Hyperball was used again at KEK in
2002 (see the E509 and E518 sections).
References[1] H. Tamura, Nucl. Phys. A 639 (1998) 83c.
[2] H. Tamura et al., Phys. Rev. Lett. 84 (2000)
5963.
[3] K. Tanida et al., Phys. Rev. Lett. 86 (2001)
1982.
[4] T. Motoba, H. Bando and K. Ikeda, Prog.
Theor. Phys. 80 (1983) 189.
[5] J. Sasao et al., Phys. Lett. B 579 (2004)
258.
[6] H. Akikawa et al., Phys. Rev. Lett. 88

7
Fig.4. Level scheme and energies of Λ Li and physics output

obtained in E419.

(2002) 082501; H.Tamura et al., Mod. Phys.
Lett. 18 (2003) 85.

April,2004

Experiment 438
Study of Σ-Nucleus Potential by the (π-, K+) Reaction on
Heavy Nuclei(KEK, Tohoku, Osaka, SNU, CIAE, Tokyo, Kyoto, ITEP,
NCA&T, Hampton)
E438 aims at investigating Σ-nucleus potential through analysis of the inclusive(π-, K+) spectra on Si, Ni, In, and Bi. The experiment was carried out at K6. Separated πbeam of 1.2 GeV/c was used and scattered kaons were analyzed with SKS. The energy
scale and resolution of the reconstructed inclusive spectra were carefully calibrated andmonitored by the missing mass peak in the (π-, K+) reaction on H of the CH2 target
placed about 250 mm upstream of the experimental target. The resolution is important
to maintain the sensitivity to the size of imaginary potential. The peak widths obtainedfor Si, Ni, In, and Bi were 3.3

0.3, 4.4

0.4, 4.8

0.5, and 5.2

0.5 MeV, respectively.

The SKS acceptance was carefully corrected since it affects the spectral shape.
Measured angular distribution of the elementary (π-, K+) reaction agreed well with theprevious measurement at the incident momentum of 1.225 GeV/c.
Measured inclusive spectra were shown as a function of –BΣ- (Σ- binding
energy) in Fig. 1. We found that (1) the spectra show a similar shape although themagnitude of the spectrum increases with an increase of the mass number, (2) the crosssection gradually increase with an increase of –BΣ- having the maximum at –BΣ->120
MeV, and (3) significant yields exist below –BΣ-=0 MeV. In order to extract further
information on the Σ-nucleus potential, we calculated the inclusive spectra based on the
DWIA framework. We demonstrated that a strongly repulsive potential with a non-zerosize of the imaginary part reproduces the spectral shape but an attractive potential does
not reproduce, as shown in Fig. 2. The repulsive potential reduces the cross section ataround –BΣ-=0 MeV and shifts the yield to higher –BΣ- region.
Various hyperon-nucleon potential parameters based on one-boson-exchange
model or quark one are proposed. The present result would give a constraint on
selecting these potentials.

Strangeness constituents (hyperons and/or K-) in a neutronstar are being discussed. Particularly, Σ- is a hyperon expected to appear first in dense
neutron matter due to its negative charge. The present result would affect the discussion.

April,2004

Fig. 1: Measured inclusive (π-, K+) spectra as a function of –BΣ-. The spectrum on
C is extracted from the CH2 data. These spectra are measured at the SKS current of
272A (2.2 T).

Fig. 2: Calculated spectra (curves) with various sizes of the Σ-nucleus potential
were fitted to the observed spectrum (closed circles) for Si. (Only for Si, thespectra are measured at three SKS current setting of 210, 272, and 320 A to cover a
wider kaon momentum range. The plotted data are combined for all the settings.)

E452: Study of the spin-dependent interaction in Σ + P scattering
The hyperon-scattering experiment E452 aimed at determination of the LS interaction between
Σ++-hyperon and proton to compare with the ΛN case where the LS interaction had been shown to bevery weak[1]. The experiment was successful to show that the LS interaction was strong in the caseof Σ+P in contrast to the ΛN case[2].
The experiment consisted of two elements; (1) the development of a new tracking detector,
SCITIC (SCIntillation Track Image Camera)[3], and (2) the asymmetry measurements of thescattering of hyperons produced in the SCITIC[2].
Development of SCITIC
Originally, the experiment was proposed to measure the asymmetries with the SCIFI detector and/or with the SCITIC. Since the experiment was approved by the PS-PAC with a recommendation
to use the SCITIC, all measurements were carried out with the SCITIC. The performance of the
SCITIC has been tested in detail through a number of test experiments. and the newly developed
track detector was used for the first time in the major physics experiment.
The composition of SCITIC is shown in Fig.1(a). Scintillating tracks in a scintillator are
focused with an optical lens system on a photocathode of the image intensifier (IIT), amplified
through a cascade of 3 or 4 IITs, and recorded as a digital data by a CCD Camera. Event selections
were made by gating one of the IITs with trigger signals from outside logic electronics. Typical
pictures taken by the SCITIC are shown in Fig.1(b).

(a)

(b)

Fig.1 (a) Composition of the SCITIC, and (b) typical pictures of hyperon events.
Hyperon-scattering experiments
The Σ+P hyperon scattering experiment was motivated by the old Λ-hypernuclear experiment at
CERN[1] which indicated the very small LS effect in the ΛN interaction. This result was confirmed
later by KEK and BNL experiments. On the other hand, theoretical interpretations based on quark
models indicated that a large LS effect must be seen in the case of Σ+P interaction[4]. The
experiment E452 was proposed to measure the left/right asymmetries in the scattering of a polarized
Σ+ on a proton.

The SCITIC was used as an active target to produce hyperons through p(π+, K+) Σ+ reactions in aliquid scintillator and to observe their scattering with protons. The 1.6-MeV pion beam from the K2
beam line was used, and the secondary particles were analyzed with a spectrometer to select the event
associated with kaon emission. The experimental setup was essentially same as that for E289
except for the use of SCITIC, and the change of angular coverage of the kaon spectrometer to a range
where the large polarization of Σ+-hyperon was expected from previous reports [5].
The experiments were carried out in three stages, E452A, B and C with different setups.
The first stage, E452A was a preliminary trial experiment testing the system from data taking todata analyses. The SCITIC system with a pair of IIT-CCD cascades were used at this stage[2].
The optical system composed of a commercially available camera lens. Results of analyses (Fig. 2)
have already indicated significantly large left-right asymmetry of the polarized Σ+P scatteringsuggesting a large LS effect in contrast to the ΛP case.
Spending about a year after E452A, the experimental setup and data taking system were modified
for E452B & C. Three sets of large aperture lenses were fabricated to expand the viewing field,
and the SCITIC configuration was changed drastically using three sets of IIT-CCD cascades. A
new trigger scheme was installed to improve the quality of event trigger. The qualities of the
SCITIC pictorial data and the data-taking speed were greatly improved .
The analysis of the pictorial data is under way. The selection of hyperon events on free protons
was made through various kinematical cuts. Since data from E452B & C with three pictures had
redundancy for the 3D-reconstruction of track patterns, a preliminary analysis has been done with apolicy to use the redundancy for statistical checks to make the asymmetry data more reliable.
The Σ+P−asymmetry data from E452B & C are plotted in Fig. 3 with those from E452A and E289.
The large Σ+P−asymmetry has been confirmed through the analyses.

Fig. 2 Results of the E452A experiment.
[1]
[2]
[3]
[4]
[5]

Fig. 3 Summary of the Σ+P−asymmetry data.

W. Bruckner et al., Phys. Lett. B79 (1978) 157
T. Kadowaki et al., Eur. Phys. J. A15 (2002) 295: J. Asai et al., Jpn. J. Appl. Phys. 43 (2004) 1586.
A. Sato et al. Jpn. J. Appl. Phys. 43 (2004) 1593.
J. Pirner, Phys. Lett. B85 (1979) 190: K. Yazaki, Nucl. Phys. A479 (1988 ) 217c.
D.J. Candline et al., Nucl. Phys. B226 (1983) 1: J. Haba et al., Nucl. Phys. B299 (1988) 627

Exclusive measurement of the Non-Mesonic Weak Decay of 5ΛHe
E462 collaboration

Coincidence measurement of the Weak Decay of 12ΛC
E508 collaboration

We are studying non-mesonic weak decay(NMWD) ofΛ hypernuclei(Λ+N → N+N). This NMWD is thevery unique strangeness-changing baryon-baryon weak interaction process, which can occur only
in nuclei. Concerning the ratio of two possible NMWD modes, Γn/Γp ≡ Γ(Λ+n→n+n)/Γ(Λ+ｐ→n+ｐ), theoretical calculation based on one-pion exchange model predicts Γn<<Γp, whereas
recent experiments for A=5,12 suggests large ratios close to unity [1,2]. However these reported
results have large errors of 30～100%. Up to now, most of the experiments concerning this ratio
measured only protons from Λp → n p process and Γn was determined by the subtraction of all
the other decay processes. Thus the obtained results must be much affected by small changes of
the assumptions on final state interaction(FSI) effect and by the possible existence of the
two-nucleon induced NMWD process, ΛNN→NNN.
In order to measure this ratio unambiguously, we choose light s-shell Λ hypernuclei, 5ΛHe,
so as to minimize the FSI effect in E462 experiment(2000-2001). In addition, we measured bothof n+p- or n+n-pairs emitted from Λ +p → n+p or Λ +n → n+n NMWD process. When we selecttwo-nucleon pairs which has back-to-back angular correlation and applied energy cut (Q-value for
NMWD for 5ΛHe is 153MeV) for them, we can measure Γn/Γp ratio directly only from the ratio of
n+p- to n+n-double coincidence pair numbers. The result of this measurement is free from thestrength of FSI effect and also from the possible ΛNN→NNN contribution. We took 6Li(π+,K+)
data during year 2000-2001 with SKS.
The experiment was extended to the heavier p-shell nuclear target, 12C, to study the mass-number
dependence in E508 experiment in 2002.
Fig. 1 shows the setup of the decay coincidence system in E462/E508. Charged particles are
identified by the Time-Of-Flight(TOF) of T2-T3 and neutral particles are identified by the
T1-T4 TOF, corrected for the flight time of T1-target. The top and bottom coincidence arms areplaced to maximize the acceptance for the back-to-back n+n and n+p pairs. In order to accumulate
sufficient statistics for the difficult n+n-coincidence, we identified 5 × 104 and 7 × 104
hypernuclear production with SKS for 5ΛHe and 12ΛC, respectively.
Fig. 2 shows the neutron and proton spectra from the NMWD of 5ΛHe and 12ΛC. From the π-mesonicbranching ratio measurement of E462/E508, the yield is normalized by NMWD. Energy loss of proton
inside the target is corrected. Whereas the theoretical calculation predicts the observationof the broad peak at Q-value/2 [3] especially in the neutron spectrum of 5ΛHe, the observed neutron
spectrum shows no peaking even at A=5. This suggests large contribution ΛNN→NNN decay process.
The number of neutron above 60MeV is about 2 times larger than that of proton for both of
5
12This ratio is significantly lower than the naïve expectation of Γn/Γp=1 (initial
ΛHe and ΛC.
number ratio=3), indicating the dominance of Λ+ｐ→n+ｐ decay process.

Fig.1

Experimental setup of E462/E508

Fig. 2 Neutron and proton spectra from the NMWD of 5ΛHe and 12ΛC.
Yields of nucleons are normalized per NMWD. Energy loss of protons inside
the target is corrected.

We also established the way to identify back–to-back n+p- and n+n-pairs. Fig. 3 shows theangular correlation of the n+p- and n+n-pairs from the NMWD of 5ΛHe (double coincidence acceptanceis corrected). In both of n+p/n+n, we observed clear back-to-back correlation. Also the sum energy
distributions of two nucleons show peak at the Q-value of NMWD.
From the back-to-back coincidence ratio, we obtained the ratio of two-nucleon coincidence,
Nnn/Nnp as
Nnn/Nnp = 0.45±0.11±0.04 (E462 for 5ΛHe)with condition of cosθNN<-0.8 and TN>30 MeV. One can consider that theΓn/Γp ratio is very close
to this measured ratio. The same ratio was successfully measured also in E508 experiment.
The preliminary number ratio is
Nnn/Nnp = 0.40±0.09(statitical) (E508 for 5ΛC),
which is close to the result of E462.
Recent theoretical calculation considering heavier meson predicts theΓn/Γp ratio close tothis value[4]. Both of the dominance of Λ + ｐ → n+ ｐ decay process and the significant
contribution of Λ+n→n+n process are established for the first time.

Fig. 3

Two-nucleon angular correlation(top) and energy sum(bottom) from the NWMD of 5ΛHe
for n+p(left) and n+n(right), showing the dominance of Λ+ｐ→n+ｐ process.
(a)/(b): show the sum energy of n+p and n+n, respectively.
(c)/(d): show the opening angle plots for n+p/n+n-pairs with TN>30MeV cuts.
(top)bare number plot(bottom) acceptance and efficiency corrected, normalized by NMWD

As by-products of the experiments, we are also analyzing1) Asymmetry of proton emission from the NMWD of polarized hypernuclei,(proton decay asymmetry parameter, αNM )2) Lifetime of 5ΛHe and 12ΛC and3) π- and π0 decay widths of 5ΛHe and 12ΛC
Those results(most of them are still preliminary) are summarized in Table 1. All the results
are 2-20 times more accurate over the values of previous reports.

Width unit : ΓΛ

5Total Decay Widths
Non-mesonic Decay Widths
Nn/Np ratio (above 60MeV)
Nnn/Nnp (back-to-back)αnm (asymmetry param,eter)π- Decay Width
π0 Decay Width

0.947±0.038
0.395±0.016
2.17±0.15±0.15
0.45±0.11±0.04
0.07±0.08
0.351±0.017
0.201±0.011

HeΛ

(E462)

12

C(E508)Λ

1.242±0.042
0.953±0.032
2.00±0.09±0.14
0.40±0.09-0.24±0.26
0.165±0.008

Table 1: Summary of E462/E508 results including by-products measurements. All the results are
2-20 times more accurate over the previous reports concerning the same quantities.

References[1] J. J. Szymanski et al., Phys. Rev. C33 (1991) 849.
[2] O. Hashimoto et al., Phys. Rev. Lett. 88 (2002) 042503[3] G. Garbarino et al., nucl-th/0212084 (2002)[4] A. Parreno and A. Ramos, Phys. Rev. C65 (2001) 015204

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E509: -ray spectroscopy of hyperfragments with stopped K 0
Collaborator list:
K. Tanidaa 1, H. Akikawab 2 , Y. Fukaob, E. Hiyamac , H. Hotchid 3 , K. Imaib, Y. Miurae, K. Miwab,
K. Mizunumae, S. N. Nakamurae, M. Niiyamab, S. Otab, P. K. Sahad , H. Takahashib, T. Takahashie,
H. Tamurae, S. Terashimab , M. Togawab, M. Ukaie
a RIKEN (The Institute of Physical and Chemical Research), Saitama 351-0198, Japanb Department of Physics, Kyoto University, Kyoto 606-8502, Japanc High Energy Accelerator Organization (KEK), Tsukuba 305-0801, Japand Brookhaven National Laboratory, NY 11973, USAe Department of Physics, Tohoku University, Sendai 980-8578, JapanThe success of high-resolution -ray spectroscopy of hypernuclei with germanium detectors[2, 3, 4] opened a new era of hypernuclear physics. On the other hand, it is known that stopped
K 0 enables us to perform systematic study of hypernuclei by producing much more hyperfragments, including neutron rich ones, than other methods such as (+ ; K +) or (K 0 ; 0 ), whichwere used in those experiment.
Such systematic study will not only give us valuable information on the 3N two bodyinteraction, but also enable us to discuss the e ect of 3N N three body interaction if twobody interaction which consistently explains level energies of hypernuclei cannot be obtained.
Furthermore, in stopped K 0 method, we can study the charge symmetry breaking in the 3Ninteraction as mirror hypernuclei are equally produced.
Motivated by the above discussions, we proposed an experiment to measure rays fromhyperfragments produced by stopped K 0 reactions to KEK-PS[1]. Though we requested 70
shifts (560 hours) of beam time (including setup time of 20 shifts), only about 40 shifts wereadmitted. Therefore, we mainly focused on the feasibility study of the proposed method in thisexperiment.
The experiment was actually performed in Apr. 2002 at KEK-PS K5 beamline. Setup of
the experiment was very simple, as we used several trigger counters to identify stopped K 0 anda germanium detector system (Hyperball [5, 2]) to measure rays. We used ve experimental
targets (Li, Be, 10B, 11B, C) in order to see target dependence of -ray yields.
Our analysis shows a ray from 73 Li(5=2+ ! 1=2+ ) was observed at 2050 keV (see Figure 1).
This ray was clearly observed for the 10B, and we determined the strength of the ray to be(7:5 6 1:6) 2 1004 's per stopped K 0 . This value gives valuable information on the production
mechanism of hyperfragments from stopped K 0 reaction.
We also observed another candidate for hypernuclear rays at 1303 keV using the 9 Be and
10 B targets (Figure 2). Since there is no known
ray for light nuclei (A  12) at this energy,
this is most likely from an unknown transition in a hypernucleus, perhaps 83Li or 93 Li.
These results indicate that -ray spectroscopy of hyperfragments with stopped K 0 is suciently feasible. We are planning to improve the experimental setups and acquire more statistics.
1
2

Spokesperson. Email: tanida@rarfaxp.riken.jp
Present address: Japan Atomic Energy Research Institute, Tokai 319-1195, Japan

1

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Li

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200

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1900

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400

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200
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2100

2150
2200
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1250

Figure 1: -ray energy spectra around 2 MeV
for all targets. The 2050 keV ray from the
7 Li(5=2+ ! 1=2+ ) transition was clearly ob3
served for the 10B target.

1300

1350

1400
1450
Energy(keV)

Figure 2: -ray energy spectra around 2 MeV
for Be, 10B, 11B, and C targets. Among themany peaks seen in the gure, the peak at
1303 keV has target dependence in the -rayyield and thus is coming from the targets.
Since no ray is known at this energy in light(A  12) normal nuclei, this is a candidate
for hypernuclear ray.

Further experiments using heavier targets are being considered in future. - coincidence analysis is also promising in future experiments in which more ecient -ray detector is available.

References[1] KEK-PS proposal 509, February 2002.
[2] H. Tamura et al., Phys. Rev. Lett. 84 (2000) 5963.
[3] K. Tanida et al., Phys. Rev. Lett. 86 (2001) 1982.
[4] H. Akikawa et al., Phys. Rev. Lett. 88 (2002) 082501.
[5] H. Tamura, Nucl. Phys. A 639 (1998) 83c.

2

E518: High-Precision γ Spectroscopy of 11ΛB
Y. Fujii, M. Kameoka, O. Hashimoto, A. Matsumura, Y. Miura, K. Mizunuma, S.N. Nakamura,
H. Nomura, T. Takahashi, H. Tamura, M. Ukai, H. Yamauchi (Tohoku Univ.);
K. Aoki, Y. Kakiguchi, T. Maruta, T. Nagae, H. Noumi, H. Outa, Y. Sato, M. Sekimoto, A.Toyoda (KEK);
T. Fukuda, H. Hotchi, T. Ohtaki, P. Saha (Osaka E.C. Univ.);
S. Ajimura (Osaka Univ.); W. Imoto (Science Univ. of Tokyo); A. Krutenkova (ITEP);
K. Miwa (Kyoto Univ.); T. Saitoh (GSI); K. Tanida (RIKEN)
E518 is one of the experiments for high-precision γ spectroscopy of Λ hypernuclei with Hyperball, a large
germanium detector array dedicated to hypernuclear γ spectroscopy (see the E419 section) [1-4].
In this experiment, we investigated γ-ray transitions of 11ΛB hypernucleus produced by the 11B(π+,K+)
reaction employing the K6 beam line and the SKS spectrometer. One of the purposes of this experiment is tomeasure the transition probability B(M1) of the Λ spin-flip M1 transition, 11ΛB(3/2+→1/2+) (see Fig. 1), andextract information on the magnetic moment of a Λ inside a nucleus. The magnetic moment of a Λ in a
nucleus can be a probe to investigate possible modifications of baryon properties in a nucleus. In the weakcoupling limit between a Λ and a core nucleus, the Λ spin-flip B(M1) is expressed as [5]B(M1) ∝ <ψlow | µz | ψup>|2 = <ψlow | gc Jcz + gΛ JΛz | ψup>|2 ∝ (gc - gΛ )2where gc and gΛ denote effective g-factors of the core nucleus and the Λ, respectively. Here the spacecomponents of the wavefunctions of the lower and upper states of the doublet (ψlow ,ψup) are assumed to be
identical. Since direct measurement of hypernuclear magnetic moments is extremely difficult due to its too
short lifetime for spin precession, the spin-flip B(M1) provides the only way to measure hypernuclear
magnetic moments at present.
The other purpose of E518 is to cross-check the parameters for the ΛN spin-dependent interactions (spinspin, Λ-spin-orbit, N-spin-orbit, and tensor force strengths denoted as Δ, SΛ, SN, T). We have alreadydetermined them from our γ-ray data on 7ΛLi (KEK E419), 9ΛBe (BNL E930(’98)), and 16ΛO (BNL E930(’01))
using Hyperball [1,3,4]. However, these parameters have to be conformed with other hyper- nuclear data.
11
ΛB is suitable for this purpose, since many bound states are expected to exist in this hyper- nucleus.
The details of the experiment and preliminary results are reported in Ref. [6]. The experimental setup isalmost identical to the one in E419 [1,2] (see the E419 section). We irradiated 1.6 1012 pions on a 10 cmthick 98%-enriched 11B metal target in about 80 shifts’ beam time.
When the bound-state region is gated in the 11ΛB mass spectrum (“A” in Fig.2), the γ-ray spectrum
exhibited six peaks as shown in Fig. 3. Oneof them was observed in the Doppler- shiftcorrected spectrum. Since they are notobserved when the highly unbound region(“B” in Fig.2) is selected, these γ rays are
attributed to transitions from 11ΛB. The
assignment of all these γ rays and thecomplete reconstruction of the level scheme
seem difficult because of low statistics
which does not allow γγ coincidence
Fig.1. Expected low-lying level scheme of 11Λ B. We observed
measurement.
and identified the E2 (1/2+→5/2+) transition. Level energies,
The prominent peak at 1482 keV islifetimes, and transitions expected by Millener’s calculation++[7] are shown together.
assigned as the E2(1/2 → 5/2 ) transition(see Fig. 1). It is likely an E2 transition because its narrow width indicates a lifetime of the transition longer
than 10 ps, which gives a very small B(M1) value if it is an M1 transition. The 1/2+→5/2+ transition is the
only E2 transition expected in 11ΛB, and the observed largest γ-ray yield is also consistent with thisassignment.
It is to be noted that the shell-model prediction by Millener [4] for this E2 energy with the experimentally
determined ΛN interaction parameters is 1020 keV, significantly lower than the observed energy, as shown in
Fig. 1. It suggests inconsistency of the parameters. Among all the hypernuclear level energies obtained byour γ-ray spectroscopy and other experiments, such an inconsistency is found in 10ΛB(2-,1-) and 11ΛB(1/2+,
5/2+) [7]. More theoretical and experimental studies are necessary to understand hypernuclear structure andthe ΛN interaction.
The data analysis to derive lifetimes of the observed transitions is inprogress. In addition, for further studyof the ΛN interaction and the B(M1)measurement, we are constructing“Hyperball2”, an upgraded Hyperballwith an efficiency twice larger thanthe present Hyperball. It will be readyFig.2. Excitation energy spectrum of 11Λ B in the scale of Λ
in 2004.
binding energy BΛ. The regions “A” and “B” are the bound andReferences
unbound regions for event selection (see Fig.3).
[1] H. Tamura et al., Phys. Rev.
Lett. 84 (2000) 5963.
[2] K. Tanida et al., Phys. Rev.
Lett. 86 (2001) 1982;[3] H. Akikawa et al., Phys. Rev.
Lett. 88 (2002) 082501.
[4] M. Ukai et al., Proc. 8th Int.
Conf. on Hypernuclear andStrange
Particle
Physics(HYP03),Jefferson
Lab.,
October 14-18, 2003, Nucl.
Phys. A in press; M. Ukai et al.,Phys. Rev. Lett. to be published.
[5] R.H. Dalitz and A. Gal, Ann.
Phys. 116 (1978) 167.
[6] Y. Miura et al., Proc. 28th
Mazurian Lakes Conference On
Physics, August 31 - Sepember
7, 2003 Krzyze, Poland, Acta
Physica Polonica B 35 (2004)
1019; Y. Miura et al., Proc. 8thInt. Conf. on Hypernuclear andStrange
Particle
Physics(HYP03),Jefferson
Lab.,
October 14-18, 2003, Nucl.
Phys. A in press.
[7] D.J. Millener, Proc. 8th Int.
Conf. on Hypernuclear andFig.3. Preliminary γ -ray spectra obtained in E518. The spectra
Strange
Particle
Physics“A” correspond to the bound-state region of 11Λ B, and “B” to the(HYP03),Jefferson
Lab.,highly unbound region (see. Fig.2). Six γ rays observed in the “A”
October 14-18, 2003, Nucl.
spectra are attributed to transitions in 11Λ B.
Phys. A in press.

Production of neutron-rich Λ hypernuclei by the (π − , K+ ) double
charge-exchange reaction (E521)
T. Fukuda, P. K. Saha, and H. Noumi
Study of neutron-rich Λ hypernuclei would give information concerningthe Λ-Σ coupling as well as baryon interaction in high density nuclear mattersuch as neutron star [1, 2]. However, there have been almost no experimental
efforts so far. Hence our primary intention is to produce one neutron-rich Λ
hypernucleus (10Λ Li) in the first step, which would lead us to understand thereaction mechanism and eventually to find exotic hypernuclei such as 6Λ H oreven 7Λ H so as to reveal several interesting phenomena in strangeness nuclear
physics.
A neutron-rich Λ hypernuclear production in the (π − ,K + ) reaction canproceed by the two-step mechanism with the meson charge-exchange, for example, π − p → π 0 n followed by π 0 p → K + Λ, or π − p → K 0 Λ followed by K 0 p→ K + n. Another process is the single-step process via a Σ− admixture in theΛ hypernuclear state appearing due to the Σ− p ↔ Λn coupling. Recently,
there has been a theoretical calculation, where the two-step mechanism isfound to be dominant as compared to the single-step one [3]. The groundstate cross section for the 10
Λ Li production was calculated to be 70 nb/sr.
The incident energy dependence of the cross section shows the maximum at
1.05 GeV/c, same as the Λ production in the (π + ,K + ) reaction. However,
the other two-step process through the real Σ− production ( π − p → Σ− K +then, Σ− p → Λn) may also contribute to the production cross section, which
is not taken into account in the above calculation. A bound hypernucleus
production depends on the momentum transfer in each step, i.e. the sticking
probability of the Λ. The incident beam momentum in this case should behigher than 1.05 GeV/c in order to produce a Σ− .
The present experiment was carried out at the K6 beam line of KEK
12-GeV proton synchrotron (PS) together with the superconducting kaon
spectrometer (SKS) system. The total beam time was 30 days in two cyclesof the year 2002 and 2003. We used a 10 B target of 3.5 g/cm2 thickness.
Naturally, we started with a beam momentum of 1.05 GeV/c, where production cross section was expected to be the maximum as explained above.
We clearly identified scattered kaons separated from both pions and protons.
Then, by gating good kaons, the missing mass spectrum was obtained. The
total number of π − beam injection was about 440 × 109 but there were only
7 counts found in the bound region and quasi-free(QF) events were also verysmall. The yield in the bound region is about one order magnitude lower
than expected from the theoretical calculation. As for the calibration, we
measured (π + ,K + ) reaction on 10 B and 12 C targets of same thickness (3.5g/cm2 ). We obtained about 850 and 900 counts in the bound region of 10
ΛB
12
9
9 +
and Λ C production with 31.2 × 10 and 15.4 × 10 π injection, respectively,
which is quite consistent with previous measurements at SKS. The energy
resolution was obtained to be 2.5 MeV in FWHM.
We then changed the beam momentum to 1.2 GeV/c, where the Σ−
production channel is open and may have contribution to the hypernuclear
production as explained above and took data for about 450 × 109 π − injection. We found that the yield in the bound region was increased about
1

0.07

0.006

0.06

0.005

0.05

µb/sr/2MeVµb/sr/2MeV

0.004
0.04

0.003

0.03

0.002
0.02
0.001

0.01
0

-50

-25

0

25

50

75

100

125

0

150

-BΛ(MeV)

-40

-20

0

20

40

-BΛ(MeV)Figure 1: Missing mass spectrum

Figure 2: An expanded view

twice, whereas the QF yield was about one order of magnitude higher ascompared to the 1.05 GeV/c data. In year 2003 run, we continued with thebeam momentum at 1.2 GeV/c and accumulated data for another 700 × 109
π − injection. The spectrum in year 2003 was found to be very identical withthat in year 2002 and we simply added two spectra together. Figure 1 shows
the missing mass spectrum on 10 B at 1.2 GeV/c for all data we took in E521,
whereas figure 2 is an expanded view for the bound region. In total, we got
about 40 counts in the bound region, although no significant discrete peaks
were observed, which may be due to the limited statics, the experimental
resolution and possible complicated nuclear structure. In the off-line analysis backgrounds were reduced as much as possible and almost all eventsin the bound region are expected to come from the signal of the produced
hypernucleus, 10
Λ Li. From a plastic scintillator trigger counter (1.2 cm thick),
we also obtained the reaction events on 12 C so as to produce 12
Λ Be.
From the present experimental result, we obtained the Λ hypernuclear
production rate in the (π − ,K + ) reaction as compared to that in the (π + ,K + )reaction. Here, we compared yield in the bound region of (π − ,K + ) reaction
on both 10 B and 12 C targets at 1.2 GeV/c to that in the (π + ,K + ) reaction
on respective target from the present experiment. The SKS acceptance andanalysis efficiencies are taken into consideration in the comparison. We foundthat the Λ production rate in the (π − ,K + ) reaction is roughly 3 order ofmagnitude smaller (10−3 ) as compared to that in the (π + ,K + ) reaction.
In summary, we observed a neutron-rich Λ hypernucleus ,10
Λ Li, for the firsttime. The yield at the beam momentum of 1.2 GeV/c was higher as comparedto that at 1.05 GeV/c. The theoretical calculation via the meson chargeexchange mechanism overestimated the reaction cross section as comparedto the present experimental result. More studies are necessary for the detailed
understanding of the reaction mechanism.
[1] Y. Akaishi et al ., Phys. Rev. Lett. 84 (2000) 3539.
[2] S. Shinmura et al ., J. Phys. G 28 (2002) 1.
[3] T. Yu. Tretyakova and D. E. Lanskoy, Phys. of Atomic Nuclei
66 (2003) 1651.
2Enhancement of the 33 near the threshold in the 12C (K 0; K + 33) reactions
C.J.Yoon,1, y K. Aoki,1 K. Shoji,1 Y. Fukao,1 H. Funahashi,1 H. Okada,1 H. Sato,1 K.Takedani,1K. Imai,1, z K. Miwa,1 N. Saito,1 J. Asai,2 T. Kadowaki,2 M. Kurosawa,2 K. Nakai,2 M. Ieiri,3H. Takahash,3 T. Hibi,4 H. Nishikawa,4 H.N. Kyaw,4 K. Nakazawa,4 M. Okuda,4 T. Wint,4 T. Hayakawa,5T. Kishimoto,5 A. Sato,5 K. Yamamoto,6 T. Yoshida,6 J.K. Ahn,7 B.H. Choi,7 S.J. Kim,8 S.H.Kim,9I.G. Park,8 J.S. Song,8 C.S. Yoon,9 A. Akikawa,10 K. Tanida,11 T. Miura,11 and B.D. Park12
1
Department of Physics, Kyoto University, Kyoto, 606-8502, Japan
2 Department of Physics, Tokyo University of Science, Noda, Chiba, Japan
3
Insitute of Particle and Nuclear Studies, KEK, Tsukuba, Ibaraki, 305-0801, Japan
4
Department of Physics, Gifu Uniersity, Gifu, 501-1063, Japan
5 Department of Physics, Osaka University, Toyonaka, Osaka, Japan
6
Department of Physics, Osaka City University, Osaka, 558-8585, Japan
7
Department of Physics, Pusan National University, Pusan, Korea
8 Department of Physics, Gyeongsang National University, Jinju, 660-701, Korea
9
The Research Institute of the Natural Science, Gyeongsang National University, Jinju, 660-701, Korea
10
Advanced Science Research Center, Japan Atomic Energy Research Institute, Tokai, Ibaraki, 319-1195, Japan
11 Department of Physics, Tohoku University, Sendai, 980-8578, Japan
12
Department of Physics, Nagoya University, Nagoya, 464-8601, Japan
A 1.67 GeV=c K 0 beam injection on a scintillating ber active target has been used to study
for the nal states of K + 33 through 12 C (K 0 , K +33) reactions and search for the unstable
H 0dibaryon discussed by S.V.Bashinsky and R.L.Ja e in 1997. We observed the enhanced
production of 33 near the threshold. The statistical signi cance of the enhancement respect
to the `combinatorial background' was found to be about 2.7 standard deviations which isconsistent with previous result.
The measurement of the invariant mass spectrum of
33 in 12 C (K 0 ; K +33) reactions by the KEK-E224
shows an enhancement near the 33 threshold respect
to the result of the microscopic cascade model calculation(Intra Nuclear Cascade Model;INC) and also to theestimated phase volume of K +3310 Be [3]. They suggest
a possible exsistence of unstable H -dibaryon. In order
to con rm previous result of the KEK-E224 with highstatistics, we have performed a new experiment.
The experiment was carried out at K2-beam line using 1.67GeV=c K 0 -beam. About 1 2 1010 K 0 beam
was injected on the scintillating ber(SCIFI) target detector through the year of 2002, and 2003. The data fromthe SCIFI target was identi ed with a K + -spectrometer
which consists of dipole magnet, Aerogel counters, tracking chambers [10]. About 45,000 (K 0 , K +) eventswere scanned by human-eye and categorized according totheir event topologies, de ned by the number of charged
prongs, kinks and V-shapes. Typical image of the 33event is shown in Figure 1. For the 208 events, we havemeasured their coordinates. Among them, 28 events havea charged prong in the (K 0 , K +) primary vertex. They
are expected to be 40 (p) ! 33 candidates. Remaining 180 events have no prong. This report covers only
for the no pronged events. The statistics is much more
improved to the previous experiment. Figure 2 shows
K + momentum distribution for the 33 events. The in-tegrated di erential cross section of 12 C (K 0 ; K +33) forthe momentum region of 0.9  PK +  1.3 GeV=c, wasfound to be about 7.3 b=sr .
Invarint mass spectrum of 33 was shown in Figure 3.
Detection eciency was corrected. Theoretical prediction curves are compared to the data. The `combinatorial
background', which was produced by event mixing where
3-pairs are taken from di erent event each other. Thismethod ensure that the observed enhancement near thethreshold is real since the distribution has same trigger
bias and experimental acceptance as real one. This removes the systematic errors. In addition, two theoreticalmodel calculations also compared. They are quite consistent each other and the shapes are similar to the experimentaly deduced `background' for the over the 2261
MeV=c2 region, the data shows an enhancement nearthe 33-threshold. The statistical signi cance of the enhancement below the 2261 MeV=c2 region was found tobe 2.7 standard deviations respect to the `combinatorial
background'.
The enhanced peak was tted with Breit-Wigner parameters after the subtraction of the `background'. They
are found to be MR  2241 MeV=c2 and 0R  22
MeV=c2 and about 2 b=sr for the cross section inrough estimation. S.D.Paganis et al. showed if the
H -dibaryon is non-singlett, J = 0 and J = 1 H dibaryons appeared above the 33 threshold, but below

Report on KEK-E522, May 13,2004

2

mm

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〈d σ/dΩdm〉(µb/sr7.5 MeV/c )11111111111111111111111111111111111111
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Phase Space for
K-12C→10Be(g.s.)ΛΛK+

1.5

INC Calculation

2

Combinatorial Background
1

Preliminary

FIG. 1: Two 3-emission from the (K 0 ,K +) interaction
point in the SCIFI block.

0.5

5

0
2220 2240 2260 2280 2300 2320 2340 2360
2ΛΛ Invariant Mass (MeV/c )4

FIG. 3: Invariant mass spectrum of 33 was comapredwith the INC calculation and it's phase space for theK 0 12 C !10 Be(g :s :)33K + . And the `combinatorial
background' was deduced from event mixing for the different pair of 3's. These calculations are normalized withthe data over the 2261 MeV=c2 < m33 < 2300 MeV=c232d σ/dΩLdpK (µb/sr/50MeV/c)

2

2

1

0
600

700

800

900

1000

1100

1200

1300

+

K momentum (MeV/c)

FIG. 2: K + -momentum distribution for the 33 events.
Boxed one is presented as the E522. It is compared tothe previous data.
the 40n-threshold(2254 MeV=c2 ) in the calculation withthe framework of QCD-sum rule[2].
Besides, the origin of the enhancement could be the attractive nal state interaction of 33. Quantitative studies are underway as well as careful checking of the data.
We are indebted to the KEK-PS experimental supporting group, operation crew for their e orts to provide excellent experimental conditions.

yzElectronic address: cjyoon@nh.scphys.kyoto-u.
ac.jpElectronic address: imai@nh.scphys.kyoto-u.ac.
jp[1] S.V.Bashinsky and R.L.Ja e,Nucl. Phys. A
625;hep-ph/9705407[2] S.D.Paganis et al., Phys. Rev.56,570-573(1997)[3] J.K.Ahn et al., Phys. Lett. B444,267(1998)[4] G.I.Kopylov and V. Ye. Komolova, Nucl. Phys.
47,33-41(1963)[5] A.Ohnishi et al. Nucl.Physics. A691,242c(2001)[6] Y.Nara et al. Nucl. Phys. A614,433(1997)[7] A.Ohnishi et al. Proc. of APCTP Workshop on
Strangeness Nucl. Phys.(SNP'99),175-182(2000)[8] Y.Fujiwara et al. Phy. Rev.64, 054001(2001)[9] J.K.Ahn Ph.D Thesis at Kyoto Univ.
[10] A.Ichikawa Ph.D Thesis at Kyoto Univ.

Kaonic Hydrogen X-Ray Experiment
E546
Measurement of Electronic X rays correlated with Pionic X rays

E546：Measurements of Electronic X rays correlated with Pionic X rays
Negative pions stopped in matter result in the formation of pionic atoms. In theprevious studies, the LMM model describing the capture process was confirmed by theexperimental results based on the measurements of pionic X rays (πX rays) and neutral pion
decays. We have studied pion capture process in molecules, especially focused on hydrogencompound, in liquid and gas phases in E262/E360. Negative pions exist in highly excitedstates at the beginning of the capture process, and then they cascade down to lower excitedstates by emitting πX rays or Auger electrons. Moreover Auger electrons emission isfollowed by electronic X-ray (eX-ray) emission from pionic atom. Since pion mass is about
270 times as large as electron mass, pionic orbital have small radii; the pion strongly shields
electrons from nuclear charge. Therefore characteristic X-ray energies of pionic atoms are
not those of target atom (atomic number Z) but close to those of Z-1 atom. In E546 andseveral test experiments, we examined electronic X-ray emission correlated with each pionic
X ray to understand electronic rearrangement in the pionic atom during the pionic cascade.
The measurements were performed at πµ-channel of KEK-PS. As illustrated in Fig 1,
the measuring system consists of four plastic scintillation counters and three Ge detectors.
The signals from Ge detectors were taken in the coincidence with the stop events by PS1, PS2,
PS3 and veto PS4. To examine the correlation between πX rays and eX rays, the photon
events are recorded in a list mode. We used Mo, Sn, Dy, Ho, Ta and Pb metals and Mo, Sn, Ba,
Nd, Gd, Ho, Yb and Hg oxides as the target. The typical pion beam intensity was about
5x104 π- per pulse. In addition, we measured eX-ray energies of these atoms and their Z-1
atoms induced by photo ionization with a Co-57 source in the same apparatus.
We tried to measure the electronic X rays emitting in each excited state of a pionicatom. However, the number of correlated events is too small to discuss the difference ofelectronic X-ray energy between pionic states quantitatively. Here, we used the grossspectrum for analysis below. For as shown in Fig 2, we can find clearly energy difference
between characteristic X-ray energies of pionic Mo atoms (measured in the beam experiment)
and those of Nb atoms (measured in photoionization experiment). The energy shifts werealso found in both Kα and Kβ lines of other targets. We show atomic number dependence ofthe energy shift of the Kα ray in Fig 3, together with that for muonic atoms obtained by othergroup.
We should notice that there are some difference in the energy shifts between pionic
atoms and muonic atoms. Muons in muonic atom are stay in the 1s state for a relatively longtime. In the experiment of muonic atoms, the authors measured electronic X ray only within
10 - 20 ns after the muon was stopped in target to examine the electronic configuration during
the muonic cascade. On the other hand, since pions have short life time (26ns) and pions areabsorbed by nucleus before their decay, the eX rays that we measured were emitted frompionic atoms in an excited state. Theoretical estimate for dynamical shielding effects, taking
into account the pionic cascades, will be needed to understand the atomic number dependence
observed for pionic atoms.
We also try to reveal the atomic and molecular states after the pion capture bymeasuring eX rays correlated with πX rays. In this purpose, we extracted eX ray signals

correlated with πX ray from the obtained spectra
and examined the difference in the fine structureof KX rays (the ratio of Kα to Kβ X rays) between
metal and its metal oxide for Sn and Mo.
However, the number of correlated events is too
small to discuss on the difference quantitatively.

Fig. 1.
Schematic drawing ofexperimental arrangement. “PS” means
plastic scintillation counter.

Fig. 2. X-ray spectrum for pion capture in Mo
target. The open squares and triangles show Nb
and Mo eX-ray spectra, respectively.

Fig. 3. Energy shifts of the eX rays. This work is showed in filledsquares and triangles. The shifts in muonic atoms are also illustrated
with open squares.