Institute of Particle and Nuclear Studies (IPNS)

4.2 Experiments at the 12 GeV Proton Synchrotron
matter of normal density. It measures both K+K− ande+e− decays of φ mesons produced in 12 GeV protoninteractions with nuclei. The earlier results of theexperiment have shown a signature of the mass shift inthe e+e− decay mode. The experiment is now moving to
the final analysis stage.
The hybrid emulsion experiment E373 studies the
singlet 1S sector of a doubly strange system. The
physics interest is a unified description of the baryonbaryon interaction by the SU(3)f symmetry. About
40% of the data so far taken have been analyzed and103 Ξ hyperon stopping events have been found.
Among them there are two events of sequential decay
topology of a double-Λ hypernucleus. One of the two
events has the topology of formation and decay of a6ΛΛHe nucleus. This event confirms the attractive ΛΛinteraction with the binding energy ∆B ΛΛ = 1.01 ±0.20 +0.18−0.11 MeV. The other event, a twin single-Λ hypernuclei event, shows a very clean topology as seen in
Fig. 4-2-1. From these type of events, the binding
energy of Ξ− in nuclei should in principle be calculable.
Unfortunately, one track (#5 in Fig. 4-2-1) of the event
escaped from the emulsion stack and is not recorded in
the fiber-block detector; therefore, the reconstruction of
the event is not unique. The group is now investigating
three possible interpretations of the event.
Experiment E391, which has now been renamed
E391a, is searching for the CP-violation effect in K 0L →π0νν rare decays. The group has been expanded significantly since last year and is now an international
collaboration with physicists from Japan, Korea,

In FY2001, the operation of the KEK 12 GeV
Proton Synchrotron (PS) started in April 2001 in thefast beam-extraction mode and the K2K experiment
took data through July 2001. After the summer shutdown, the PS operation was resumed in the slow
extraction mode and five experiments were run until
March 2002. Out of these five experiments, four
experiments, E325, E452, E462, and E470, finishedtheir data taking in FY2001.
Experiment E246 is searching for T-violating muonpolarization in K+ → π0µ+ν (Kµ3) decays. In FY2001,
the group concentrated on data analysis. They
reviewed the systematic errors of their 1998 results andanalyzed new data from 1999 and 2000. A preliminary
result has been obtained and will be reported elsewhere. The group also studied K+ → π0e+ν (Ke3) decays
to further constrain the exotic scalar and tensor
couplings. The group has obtained fs /f+(0) = 0.0040 ±0.0160(statistical) ± 0.0067(systematic) and fT / f +(0)=0.019 ± 0.080(statistical) ± 0.038(systematic). They
have also looked at the ratio Γ(Ke3)/Γ(Kµ 3) and obtained
the q2-dependent coefficient λ0 of the f0 (Kl3) form factor
as λ0 = 0.019 ± 0.005(statistical) ± 0.004 (systematic).
To study the direct emission component in radiative
K+ decays, K+ → π+π−γ, the same group proposed a newexperiment designated E470 using the E246 detector
but with optimized triggers. Radiative K+ decay isdominated by the inner bremsstrahlung, that is, theQED radiative correction to K+ → π+π0 decay, and the
direct emission is to be found as a deviation of themeasured spectrum from the inner bremsstrahlung
spectrum. The direct emission is interesting because its
magnetic component originates in the chiral anomaly
and may be calculated by low energy effective theories
such as the chiral perturbation theory. The E470 experiment collected data for 120 shifts in autumn 2001.
The branching ratio of the direct emission is expected
to be O(10−5) and the statistical accuracy of this experiment to be around 10%.
The E325 experiment is the oldest among the active
experiments at the KEK PS. It was approved in 1995
and started physics runs in 1997. Since then, the total
beam time has exceeded 400 shifts. In FY2001, theexperiment finally completed its data taking. The
objective of the experiment is to find signatures ofmass shifts of vector mesons decaying in the nuclear

#7

#4

#6
#3

C
#2

A

B
#1

Ξ-

#5

(Top View)
Fig. 4-2-1 Emulsion image of the twin single-Λ hypernuclei event (left) in E373. The event topology
is also shown (right).

23

Institute of Particle and Nuclear Studies (IPNS)

Fig. 4-2-2 Engineering design of the E391a detector to measure K0L → π0νν rare-decays. Most of the detectors including a
CsI calorimeter are placed inside one large vacuum vessel.

Russia, and the United States. In FY2001, the groupconducted an extensive engineering study of thedetector system in collaboration with a Russian design
company. The design has been almost finalized asshown in Fig. 4-2-2. Most of the detectors are placed
inside one large vacuum vessel that consists of three
sections: upstream section, middle section, and downstream section. The downstream section was fabricatedin FY2001 (Fig. 4-2-3). The main detector of theexperiment, CsI crystal calorimeter, will be mounted on
it and tested in autumn 2002. The experiment foresees
pilot runs at the KEK PS starting in FY2003 and then ahigh statistical experiment at the JHF 50 GeV PS(HIPA) in the future.
In order to study the spin-orbit interactions of thehyperon-nucleon system, experiment E452 ismeasuring the left-right asymmetries of the Σ+ and Λ
elastic scatterings on protons. Polarized Σ+ and Λ were
produced through ( π + , K + ) interactions in an active
liquid-scintillator target at pπ =1.6 GeV/c. The polarization of Σ+ was known to be as large as 0.8 (Fig. 4-24 (a)) from previous experiments. The polarization ofΛ was measured and found to be also large in thisexperiment (Fig. 4-2-4 (c)). In the experiment, candidates for the hyperon event were selectively recorded

Fig. 4-2-3 Downstream section of the E391a vacuum
vessel placed in the PS experimental hall.

24

Institute of Particle and Nuclear Studies (IPNS)Σ(c)Haba et al.
Candlin et al.
P = 0.8

1.0

Polarization

Polarization

(a)

Λ

0.5

0.5

Λ Production
0

0.8

(b)

(d)

Σp Scattering

E452
E289

0.4

0.2

0

-0.2

0

E452
E289

0.6

Asymmetry (N L-N R)/(N L+N R)

0.6

Asymmetry (N L-N R)/(N L+N R)

1.0

Σ Production
From previous work

0

-0.4

E452

10

20

30

0.4

0.2

0

-0.2

-0.4

40

Λp Scattering

0

10

20

30

θ KLab (deg)
0.6

0.8

1.0

40

θ KLab (deg)
1.2

0.6

PΣ (Gev/c)

0.8

1.0

1.2

PΛ (Gev/c)

Fig. 4-2-4 E452 measured polarizations of Σ (a) and Λ (c) produced in (π+, K+) interactions (top) and left-right asymmetries ofthe Σ+p (b) and the Λp (d) elastic scatterings (bottom).

Cloud Chamber (ECC) to measure charged particle
momentum and thereby identify electrons. The goal is
to develop a new technology to measure neutrino flux.
The momentum resolution measured by multiple scatterings in the ECC was found to be about 20%. The
T489 group has been developing a position-sensitive
Cerenkov detector (POSCH) for the search of η and ω
mesonic nuclei at GSI and the T494 group tested theperformance of prototype detectors for a new spectrometer to be used in (e,e’K+) Λ-hypernuclei spectroscopy at the Jefferson Lab. The former group has
obtained good p/K + separation by a pure-water
Cerenkov counter (Fig. 4-2-5). To improve the particle
identification capability of the PHENIX experiment at
BNL-RHIC, the T496 group tested aerogel Cerenkov
counters (Fig. 4-2-6). Together with TOF and RICH,
an aerogel Cerenkov counter of n = 1.01 provides good
π /K/p identification up to 10 GeV/c without any
momentum gap. The T499 group has been designing
an experiment to measure Auger electrons emitted inthe π capture and electronic X-rays (eX-rays) in coincidence with pionic X-rays (πX-rays). In FY2001, theyby the SCIntillating Track Image Camera (SCITIC)
with a kaon trigger signal from the forward spectrometer. The experiment started in FY2000 and finisheddata taking in February 2002. About 800k hyperon
event candidates were recorded. The immediate
interest of the experiment is to compare the spin-orbit
interaction between the Σ+p and Λp elastic scatterings.
So far only 10% of the events have been analyzed.
Preliminary results of the left-right asymmetries of the
Σ+p and the Λp elastic scatterings are shown in Fig. 42-4 (b) and (d), respectively. Reanalyzed data from theprevious experiment E289 are included here. A larger
spin-orbit interaction is seen in the Σ+p scatterings thanin the Λp scatterings.
Beside the above five physics experiments, 23 test
experiments were carried out in FY2001. ExperimentT484 tested the basic performance of a new detector,
called Time-Of-Propagation (TOP) counter, for a future
Belle detector upgrade. It is a detector for particle
identification using the spacetime information ofCerenkov lights emitted in quartz bars. Experiment
T487 studied the feasibility of using an Emulsion

25[count]

Institute of Particle and Nuclear Studies (IPNS)p
10

Water: Pure Water (18.3MΩ·cm)
Reflection material: Titaniumdioxide

4Central momentum: 1.2GeV/c

10

3

10

2

e++µ++π+

K+

10

1

0

20

40

60

80

100

120

140

[Number of photoelectrons]
Number of photoelectron for water c̆erenkov

Fig. 4-2-7 TOF resolutions obtained by a fine-segmented
TOF hodoscope (5 mm × 10 mm × 1,000 mm)
made of fiber bundles read out by multi-anode
PMTs (T502).

Fig. 4-2-5 Number of photoelectrons measured by theT494 pure-water Cerenkov counter. Morethan 99% of protons are separated without
missing K+s.

measured the eX- and π X-rays using a pair of Gedetectors for low-energy photon spectrometry. The
T502 group tested the performance of two drift chambers and a scintillation fiber detector for the BESS
2002 balloon-borne experiment. Also tested was a new
TOF counter of fiber-bundles for the BESS-Polarexperiment. The timing resolution of the TOF counter
was found to be 0.2 − 0.6 ns, which is sufficient to separatep from negatively charged low-energy muons (Fig.
4-2-7). For an upgrade of the K2K near detector, four
test experiments were performed. Experiment T488
tested a tracking detector consisting of plastic-scintillator plates with wavelength-shifting (WLS) fibers.
The scintillator plates are doped with light-scattering
material to improve the position resolution while maintaining good detection efficiency. Experiments T491
and T503 tested the basic performance of a scintillator
tracker called SciBar. The tracker uses extruded scintillators read out by wavelength-shifting fibers. And
experiment T501 was conducted to calibrate the energy
scale of the neutrino spectrum measured in the present
K2K near detector.

Fig. 4-2-6 Study of particle identification by a combination of aerogel counter and TOF for the
PHENIX experiment (T496).

26

Institute of Particle and Nuclear Studies (IPNS)

4.3 K2K Experimenttable also includes numbers for 1-ring µ-like and e-likeevents and multi-ring events.
Figure 4-3-1 shows the overall history of event
arrival time through all runs. The horizontal axis represents POT rather than the elapsed time. The uniformityof the arrival time was tested by the KolmogoroffSmirnov (KS) probability and found to be 43.1%, indicating a satisfactorily uniform distribution.
The goal of the oscillation analysis is to compare
the spectrum observed at the Super-Kamiokande tothose extrapolated from the spectrum measured at the

The KEK-to-Kamioka (K2K) long-baseline
neutrino oscillation experiment is the first acceleratorbased experiment with a neutrino path length ofhundreds of kilometers. The intense, nearly pureneutrino beam (98.2% νµ, 1.3% νe, and 0.5%νµ) has anaverage L/Eν ~ 200 (L = 250km, < Eν > ~ 1.3 GeV).
The K2K experiment focuses on studying the existenceof neutrino oscillation in νµ disappearance, which wasobserved for the first time in atmospheric neutrinos,
and on the search for νµ → νe oscillation with the KEK
neutrino beam, whose flux and neutrino composition
have been well studied in the m2 ≥ 2 × 10−3 eV2 region.
In FY2001, the K2K experiment took data from
April to July. Since the start of the experiment in 1999,
the total number of protons delivered on the pion
production target (protons on target: POT) has reached
5.6 × 10 19 . The net POT successfully used for the
analysis of the Super-Kamiokande (SK) data is 4.8 ×1019. The balance is due to various reasons: tuning of
the beam channel and its horns, calibration of theSuper-Kamiokande, power failure at the SuperKamiokand, and failure in the online data acquisitionof the beam-monitoring data. As of July 2001, theK2K experiment is still midway toward its original
goal of 1020 POT.
With all the available data analyzed, 56 neutrinos
coming from KEK were observed inside the SuperKamiokande fiducial volume of 22,500 tons of water,while, from the observations with the 1,000-ton water
Cerenkov detector located at the KEK site, 80.6 +7.3
−8.0
neutrino events were expected to be observed if
neutrinos did not oscillate. Table 4-3-1 compares theseresults with the expected number of events calculated
for the maximal neutrino oscillation (sin22θ = 1). The

56 events @ 48; 1018 POT

Fig. 4-3-1 POT (number of protons delivered onto thetarget) versus accumulated number of eventsat the Super-Kamiokande.

Table 4-3-1 Observed numbers of the fully-contained (FC) events in the SK with the vertices located inside the 22,500-ton
fiducial volume (FV) compared to the two expected numbers calculated from the observations in the 1,000-ton
water Cerenkov detector at the KEK site for no oscillation and for neutrino oscillation with sin22θ = 1
observed
N SK

Null oscillation

FC event in FV

56

1-ring∆m2 (eV2)−3

3 × 10

5 × 10−3

7 × 10−3+7.3
80.6 −8.0

52.4

34.6

29.2

32

48.4 ± 6.7

28.1

17.8

16.0

µ-like3044.0 ± 6.8

24.4

14.6

13.5

e-like2

4.4 ± 1.7

3.7

3.2

3.0

Multi-ring24

32.2 ± 5.3

24.3

16.8

12.6

27

Institute of Particle and Nuclear Studies (IPNS)
the data and the extrapolation of the spectrum are very
preliminary, the data, which also include the non-QE
background, seem to present the minimum at the rightEν energy.
On November 12, 2001, the Super-Kamiokande,
the far detector of the K2K experiment, suffered from a
sad accident, in which about 60% of both the 20-inch
photo-multiplier tubes (PMTs) for the inner detector
and the 8-inch PMTs for the outer detector weredestroyed. Before the accident, the Super-Kamiokande
had been drained since July 2001 after five years ofoperation and dead PMTs replaced with new ones. The
accident occurred when water was being filled after the
maintenance work was completed. The water level at
the time of the accident was about 30 m. The cause of
the accident was investigated and found to be theimplosion of one PMT of the inner bottom PMT plane.
The implosion generated shock waves that destroyedneighboring PMTs, resulting in a chain reaction andthereby the mass destruction of PMTs. It is speculated
that the first PMT that imploded was one that sufferedslight damage to the PMT glass when it was installed
as a replacement for a dead PMT or one that was
subjected to strain when nearby bottom PMTs were
replaced.
As a result of the accident, the K2K runs originallyscheduled after January 2002 had to be deferred. As
the spokesperson for the Super-Kamiokande Collaboration, Professor Totsuka, announced immediately after
the accident, in order to resume the K2K experiment at
the earliest possible time, possibly within a year, it was
decided to rebuild the Super-Kamiokande with existing
PMTs; that is, with a PMT density of about one-halfthe original density. It should be noted, however, thatfor the detection of GeV neutrinos from KEK, thereduced PMT density should not cause any serious
deterioration of the detector performance.

near detector with and without neutrino oscillation.
Figure 4-3-2 shows the reconstructed Eν spectrum of 30
fully contained 1-ring µ -like events in the SuperKamiokande fiducial volume. Here, the kinematics ofa two-body quasi-elastic (QE) reaction are assumed.
The observed spectrum, therefore, mostly consists ofreal muon neutrinos and non-QE background. The
figure also shows the spectrum extrapolated from themeasurement at the near detector. The non-QE background is estimated by a Monte Carlo simulation with
detector response. It should be noted that the neutrino
oscillation with ∆m2 = 3 × 10−3 eV2, which is consistent
with the atmospheric neutrino oscillations, predicts theoscillation minimum in the Eν spectrum at about 600
MeV. Although the systematic errors associated withE ν F.C. 22.5 kt 1-ring µ-like

Events/0.5 GeV

Note: ∆m2=3×10-3eV2
corresponds to600 MeV Eν

ry

na
i
m

i

l
Pre

Reconstructed Eν(GeV)
Fig. 4-3-2 The neutrino spectrum observed at theSuper-Kamiokande. The histogram shows
the spectrum extrapolated from the observation at the near detector assuming noneutrino oscillation (preliminary).

28