Θ+ study at LEPS and Conclusive
Experiment at J-PARC
T. Nakano（RCNP, Osaka University）

Workshop on Hadron Structure at J-PARC@ KEK, November 30, 20051

Outline•Introduction•Search in γ d Æ K+ K- X
•Search in γ d Æ Λ(1520) X•Formation experiment2

Prediction of the Θ+ Baryon
D. Diakonov, V. Petrov, and M. Polyakov,
Z. Phys. A 359 (1997) 305.

••••

Exotic: S=+1
Low mass: 1530 MeV
Narrow width: ~ 15 MeV
Jp=1/2+M = [1890-180*Y] MeV

3

Experiements on the Θ+• Similar number of positive and negative results have been reported.

But most of the recent ones are negative.
And there are many unpublished negative results from the high energyexperiments.
Positive result

Negative result

K.Hicks, hep-ex/0504027

4

The LEPS Collaboration
Research Center for Nuclear Physics, Osaka University ：T. Nakano, D.S. Ahn, M. Fujiwara, K. Horie,
T. Hotta, K. Kino, H. Kohri, N. Muramatsu, T. Onuma, T. Sawada, A. Shimizu, M. Uchida, R.G.T. Zegers
Department of Physics, Pusan National University ：J.K. Ahn, J.Y. Park
School of Physics, Seoul National University ：H.C. Bhang, K.H. Tshoo
Department of Physics, Konan University ：H. Akimune
Japan Atomic Energy Research Institute / SPring-8 ：Y. Asano, A. Titov
Institute of Physics, Academia Sinica ：W.C. Chang, D.S. Oshuev,
Japan Synchrotron Radiation Research Institute (JASRI) / SPring-8 ：H. Ejiri, S. Date', N. Kumagai,
Y. Ohashi, H. Ohkuma, H. Toyokawa, T. Yorita
Department of Physics and Astronomy, Ohio University ：K. Hicks, T. Mibe
Department of Physics, Kyoto University ：K. Imai, H. Fujimura, T. Miwa, M. Miyabe, Y. Nakatsugawa,M. Niiyama, N. Saito, M. Yosoi
Department of Physics, Chiba University ：H. Kawai, T. Ooba, Y. Shiino
Wakayama Medical University ：S. Makino
Department of Physics and Astrophysics, Nagoya University ：S. Fukui
Department of Physics, Yamagata University ：T. Iwata
Department of Physics, Osaka University ：S. Ajimura, M. Nomachi, A. Sakaguchi, S. Shimizu, Y. Sugaya
Department of Physics and Engineering Physics, University of Saskatchewan ：C. Rangacharyulu
Department of Physics, Tohoku University ：M. Sumihama
Laboratory of Nuclear Science, Tohoku University ：T. Ishikawa, H. Shimizu
Department of Applied Physics, Miyazaki University ：T. Matsuda, Y. Toi
Institute for Protein Research, Osaka University ：M. Yoshimura
National Defense Academy in Japan ：T. Matsumura
5

LEPS spectrometer
Charged particle spectrometer with forward acceptance
PID from momentum and time-of-flight measurements
SVTX

TOF

DC1

AC(n=1.03)

Photons

TargetStart Counter

Dipole Magnet
0.7 Tesla

DC2DC36

First evidence from LEPS
γ n→K+K-n

Phys.Rev.Lett. 91 (2003) 012002
hep-ex/0301020

Low statistics: S = 4.6
Bbut

S= 3.2
S+B

Θ+Tight cut: 85% of events are rejectedby the φ exclusion cut.

Unknown background: BG shape is
not well understood. Events from a
LH2 target were used to estimate it.
Possible kinematical reflections.

Events/(0.02 GeV/c2)

15

1050Correction: Fermi motion correction
is necessary.

1.45 1.5 1.55 1.6 1.65 1.7 1.75 1.8
MMcγK− (GeV/c2)

7

LEPS LD2 runs
• Collected Data (LH2 and LD2 runs)
Dec.2000 – June 2001 LH2 50 mm ~5×1012 photons
published data
May 2002 – Apr 2003 LH2 150 mm ~1.4×1012 photons
Oct. 2002 – June 2003 LD2 150 mm ~2×1012 photons• #neutrons × #photons in K＋K− detection mode
LD2 runs = 5mm-thick STC in short LH2 runs × ~5
• K−p detection mode w/o Fermi correction :γd→Θ＋K−pK-p mode will be intensively presented today.
8Search for Θ+ in γ n→K+K-n•A proton is a spectator (undetected).
• Fermi motion is corrected to get the missing mass spectra.
•Tight φ exclusion cut is essential.
•Background is estimated by mixed events.
200

175

L(1520)

150

125

10075

aryni
imprel50

γ n→K+K-nCounts/12.5 MeVCounts/12.5 MeV

γ p→K+K-p
1008060

aryniimprel

40

20250

1.4

1.45

1.5

1.55

1.6

1.65

1.7

1.75

MMγΚ+ (GeV)

1.8

1.85

1.9

0

1.4

1.45

1.5

1.55

1.6

1.65

1.7

1.75

1.8

MMγΚ− (GeV)

1.85

1.9

9

Θ+ search in γ d Æ Λ(1520) KNreaction
Θ＋ is identified by K−p missing mass from deuteron.
⇒ No Fermi correction is needed.
K- n and pn final state interactions are suppressed.
If ss(I=0) component of a γ is dominant in the reaction,
the final state KN has I=0. (Lipkin)γ

Θ＋Λ(1520)pK−ndetectedp10

Background processes• Quasi-free Λ(1520) production must be the majorbackground.
• The effect can be estimated from the LH2 data.
γΛ(1520)

pnnK+• The other background processes which do not have astrong pK- invariant mass dependence can be removed
by sideband subtraction.
11

Sideband subtraction to remove nonresonant backgroundEγ > 1.75 GeV
Λ(1520)300

Λ(1520)LD2LH2120

250
100

200
80

150

60

100

4050

20

0

1.46

1.48

1.5

1.52

1.541.56

1.58

0

1.46

1.48

M(K−p) GeV/c2

1.5

1.52

1.54

1.56

1.58

M(K−p) GeV/c2

1.50 < M(K−p) < 1.54
1.45 < M(K−p) < 1.50 or 1.54 < M(K−p) < 1.59

S=- 0.4

12

Counts/5 MeV

BG estimation with two independentsideband regions
300

250

200

150

70

60

50

40

30

100

20

correction forφ contribution

50

100

1.46

1.48

1.5

1.52

1.54

1.56

1.58

0

1.4

1.45

M(K−p) GeV/c2•Validity of the sideband method with Eγsmearing was checked by using two
independent regions of the sideband.

1.5

1.55

1.6

1.65

1.7

1.75

1.8MMd(γ,K−p) GeV/c2
420•Channel-to-channel comparison gives-2

mean=-0.04 and RMS=2.0.

-4-6
1.4

1.45

1.5

1.55

1.6

1.65

1.7

1.75

1.8

1.85

MMd(γ,K−p) GeV/c2

1.9

13

Remove fluctuation by smearing EγCounts/5 MeVCounts/5 MeV

60
160

140

120

100

80

50

4030

60
2040

correction forφ contribution

10200

1.4

1.45

1.5

1.55

1.6

1.65

1.7

MMd(γ,K−p) GeV/c2

1.75

1.80

1.4

1.45

1.5

1.55

1.61.65

1.7

1.75

1.8

MMd(γ,K−p) GeV/c2•Fluctuations in the sideband spectra are removed by smearing Eγ with 10 MeV
smearing (nearly equal to the resolution).
•Eγ smeared spectrum gives χ2/n.d.f ~ 1 when compared with the originalspectrum.
• φ contribution in the signal region is slightly larger than that in the the 14
sideband region. The underestimtion is corrected by using the MC simulation.

•Estimate quasi-free Λ* contribution
using LH2 data.
•Missing mass is calculated byassuming deuteron mass in the initialstate.
•MC study shows the Fermi motion
effect is small.
•Non-resonant and φ contributions aresubtracted by sideband subtraction
method.
•Small fluctuations in the large missing
mass region (MM>1.55 GeV) could not
be completely removed.

Counts/5 MeV

Estimate Λ* contribution from LH2 data
50

40

30

20

100

1.4

1.45

1.5

1.55

1.6

1.65

1.7

1.75

1.8

MMd(γ,K−p) GeV/c2

MC

15MMd(γ,K−p) GeV/c2

Excesses are seen at 1.53
GeV and at 1.6 GeV above
the background level.

sideband100

Λ*80sum60

preliminary

Counts/5 MeV

K−p missing mass spectrum

1.53-GeV peak:(in the 5 bin = 25 MeV)S∼5S+B

0

1.4

1.45

1.5

1.55

1.6

1.65

1.7

1.75

MMd(γ,K−p) GeV/c2•Normalization factor for
LH2 data (green line) is 1.3.
Æ Large p/n asymmetry.

1.8

preliminary

20Counts/5 MeV

40
100

80

60

40

200

1.4

1.45

1.5

1.55

1.6

1.65

1.7

1.75

1.8

MMd(γ,K−p) GeV/c2
16

Normalization of Λ* is obtained by fit in the region of MMd < 1.52 GeV.

K−p missing mass in sideband regions
Counts/5 MeV

160

1.45<MpK<1.50140120

γor1.54<MpK<1.5980

K-60

K+/K040Θ+

200Counts/5 MeV

p100

1.4

1.45

1.5

1.55

1.6

1.65

1.7

1.75

1.8

140

1.44<MpK<1.49120or

100

1.55<MpK<1.60

10 MeVaway
from theΛ* region

Θ+ formation cross-section bysimple kaon re-scattering issmall.

80

60

40

200

1.4

1.45

1.5

1.55

1.6

1.65

1.7

1.75

1.8

MMd(γ,K−p) GeV/c217Search for γ d Æ Λ(1116) Θ+γΘ＋Λ(1116)p

π−np

forward angle
detection

1.5 GeV < Eγ < 2.4 GeV

18γ d Æ Λ(1116) X
3000

8000

7000

LD2K*

LD22500

K+6000
2000
5000

1500

4000

3000
1000
2000
500
1000

01.105

1.11

1.115

1.12

1.125

00

M(π−p) GeV/c2• ~100k Λ events are identified
in the deuteron data.
•The missing mass wascalculated by assuming anucleon at rest.

0.2

0.4

0.6

0.8

1

1.2

1.4

MMp(γ,π−p) GeV/c2

Λ(1116)

γK+/K*19

Close up of the Missing Mass300

700

LH2600250

LD2500200
400150300100200Σ decay
5001000.350.40.45

0.5

0.55

0.6

00.35

0.4

0.45

0.5

0.55

0.6

MMp(γ,π−p) GeV/c2

MMp(γ,π−p) GeV/c2• Background due to Σ decay
cannot be removed completely.

• The missing mass distribution issmeared by Fermi motion.

•The missing mass resolution isworse than the Λ(1520) one
because of higher momentum of
Λ(1116).

• Events with 0.40 GeV < MM< 0.58
GeV were selected for the Θ+search.

20

MMd(γ,π-p) spectra•Normalization factor for
LH2 data (green line) is 2.6.
Æ No large p/n asymmetry.

350• No excess at 1.53 GeV nor
1.6 GeV.

300

250200150100500•Quasi-free process can be
reproduced by free process.
Æ small effect from Fermi
motion.

preliminary

Counts/5 MeV

400

1.4

1.45

1.5

1.55

1.6

1.65

MMd(γ,π−p) GeV/c2

1.7

1.75

1.8

•Large cross-section
compared with Λ(1520).
•Missing Mass resolution isworse.
21

LEPS summary• We searched for Θ+ in the in the γ d Æ Λ∗(1520) X,
γ d Æ K+ K- X, and γ d Æ Λ(1116) X reactions• A ~ 5 σ Peak is seen at ~1.53 GeV/c2 in the missing mass of the(γ,Λ*).
• The peak is not be seen in the K-p invariant mass region
outside of the Λ(1520).
• If the peak is due to the Θ+, its production by re-scattering
seems to be small in our kinematical region.
• Bump structure) around 1.6 GeV was also observed in the(γ,Λ*) reaction in the low energy region.
• 1.53 GeV/c2 peak was seen in γ d Æ K+ K- X.
• No peak was seen in γ d Æ Λ(1116) X.

Cross-sections and upper limits will be given shortly.
New experiment with improved acceptance andincreased beam intensity will start in March, 2006.
22

Experimental evidence for the Q+
is inconclusive.
If Θ+ exists, its production ratedepends on reaction mechanisms
heavily.
Model independent confirmation isneeded!
23

Pentaquark production with kaon beam
• formation reaction : K＋n→Θ+→KS0p→π＋π−p417 MeV/c

DIANA
K+Xe→K0pX

Belle• Width of Θ+ (≲1 MeV) can be extracted from cross section.
• Spin of Θ+ can be extracted from angular distribution ofkaon.
S=1/2 : flat, S=3/2 : 1+3cos2θ
24

Kaon Scattering
The most direct experiments for exciting pentaquarks would bewith kaon beams for which there is a surprising dearth of data.
The most cited positive signals are from Bowen et al (1970), which
actually used kaon beams.

K++D (total)
The consensus now is thatthe upper limit on the widthis less than 1 MeV.
ΓΘ < 1 MeV

New measurements with highintensity kaon beam andmodern detectors should bedone!
25Pentaquark Search at BNL-E949
BarrelVeto•

π＋π− detection at UTC
M(π＋π−)=M(KS0)＋πM(Θ+)=M(K＋n)RSSC[MM(K＋,π＋π−)-M(p)]EndcapΔP/P=1.3%
TVeto
Fermi-correction＋K beam+• π＋π− detection at UTC &K Beam
417 MeV/cTarget
proton detection at Sci. Tgt.
σ=8.4 MeV
B4CI
M(π＋π−)=M(KS0)
DegraderDrift Chamber protonM(Θ＋)=M(KS0p)
π−TOF
Range
Stack

Δv/v=Δt/t, v=L/t, Δt=200 psec
⇒ L=4 m is equivalent to σ=8.4 MeV
K/π separation : ~6 nsec
Or K+ beam may be degraded until 417 MeV/c.
No problem in case of both Ks & p detection

26

(Study done by N. Muramatsu)

True CMS energy M(K＋n)

Mass Resolution• ππ detection withFermi correction
MM(K＋,π＋π−)11.228±0.226 MeV• Select backward production of Ks in CMS
mass res.
mean
120-180 degree 7.568±0.179 -0.599±0.190 MeV
150-180 degree 4.852±0.154 -1.047±0.212 MeV• ππp detection with angle mes. error = 6 degree
7.081±0.116 MeV• Kinematic fit (Ks mass & initial neutron mass)
4.099±0.242 MeV
27

Yield Estimation• In case that π＋π− are detected at UTC⇒ Acc. ~12 %⇒ 195 events/mb/pulse x 0.12 = 23 events/mb/pulse(Acceptance will be further reduced whena kinematical region where Fermi correction
works well is selected.)• In case that a proton Is detected at sci. target.
(Proton range < distance to target surface)⇒ Acc. ~8%⇒ 16 events/mb/pulse (without nuclear interaction)
26 mb for Γ=1 MeV if J = ½.
CEX BG ~ 7 mb (PRD15 (1977) 1846)

28

At J-PARC• We need low momentum K+ beamline.
• We need large acceptance detector and LD2or Active target, and good collaboration.

rell Experiment.
i
We can do truly conclusive KeyK29Variation of M(pK−) gate width
300

300

250

250200

200

150

150

100

100

50

50

0

1.46

1.48

1.5

1.52

1.54

1.56

0

1.58

1.46

1.48

BG: +25%

80

60

40

200

1.4

1.45

1.5

1.55

1.6

1.65

1.7

MMd(γ,K−p) GeV/c2

1.52

1.54

1.56

1.58

1.75

1.8

Λ* : -17%

7050

BG: -50%4030

20

100S/N gets better.

EX: -12%60

preliminary

EX: +22%

100Counts/5 MeVΛ* : +13%120

1.5M(K−p) GeV/c2

preliminary

Counts/5 MeV

M(K−p) GeV/c2

1.4

1.45

1.5

1.55

1.6

1.65

1.7

1.75

1.8

MMd(γ,K−p) GeV/c2

Possible leakage of Λ* in thesideband region30