1

• Introduction

• Experiment

• Evidence for

• Confirmation from other labs

• Conclusion & Outlook

Evidence for S=+1 Pentaquark StateT. Nakano (RCNP, Osaka Univ)

September 17, 2003 @ Kyoto Univ.

2

(Z+) Baryon

M1890-180*Y] MeV

D. Diakonov, V. Petrov, and M. Polyakov, Z. Phys. A 359 (1997) 305.

suudd

• Exotic: S=+1

• Low mass: 1530 MeV

• Narrow width: < 15 MeV

• Jp=1/2+

3

Exotic S=+1 BaryonNOTE ON THE S = + 1 BARYON SYSTEM

(PDG 1986; Phys. Lett. B170, 289)

The evidence for strangeness +1 baryon resonances was reviewed in our 1976 edition,1 and more recently by Kelly2 and by Oades.3 Two new partial-wave anaIyses4 have appeared since our 1984 edition. Both claim that the P13 and perhaps other waves resonate.

However, the results permit no definite conclusion- the same story heard for 15 years. The standards of proof must simply be much more severe here than in a channel in which many resonances are already known to exist. The general prejudice against baryons not made of three quarks and the lack of any experimental activity in this area make it likely that it will be another 15 years before the issue is decided.References

•1. Particle Data Group, Rev. Mod. Phys. 48, SI88 ( 1976).

•2. R.L. Kelly, in Proceedings of the Meeting on Exotic Resonances (Hiroshima, 1978), ed. I. Endo et al.

•3. G.C. Oades, in Low and Intermediate Energy Kaon-Nucleon Physics (1981), ed. E. Ferrari and G. Violini.

•4. K. Hashimoto, Phys. Rev. C29, 1377 (1984); and R.A. Arndt and L.D. Roper, Phys. Rev. D31, 2230 (1985).

4

Possible + Production Reactions

DIANA/ITEP KEK-PS/E522

CLAS/JLABLEPS/SPring-8

5

Super Photon ring-8 GeV SPring-8

• Third-generation synchrotron radiation facility

• Circumference: 1436 m

• 8 GeV

• 100 mA

• 62 beamlines

6

Laser Electron Photon facility at SPring-8

in operation since 2000

7

The

Wakayama Medical University

S. Makino

Nagoya University

T. Fukui

Osaka University

H. Nakamura, M. Nomachi, A. Sakaguchi, Y. Sugaya,

University of Saskatchewan

C. Rangacharyulu

Institute for High Energy Physics (IHEP), Moscow

P. Shagin

Laboratory of Nuclear Science, Tohoku University

H. Shimizu, T. Ishikawa

University of Michigan

K. Yonehara

Michigan State University

R.G.T. Zegers Seoul National University

H. Fujimura Miyazaki University

T. Matsuda, Y. Toi

8

LEPS collaboration

Research Center for Nuclear Physics, Osaka University T. Nakano, D.S. Ahn, M. Fujiwara, T. Hotta, K. Kino,H. Kohri, T. Matsumura, T. Mibe, A. Shimizu, M. Sumihama Pusan National University J.K. AhnKonan University H. Akimune Japan Atomic Energy Research Institute / SPring-8 Y. Asano, N. MuramatsuInstitute of Physics, Academia Sinica, Taiwan W.C. Chang, T.H. Chang, D.S. Oshuev, C.W. Wang, S.C. Wang Japan Synchrotron Radiation Research Institute (JASRI) / SPring-8 S. Date, H. Ejiri, N. Kumagai, Y. Ohashi, H. Ookuma, H.Toyokawa, T. Yorita Ohio University K. Hicks Kyoto University K. Imai, M. Miyabe, M. Niiyama, T. Sasaki, M. Yosoi Chiba University H. Kawai, T. Ooba, Y. Shiino Yamagata UniversityT. Iwata

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Energy Spectrum and Polarization

E (GeV)

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

00 .5 11 .522 .5 3

Intensity : 2.5 * 106 /sec

> 1.5 GeV: 1.0 * 106 /sec

2.4 GeV 8 GeV

Polarization : 95 % at 2.4 GeV

10

LEPS detector

1m

11

LH2 Target

Cerenkov Detector

SSD

Drift Chamber

Start Counter

12

Charged particle identificationPhoton beam asymmetries for K+ photoproduction

Mass(GeV)

Mo

men

tum

(G

eV

)

K/ separation (positive charge)

K++

Mass/Charge (GeV)

Eve

nts

Reconstructed mass

d

p

K+

K-

+-

(mass) = 30 MeV(typ.) for 1 GeV/c Kaon

13

Summary of data taking

Target(LH2)

Start counter (SC)

AC(n=1.03)

30 Hz for 800 kHz@tagger

Charge veto

•Total number of trigger1.83*108 trigger

Dec, 2000 to Jun, 2001

•Number of events with reconstructed charged tracks

4.37*107 events

•About a half of events were produced in SC

Optimized for p p K+ K- p

Small distance between LH2 and SC

High index of AC

p K0 + + - K+ n

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Identification of +

n → K- + → K- K+ n

• K- missing mass gives + mass• K+K- missing mass gives n

Non-resonant KK

(1520)

+?

Problems:• “background”

K+K-

(produced from n & p)• Fermi motion distorts a missing mass spectrum.

15

Test-case:n → K+ -→K+ - np → K+ →K+ - p

K+-

mis

sin

g m

as

sK+ missing mass

K+ missing mass

K+ missing mass (corrected)

K+ missing mass (corrected)

Sta

rtc

ou

nte

r (C

H)

LH

2 targ

et

Correction: MMcorrected)= MM- MM+ Mn

Fermi motion correction

Correction works

better when

is small.

p~n áp~K

16

Proton-recoil cutnK+K-n no recoil proton 　 (proton is a spectator)pK+K-p slow recoil proton is present proton is too slow to be seen in full detector, but might be seen in SSD vertex detector.

K+

K-

ps

sd dc

dc

dc to

f

dipole

target(SC)

Remove all events for which proton is detected in SSD, or for which predicted proton track does not hit SSD.

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Effectiveness of proton recoil cut p(n)K+K-p(n)

• select KK events from startcounter• construct K+ missing mass (corrected) plot

• apply p-recoil cut

• reverse p-recoil cut

（ proton is present)

p(1520)K+

K-p

MMc= MM- MM+ Mn

peak in events with a spectator proton.

The cut enhances nK+K-n

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n-) missing mass

• select KK events from SC• apply KK invariant mass cut ()• apply KK missing mass cut (0.9<MMkk<0.98)• apply proton recoil cut• construct K- missing mass plot

Background?

+

n → K- + → K- K+ n

Background shape can be determined from events from LH2 target using the same cuts except for:

• Proton-recoil cut is removed• (1520) events are removed (only from p)

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+ identification

Background level is estimated by a fit in a mass region above 1.59 GeV.

Assumption:• Background is from non-resonant K+K- production off the neutron/nucleus• … is nearly identical to non-resonant K+K- production off the proton

1.540.01 MeV< 25 MeVGaussian significance 4.6

background

Phys.Rev.Lett. 91 (2003) 012002

hep-ex/0301020

20

CLAS/JLAB

hep-ex/0307018

Confirmation from other labs

DIANA/ITEP

hep-ex/0304040

MeVMeV

MeVMeV

d→p nK+Xe→p X n→p)

21

Questions to be answered

• Why is it so light?

• Why is the width so narrow?– S.Nussunov (hep-ph/0307357) based on K+d scattering data (+)

<6MeV– Arndt,Strakovski & Workman (nucl-th/0308012) based on existing

K+N elastic scattering data estimate that (+) can/should be as small as 1 MeV

• Pentaquark or KN molecule?

• Excited S=+1 states?

• Is this really an Jp=1/2+ state?– Lattice QCD calculations suggest positive parity (S. Sasaki, Hadron0

3) , (F. Csikor et al, hep-lat/0309090)

22

Theoretical activities• Exotic baryon states in topological soliton models Walliser, H ; Kopeliovich, V B, hep-ph/0304058• Interpretation of the Theta+ as an isotensor resonance with weakly decaying partners Capstick, Page, Roberts, hep-ph/0307019• Stable $uudd\bar s$ pentaquarks in the constituent quark model Stancu, Fl ; Riska, D O, hep-ph/0307010• The Constituent Quark Model Revisited - Quark Masses, New Predictions for Hadron Masses and KN Pentaquar

k Karliner, Marek; Lipkin, Harry J, hep-ph/0307243• Pentaquark states in a chiral potential Hosaka, Atsushi hep-ph/0307232• Group theory and the Pentaquark Wybourne, B G, hep-ph/0307170• Diquarks and Exotic Spectroscopy Jaffe, R L ; Wilczek, F, hep-ph/0307341• Understanding Pentaquark States in QCD Zhu, Shi-Lin, hep-ph/0307345• The anticharmed exotic baryon Theta_c and its relatives Karliner, Marek; Lipkin, Harry J hep-ph/0307343• Determining the $\Theta^+$ quantum numbers through the $K^+p\to \pi^+K^+n$ reaction Hyodo, T ; Hosaka, A ; Oset, E nucl-th/0307105

23

Very recent results with proton target

MeVMeV

MeVMeV

Require cos K > 0.5SAPHIR/ELSA

hep-ex/0307083

CLAS/Jlab

hep-ex/0307088

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: Ks photoproduction at CLAS

• Even with very little background, the is not seen in p Ks X.

• Upper limit on the production cross section: <20 nb. (MC efficiency with t-channel production at 3 confidence level)

• It is possible that the coupling of the pK* vertex is smaller than the pK vertex or that the mass of the virtual meson exchanged is a factor.

INFN group analysis

25

: Ks production (forward angles)

• Still cut on forward angles (coscm > 0.5) as done by SAPHIR

• Additional requirement: only 3 charged particles (+ 1 neutral)

• Note: 2/3 of the data (Ee = 2.4 and 2.9 GeV)

• A small peak appears at ~1.54 GeV, but with only 3-4 , depending on background

• Still small cross section!

Ohio Univ. analysis

26

Neutrino scattering

hep-ex/0309042

Reanalysis of bubble chamber experiments fromWA21, WA25, WA59,E180, E632

M(Ksp) spectrum

27

New proton target result from CLAS

D. P. Weygand HADRON03

4.8 < E < 5.4 GeV (g6c)

p n

28

To determine Spin and Parity• Polarize + and measure the K+ direction and the

neutron spin.

• Double or triple polarization experiment?

p

2

1,

2

1

2

1,

2

1 00 Y

2

1,

2

1

3

1

2

1,

2

1

3

2

2

1,

2

1 01

11 YY

ieY

Y

sin8

3

cos4

3

11

01

29

Photoproducion by linearly polarized photon Decay Plane //

if natural parity exchange (-1)J

Polarizationvector of

+

K-

+

Decay Plane if unnatural parity exchange -(-1)J (Pseudoscaler mesons)

In rest frame (Helicity frame)

Decomposition of• natural parity exchange• unnatural parity exchange

p K*+Eth = 2.65 GeV

4 detector TPC

p

or

K-

30

Conclusion & Outlook• Observation of Nariirow Peak in Missing

Mass of n → K- X.– Evidence for Narrow S=+1 baryon at LEPS at 1.54

GeV with a narrow width.– Confirmation from other facilities (CLAS(d)/Jlab,

　 DIANA/ITEP, 　 CLAS(p)/Jlab, SAPHIR/ELSA).

– Narrow S=+1 baryon state at 1.54 GeV is getting established.

– Further data taking with LD2 target finished at LEPS and scheduled at CLAS.

• Next things to do. – Determination of Spin and Parity. Is this really +?– Other pentaquark resonances ? (S=+1 or not)– More theoretical works including lattice are needed.

31

Conclusion & Outlook (cont.)• For further experimental study

– 4 Coverage . A new TPC (Readout system will be ready in a few month.)

– Photon energy upgrade (Max. 3 GeV) to study in K*(892) photo-production. (Use linearly polarized photons as a parity filter.)

– Measurement of a recoiled nucleon polarization OR a Polarized target. (Technically the latter is easier.)