1

Overview of Pentaquark Searches

T. NakanoRCNP, Osaka University

SQM2006 @ UCLA, March 27, 2006

2

Outline

• Introduction

• Status of + study

• New results from LEPS

• Other recent results

• Summary

3

Pentaquark

M1890-180*Y] MeV

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+

The antiquark has a different flavor than the other 4 quarks.

4

•DPP predicted the with M=1530MeV, <15MeV, and Jp=1/2+.

•Naïve QM (and many Lattice calc.) gives M=1700~1900MeV with Jp=1/2-.

•But the negative parity state must have very wide width (~1 GeV) due to “fall apart” decay.

Theory

For pentaquark

Fall apart

Ordinary baryons

qq creation

•Positive parity requires P-state excitation.

•Expect state to get heavier.

•Need counter mechanism.

diquark-diquark, diquark-triquark, or strong interaction with “pion” cloud?

Positive Parity?

5

Time dependent experimental status of +

γ + d (n ) reactions

γ + p → p Ks0 　

γ + p → n K+ K- p+

K + (N) → p Ks0

lepton + D, A → p Ks0

p + A → pKs0 + X

p + p → pKs0 + +

Other + Upper Limits

BaBar

CLAS-d2

BELLE

ALEPH, ZSVD2

LEPS-d2LEPS-C CLAS-d1

DIANA

SAPHIR

SVD2

COSY-TOF

Hermes

JINR

CLAS-p

LEPS-d

BCZEUS

BES J,

CLAS g11

SPHINXHyperCP

HERA-B

FOCUS WA89CDF

: Positive result

: Negative result

2002 2003 　 2004 2005

Non-evidence for PentaquarksNon-evidence for Pentaquarks

FOCUS BABARBES

CDF

FOCUS

SPHINX

CDF DELPHI

HyperCPHERA-B

CDF

0c

--

--

+ more

7

Slope for mesons

Slope for baryons

Slope for pentaquarks??

8

M(nK+)(GeV)

Cou

nts

/4 M

eV

M(nK+)(GeV)

M(nK+)(GeV)

Cou

nts

/4 M

eV

Cou

nts

/4

MeV

-0.8 < cosCM < -0.6

preliminar

y

0.6 < cosCM < 0.8

CLAS: New high statistics exp.Search for + in pK+Ksn

R. De Vita, APS April meeting, 2005

0.2%)N(Λ

)N(Θ*

9

Impact of the CLAS proton result• A 5 evidence turned out to be wrong!

• If there is no large isospin asymmetry in the elementary process, the d and A experiments with lower statistics should not be able to see the signal, and they are also wrong.

t-channel K* exchange amplitude does not have isospin asymmetry. However, many quark model predicts . Moreover, suppression of K* exchange leads to small cross-section in general.

* KNK Ng g >

Theoretical attempts to explain large isospin asymmetry:

Nam, Hosaka and Kim, hep-ph/0502143, hep-ph/0503149[PRD], hep-ph/0505134

Lipkin and Karliner, hep-ph/05060840

(1520) (1520)p K n K → Λ → Λ?

K0

p

K*

+

K+

n

10

New and old CLAS data

Two distributions statistically consistent with each other:

26% c.l. for null hypothesis from the Kolmogorov test (two histograms are compatible).

Reduced 2=1.15 for the fit in the mass range from 1.47 to 1.8 GeV/c2

G10 mass distribution can be used as a background for refitting the published spectrum.

Pre

limin

ary

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Final state interactions

Minimum momentum of protons in CLAS >0.35 GeV/c.

Detecting the high energy spectator, proton in the reaction d’pK-K+n requires re-scattering (FSI).

The upper limit on the measured cross section in the reaction gd+pK-, with Pp>0.35 GeV/c, is about 450 pb (95.4% CL).

The upper limit on the cross section of the elementary process gn+K- is 4-20 nb, model dependent.

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From Carl Carlson’s talk at Hawaii pentaquark workshop

Don’t give up so easily...

pentaquark

negativeevidence

13

First evidence from LEPS

nK+K-nPhys.Rev.Lett. 91 (2003) 012002

hep-ex/0301020

+Low statistics: but

Tight cut: 85% of events are rejected by the exclusion cut.

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

Correction: Fermi motion correction is necessary.

4.6S

B= 3.2

S

S B=

+

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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

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Search for + in nK+K-n

MM (GeV) MM (GeV)

•A proton is a spectator (undetected).

• Fermi motion is corrected to get the missing mass spectra.

•Tight exclusion cut is essential.

•Background is estimated by mixed events.

preliminary

preliminary

L(1520)

pK+K-p nK+K-n

Co

un

ts/1

2.5

MeV

Co

un

ts/1

2.5

MeV

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+ search in d Λ(1520) KN reaction

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 is dominant in the reaction, the

final state KN has I=0. (Lipkin)

γ

p

n

Θ ＋

K －

p

Λ(1520)

detected

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A possible reaction mechanism

• + can be produced by re-scattering of K+.• K momentum spectrum is soft for forward going Λ(1520).

γ

p/n

n/p

Λ(1520)

K+/K0

missing momentum

Formation momentum

LD2

Pmiss GeV/c

•LEPS acceptance has little overlap with CLAS acceptance.•Exchanged kaon can be on-shell.

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Background process

• Quasi-free (1520) production must be the major background.

• The effect can be estimated from the LH2 data. γ

p

n

Λ(1520)

K+

n

• The other background processes which do not have a strong pK- invariant mass dependence can be removed by sideband subtraction.

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Co

un

ts/5

MeV

Sideband subtraction to remove non-resonant background

S = - 0.4

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

LD2 LH2

M(K － p) GeV/c2M(K － p) GeV/c2

Λ(1520) Λ(1520)

MMd(γ,K － p) GeV/c2

LD2

Fluctuations in the sideband spectra are removed by smearing Eg by 10 MeV (nearly equal to the resolution).

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BG estimation with two independent sideband regions

M(K － p) GeV/c2 MMd(γ,K － p) GeV/c2

MMd(γ,K － p) GeV/c2

correction for contribution

•Validity of the sideband method with E smearing was checked by using two independent regions of the sideband.

•Channel-to-channel comparison gives

mean=-0.04 and RMS=2.0.

Co

un

ts/5

MeV

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K － p missing mass spectrum

MMd(γ,K － p) GeV/c2

sideband

Λ*

sum

5S

S B+:

Co

un

ts/5

MeV

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

1.53-GeV peak:

pre

lim

inar

y

MMd(γ,K － p) GeV/c2

Co

un

ts/5

MeV

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

(in the 5 bin = 25 MeV)

pre

lim

inar

y

No visible signal in sidebands.

+

~

22

Remove high frequency fluctuations by 10-MeV E smearing

MM(K － p) GeV/c2MM(K － p) GeV/c2

Co

un

ts/5

MeV

Co

un

ts/5

MeV

pre

lim

inar

y

pre

lim

inar

y

23

M(K+K-) GeV/c2

Remove f background by rejecting events with Pp<0.55 GeV/c

MM(K － p) GeV/c2MM(K － p) GeV/c2

Co

un

ts/5

MeV

Co

un

ts/5

MeV

pre

lim

inar

y

pre

lim

inar

y

The + peak nor the bump at 1.6 GeV is not associated with events.

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•Normalization factor for LH2 data (green line) is 2.6. No large p/n asymmetry.

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

•Large cross-section compared withΛ(1520).

•Missing Mass resolution is worse.

• No excess at 1.53 GeV nor at 1.6 GeV.

MMd(γ, － p) GeV/c2

Co

un

ts/5

MeV

pre

lim

inar

y

Search for d Λ(1116) +

γ

pn

Θ ＋

－

p

Λ(1116)

forward angle detection

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K+ + Xe ++Xe’ (K0++p) +Xe’

DIANA/ITEP

Kaon Scattering

Secondary kaons produced in the detector materials and then interacting within bubble chamber.

More pictures were under analysis.

pbeam<445 MeV/c pbeam>525 MeV/c

445<pbeam< 525 MeV/c

hep-ex/0603017

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momentum, GeV/c1

/ 50

MeV

momentum spectra of K+ and K-

K+

n +

Momentum range possibly contributing to + formation.

BELLE – Low energy K+N scatteringe+e- K+/- X, K+/-A pK0, pK-

R. Mizuk

Detector Tomography

Determine resonance width

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Belle – Limit on + Width

397 fb-1

K+A pK0s Belle limit 90%CL

MeV (90% CL) @ M = 1.525–1.545 GeV

Not inconsistent with previous results.

from K+A pK0sX &

K+D inclusive analysis

Belle: < 0.64 MeV (90% CL) @ M = 1.539 GeVΓΘ+= ΔmNΘ+

Nch

σch

107mb Bi Bf

Cahn,Trilling,PRD69,11501 (2004).

mpKs (GeV/c2)

N /

2 M

eV/c

2

DIANA

28

Existence of the 3/2(1862) and the c0 is questionable. For

the both cases, only one experiment has observed the positive evidence, while other experiments claim that their null results are incompatible with them.

Other pentaquarks

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pK+ and pK- from 18.6 M d+Au at 200 GeVBackground – Combinatorial and Correlated Pairs

dAu results

M (GeV/c2)

++ p + K+

STAR Pentaquark Search

30

dAu results

The invariant mass distribution is fitted to a Gaussian plus a linear function. A 3.5-5.0 sigma signal is seen Measured mass is about 1.53 GeV/c2. Full width is about 15 MeV

++

31

Summary

• Recent null results restrict the possibility that a pentaquark exists severely. •If it exists, its production mechanism should be also exotic. •There are still some positive evidences which cannot be excluded completely.•Experiments with positive results should be repeated with higher statistics.•Formation experiment with low energy K+ beam will conclude the case for +.