Pentaquark search and other multiquark candidates at BES

Xiaoyan SHEN

Representing BES Collaboration

Institute of High Energy Physics, CASshenxy@ihep.ac.cn

ICHEP 2004August 16 - 22, 2004, Beijing, P. R. China

Outline Introduction

Pentaquark searches at BES

Other multiquark candidates at BES

* pp threshold enhancement in

* p threshold enhancement in

* K threshold enhancement in

Summary

ΛpKJ/ψ ΛpKJ/ψ

pγpJ/ψ

0

10

20

30

40

50

60

CBAL MK II MK III DM2 BES I BES II

BESII Detector

VC: xy = 100 m TOF: T = 180 psMDC: xy = 220 m BSC: E/E= 21 % dE/dx= 8.5 % = 7.9 mr p/p=1.78(1+p2) z = 2.3 cm counter: r= 3 cm B field: 0.4 T z = 5.5 cm

Introduction

0

2

4

6

8

10

12

14

MKI MKII MKIII CBAL BESI BESII CLEOc

J/

(2S)

World J/ and (2S) Samples (×106) BESII

58M J/

BESII 14M (2S)

Pentaquark searches at BES

not forbidden by QCD:

definite evidence of pentaquark states would be an important addition to our understanding of QCD.

a baryon with S=+1 is a natural candidate

+(1540)

experimental status of +(1540)

(LEPS) nK

MΘ

0SpK

0SpKnK

“Positive” experiments: many

“Negative” experiments: many

BES: ALEPH: RΛ*(1520) < 0.1 DELPHI: L3: HERA-B: RΛ*(1520) < 0.02 SPHINX: RΛ*(1520) < 0.02 CDF:

Need more experimental facts (through different processes)

BES: e+ e- collision; has relatively clean data samples with less backgrounds

investigate the pentaquark state in the hadronic decays of charmonium

pKnK

nKpK

nKpKnKpKJS

s

s

ss

or

or

))(( & ))((/ & )2(

(2S) KSpK-n and KSpK+n Decays

NK

pentaquarkNK

J/ KSpK-n and KSpK+n Decays

No clear pentaquark state (1540) (or ) is observed.

J/(2S)

Upper limits @ 90% C.L.

5106.1)/( nKpKpKJB ss

Upper limits @ 90% C.L.

51084.0

Multiquark candidates at BES

1. Near pp threshold enhancement in

enhancement

c

Phys. Rev. Lett., 91 (2003) 022001

ppJ /

Fit results

Mass: M=1859 MeV/c2

Width: < 30 MeV/c2 (90% CL)

J/pp

M(pp)-2mp (GeV)0 0.1 0.2 0.3

BG curve Eff. curve

2/dof=56/56

Fitted peak

Fitted curve +3 +510 25

BES II

MARK-III & DM2 Results

Threshold enhancement

pp/J

Claimed inPhys. Rep. 174(1989) 67-227

Too small statistics to draw any conclusion on the threshold enhancement,

e.g., cannot exclude known particles such as (1760)

MARK-III

DM2

Belle sees low-mass pp systems in B decays

ppM

pp (GeV)M

BD0pp

BppK

Statistics is also low.

This enhancement is important:

FSI not favored mass≤2mp ， width is narrow Hard to be explained as a conventional qq meson

pp bound state (baryonium)?

+ n +

deuteron:

loosely bound 3-q 3-

q color singlets with Md = 2mp-

baryonium:

loosely bound

3-q 3-q color singlets with Mb = 2mp-

?

attractive nuclear force attractive force?

There is lots & lots of literature about this possibility

Observations of this structure in otherdecay modes are desirable.

2. Observation of an enhancement near mass threshold in process

p pKJ /

The clear Λ signal in data shows high purity of signal.

Data/MC

..ccpM ..ccpM

Phase Space

Data Data

pM pM

pK Kp

S-wave BW fit results

M = (2075 12 5) MeV

Γ = (90 35 9) MeV

BR = (5.9 1.4 2.0) 10-5

• M = (2044 17) MeV Γ = (20 45) MeV• 2/d.o.f = 32.5/26

P-wave BW fit results

The systematic errors arecarefully studied in S-wave case.

2/d.o.f = 31.1/26 About 7σ statistical significance high L hypotheses fail

Accepted by P. R. L.

Hep-ex/0405050

Near threshold enhancement is also observed inp pKS)2(

Fix the parameters, 4 away from phase space.

Accepted by P. R. L. Hep-ex/0405050

3. Near threshold enhancement in..ccK

../ ccpKJ

)(GeV/c2KΛM

Eve

nts

/10

MeV

Nx

Nx

Nx

)(GeV/c2 MMM KKΛ

PS, eff. corrected

(Arbitrary normalization)

PWA is performed to pKJ /

possible N* and *states listed in PDG are fitted N(1720), N(1900), (1520), (1690), …

many different combinations are tried

different form factors are used

different JP of Nx is tried

also tried N(1535) to fit Nx

Mass and Width scan

M 1520 – 1620 MeV

110 MeV

JP = 1/2-

Total fit (S=-952)

Nevent:

Fraction Nevent

NX 22% 1210

Mass scan(GeV/c2)

An example:

Width scan(GeV/c2)L

n

L

N(1720), N(1900), (1520), (1690) …. included in the PWA fit

Dalitz plot (data)

Dalitz plot (MC)

Eve

nts

/10M

eV Crosses: data

Hist.: PWA fit projection

JP check with various combinations

JP ½- ½+ 3/2- 3/2+ non A -940 -848 -848 -930 -813 B -845 -783 -806 -833 -752 C -952 -841 -844 -916 -768 D -880 -768 -752 -822 -650 E -957 -889 -893 -944 -875 F -970 -920 -925 -963 -919 G -954 -925 -919 -944 -909

fit results

Cases Mass(GeV) Width(MeV) Fraction(%) Nevent Log Likelyhood

a 1.52 ~ 1.62 110 22.3 1210 -940

b 1.56 80 ~110 44.4 2412 -845

c 1.62 70 14.7 799 -952

d 1.6 ~1.64 70 17.1 929 -880

e 1.57 90 20.6 1119 -957

f 1.62 70 ~ 90 19.9 1081 -970

g 1.58 80 15.6 845 -954

1.50 ~1.65 70 ~110 >14.7 >800

An abnormal enhancement is observed in the mass threshold of MK

Its mass and width:

(large uncertainty near threshold, high statistics is crucial!)Mass 1500~1650MeV

Width 70~110MeV

JP favors 1/2-

large BR(J/pNx)Br(NxK) (2*10-4). What is it??

Possibly N*(1535) with large coupling to K K molecular state ??

no (1540) seen in both J/ and (2S) hadronic decays.

a near pp threshold enhancement observed in J/pp decay. Baryonium??

a near p threshold enhancement observed in both

J/ pK and (2S) pK decays.

a near K threshold enhancement observed in

J/pK. large coupling to K. K molecular state??

Summary

FSI ? —— Not favored1. Theoretical calculation (Zou and Chiang, PRD69 034004 (2003)) sh

ows that the enhancement caused by one-pion-exchange FSI is too small to explain the BES structure.

2. For FSI, the BES structure should be the same as in elastic scattering (Bugg, hep-ph/0406293): But it is NOT !

BES

elasticelasticM ~|| 2

BES

one-pion-exchange FSI

pmppM 2)(

|M|2 |M|2

Both arbitrary normalization Both arbitrary normalization

The enhancement caused by Coulomb interaction is even smaller than one-pion-exchange FSI !

With threshold kinematic contributions removed, there are very smooth threshold enhancements in elastic “matrix element” and very small enhancement in annihilation “matrix element”:

much weaker than what BES observed !

NO strong threshold enhancement in collision (at LEAR)

pp

pp

pmppM 2)(

|M|2 |M|2BES BES

elasticelasticM ~|| 2 annlabann PM ~|| 2

Both arbitrary normalization Both arbitrary normalization

Any inconsistency? NO!

For example: with Mres = 1859 MeV, Γ = 30 MeV, J=0, BR(ppbar) ~ 10%, based on:

At Ecm = 2mp + 6 MeV ( i.e., pLab = 150 MeV ), in elastic process, the resonant cross section is only ~ 0.6 mb : much smaller than the continuum cross section

~ 94 +/- 20 mb

Very difficult to observe it in collisions.

4/)(4

)(4

)12)(12(

)12(22

2

22

2

21

rescm

outin

pcmres mE

BB

mE

c

SS

J

pp

PWA of the near-threshold enhancement(NX) in mK

PWA with: a: NX,N(1720),N(1900), (1520), (1570), (1690), (1810), X(2075) b: NX,N(1720),N(1900) ， (1520), (1690), (1810), X(2075) c: NX,N(1720),N(1900), (1520), (1570), (1690), (1890),X(2075) d: NX,N(1720),N(1900), (1520), (1690), (1890),X(2075) e: NX,N(1720),N(1900), (1520), (1570), (1690), (1810), (1890),X(2075) f: NX,N(1720),N(1900),N(2050), (1520), (1570), (1690), (1810), (1890),X(2075) g: NX,N(1720),N(1900),N(2050), (1520), (1570), (1690), (1810), (1890)J(p) 1/2(-) 3/2(+) 3/2(+) 3/2(+) 3/2(-) 1/2(-) 3/2(-) 1/2(+) 3/2(+) 1(-)m(GeV)1.535,1.720, 1.900, 2.050, 1.5195, 1.570, 1.690, 1.810, 1.890, 2.080

(GeV) 0.150, 0.150 0.300, 0.200, 0.0156, 0.070, 0.060, 0.150, 0.100, 0.080 h:N(1535),N(1650),N(1720),N(1900), (1520), (1570), (1690), (1810), (1890),X(2075) i: N(1535),N(1650),N(1720),N(1900), (1520), (1690), (1810), (1890),X(2075)

J(p) 1/2(-) 1/2(-), mN(1650)=1.650, N(1650)=0.150 j: 18Res All possible N* and *states listed in PDG and N(1900)(3/2-),N(2050)(1/2+,3/2+), (1570), X(2

075)

N(1535 ） =NX