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Observation of near-threshold enhancement at BES HongXun Yang Representing BES Collaboration IHEP [email protected] September 26- 30, 2004 K p p p , ,

Observation of near-threshold enhancement at BES

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Observation of near-threshold enhancement at BES. HongXun Yang Representing BES Collaboration IHEP [email protected] September 26- 30, 2004. Outline. Introduction threshold enhancement in threshold enhancement in - PowerPoint PPT Presentation

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Page 1: Observation of     near-threshold enhancement                        at BES

Observation of near-threshold enhancement at BES

HongXun YangRepresenting BES Collaboration

[email protected]

 September 26- 30, 2004

Kppp ,,

Page 2: Observation of     near-threshold enhancement                        at BES

Outline

• Introduction

• threshold enhancement in

• threshold enhancement in

• threshold enhancement in

• Summary

pp

p

K

ppJ /

pKJ /

pKJ /

Page 3: Observation of     near-threshold enhancement                        at BES

Introduction

BESII Detector

Data Sample

Page 4: Observation of     near-threshold enhancement                        at BES

BESII

VC: xy = 100 m TOF: T = 180 ps counter: r= 3 cm MDC: xy = 220 m BSC: E/E= 22 % z = 5.5 cm dE/dx= 8.5 % = 7.9 mr B field: 0.4 T p/p=1.7%(1+p2) z = 2.3 cm

Page 5: Observation of     near-threshold enhancement                        at BES

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

J/ (2S)

0

10

20

30

40

50

60

MarkI I I DM2 BES I BES I I

0

2

4

6

8

10

12

14

MKI MKII MKIII CBAL BESI BESII

Page 6: Observation of     near-threshold enhancement                        at BES

Observation of threshold enhancement in

pp bound state (baryonium)?

ppJ /

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

pp

Page 7: Observation of     near-threshold enhancement                        at BES

Near pp threshold enhancement in

enhancement

c

ppJ /

Page 8: Observation of     near-threshold enhancement                        at BES

Fit Result

M=1859 MeV/c2

< 30 MeV/c2 (90% CL)

J/pp

M(pp)-2mp (GeV)

0 0.1 0.2 0.3

3-body phase spaceacceptance

2/dof=56/56

fitted peak location

acceptance weighted BW +3 +5

10 25

BESII

Page 9: Observation of     near-threshold enhancement                        at BES

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

Page 10: Observation of     near-threshold enhancement                        at BES

• 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 dynamical threshold enhancement in cross sections (at LEAR)

pp

pp

pmppM 2)(

|M|2 |M|2BES BES

elasticelasticM ~|| 2 annlabann PM ~|| 2

Both arbitrary normalization Both arbitrary normalization

Page 11: Observation of     near-threshold enhancement                        at BES

Any inconsistency? NO!

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

At Ecm = 2mp + 6 MeV ( i.e., pLab = 150 MeV ), in elastic process, the resonant cross section is ~ 0.6 mb : much smaller than the continuum cross section ~ 94 20 mb .

Difficult to observe it in cross sections.

4/)(4

)(4

)12)(12(

)12(22

2

22

2

21

rescm

outin

pcmres mE

BB

mE

c

SS

J

pp

Page 12: Observation of     near-threshold enhancement                        at BES

Why can it be seen in J/ decays, but not in cross sections?

• Reason is simple:

J/ decays do not suffer large t-channel “background”. It is an s-channel effect !

pp

>>

p p p p

p

p p p p

p

/J

Page 13: Observation of     near-threshold enhancement                        at BES

Final State Interaction ? —— Not favored

1. Theoretical calculation (Zou and Chiang, PRD69 034004 (2003)) shows: The enhancement caused by one-pion-exchange (OPE) FSI is too small to explain the BES structure.

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

BES

one-pion-exchange FSI

|M|2 |M|2

Both arbitrary normalization

BES

pmppM 2)(

Both arbitrary normalization

Coulomb interaction

Page 14: Observation of     near-threshold enhancement                        at BES

Final State Interaction ? —— Not favored

Theoretical calculation might be unreliable, however, according to Watson’s theorem, we can use elastic scattering experiments to check the FSI effect, i.e.,if the BES structure were from FSI, it should be the same as in elastic scattering: But it is NOT ! FSI cannot explainthe BES structure.

/J

p

p

p p

p pelastic scattering

|M|2 BES

Both arbitrary normalization

elasticelasticM ~|| 2

pmppM 2)(

Page 15: Observation of     near-threshold enhancement                        at BES

Threshold Enhancements in J/ decays and B decays

• They may come from different mechanism:

There is “fragmentation” mechanism in B decays but NOT in J/ decays.

KppB

Belle

BES II

ppJ /

“Threshold” enhancement in B decays is much wider and is not really at threshold. It can be explained by fragmentation mechanism.

Threshold enhancement in J/ decays is obviously much more narrow and just at threshold, and it cannot be explained by fragmentation mechanism.

Page 16: Observation of     near-threshold enhancement                        at BES

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 other decay modes are desirable.

E. Fermi, C.N. Yang, Phys. Rev. 76, 1739 (1949)

…I.S. Sharpiro, Phys. Rept. 35, 129 (1978)C.B. Dover, M. Goldhaber, PRD 15, 1997 (1977)…A. Datta, P.J. O’Donnell, PLB 567, 273 (2003)]M.L. Yang et al., hep-ph/0405087

Page 17: Observation of     near-threshold enhancement                        at BES

Observation of threshold enhancement in

p

pKJ /

Phys. Rev. Lett. 93, 112002 (2004)   

Page 18: Observation of     near-threshold enhancement                        at BES

High Purity of Signal after Selection

• It can be shown by the clean Λ signal

• MC background study: only 1~2%

Dominantly from

Data

..ccpK

Data/MC

Page 19: Observation of     near-threshold enhancement                        at BES

Strong enhancement near the threshold of pm

Phase Space

Page 20: Observation of     near-threshold enhancement                        at BES

Observed in both of and pmpm

pM pM

pK Kp

Page 21: Observation of     near-threshold enhancement                        at BES

S-wave BW fit results

• M = (2075 12 5) MeV

Γ = (90 35 9) MeV

BR = (5.9 1.4 2.0) 10-5

2/d.o.f = 31.1/26 ~ 7σstatistical significance

Page 22: Observation of     near-threshold enhancement                        at BES

Similar enhancement also observed in pK'

Fix the parameters, 4 away from phase space.

Page 23: Observation of     near-threshold enhancement                        at BES

• MC (phase space) also show non-uniform and asymmetric distribution of .

• The enhancement is consistent with S wave.

The distribution is consistent with S-Wave

pcos

)15.2( .. GeVM ccp pcos

Err: DataHis: MC

pcos

Page 24: Observation of     near-threshold enhancement                        at BES

Observation of threshold enhancement in

K

pKJ /

Page 25: Observation of     near-threshold enhancement                        at BES

)(GeV/c2KΛM

Eve

nts

/10

MeV

Nx

Nx

Nx

)(GeV/c2 MMM KKΛ

PS, eff. corrected

(Arbitrary normalization)

Near-threshold enhancement in MK

Page 26: Observation of     near-threshold enhancement                        at BES

We perform PWA studies on the KΛ mass threshold structure:

The most important we want to study is its production BR

Page 27: Observation of     near-threshold enhancement                        at BES

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

Page 28: Observation of     near-threshold enhancement                        at BES

• 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)

Ln

L

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

Page 29: Observation of     near-threshold enhancement                        at BES

Dalitz plot (data)

Dalitz plot (PWA)

Eve

nts

/10M

eV Crosses: data

Hist.: PWA fit projection

Page 30: Observation of     near-threshold enhancement                        at BES

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

Page 31: Observation of     near-threshold enhancement                        at BES

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

Page 32: Observation of     near-threshold enhancement                        at BES

• A strong enhancement is observed near the mass threshold of MK at BES II.

• Preliminary PWA with various combinations of possible N* and Λ* in the fits —— The structure Nx*has:

Mass 1500~1650MeV

Width 70~110MeV

JP favors 1/2-

consistent with N*(1535)

The most important is:

It has large BR(J/ψ pNX*) BR(NX* KΛ) 2 X 10-4 ,

suggesting NX*has strong coupling to KΛ.

indicating it could be a KΛ molecular state

(5 - quark system).

Page 33: Observation of     near-threshold enhancement                        at BES

• A unique very narrow threshold enhancement is observed in decays at BES II:– It is not observed in elastic cross section

it cannot be explained by FSI.

– It is obviously different from the structure observed in B decays and it cannot be explained by fragmentation.

• We need to understand the nature of the strong anomalous threshold enhancements in J/ decays: multiquark states or other dynamical mechanism ? (keeping in mind that there are no strong threshold enhancements in many cases)

pp

ppppJ /

Kppp ,,

Summary

Page 34: Observation of     near-threshold enhancement                        at BES

THANK YOU!

Page 35: Observation of     near-threshold enhancement                        at BES

BES-I Resultpp/J

Threshold enhancement

But NOT claimed in Phys. Rev. Lett. 76(1996) 3502

Too small statistics to draw any conclusion on the threshold enhancement

Page 36: Observation of     near-threshold enhancement                        at BES

• 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

Page 37: Observation of     near-threshold enhancement                        at BES

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

Page 38: Observation of     near-threshold enhancement                        at BES

• This is due to acceptance• It can be shown in distribution, where is

the decay angle of p in

Why Dalitz plot not uniform for events GeVM ccp 15.2..

pcos

p

..ccp

MCDATA

Page 39: Observation of     near-threshold enhancement                        at BES

Interference of excited baryons?

• PWA fits with pure N* and Λ* can hardly reproduce the enhancement. (with reasonable constrains production rate for excited baryons)

• PWA fit with X(2075) can easily reproduce the enhancement with high significance. (independent of constrains)

It is unlikely that the enhancement is purely from * and N* interference.

Page 40: Observation of     near-threshold enhancement                        at BES

Systematic uncertainties

Page 41: Observation of     near-threshold enhancement                        at BES

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(2075)

N(1535 ) =NX

Page 42: Observation of     near-threshold enhancement                        at BES

Is the STRONG threshold enhancement universal in J/ decays ? —— NO !

• Actually in many other cases we do NOT see STRONG threshold enhancements !

• For example: In J/ decays at BES II

np J /

)( pM

)(KKMpmppM 2)(

KKJ /

)( pKM

pKJ /

0/ ppJ

Page 43: Observation of     near-threshold enhancement                        at BES

Conclusion

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

a 1.52~1.62 110 22.3(39.5) 1210(2146) -940(-924)

b 1.56 80~110 44.4(48.5) 2412(2635) -845(-839)

c 1.62 70 14.7(26.0) 799(1413) -952(-932)

d 1.6~1.64 70 17.1(23.2) 929(1260) -880(-873)

e 1.57 90 20.6(31.3) 1119(1701) -957(-946)

f 1.62 70~90 19.9(19.3) 1081(1049) -970(-961)

g 1.58 80 15.6(16.6) 845(902) -954(-951)

h(free) 107.7 5851 -963

h(fixed) 51.5 2798 -945

i(free) 68.6 3727 -901

i(fixed) 39.8 2162 -898

j 72.0 3912 -1028

Total 1.5~1.65 70~110 >14.7 >800