observation of near-threshold enhancement at bes

Observation of near-threshold enhancement at BES

HongXun YangRepresenting BES Collaboration

IHEPyanghx@mail.ihep.ac.cn

 September 26- 30, 2004

Kppp ,,

Outline

• Introduction

• threshold enhancement in

• threshold enhancement in

• threshold enhancement in

• Summary

pp

p

K

ppJ /

pKJ /

pKJ /

Introduction

BESII Detector

Data Sample

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

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

Observation of threshold enhancement in

pp bound state (baryonium)?

ppJ /

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

pp

Near pp threshold enhancement in

enhancement

c

ppJ /

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

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

• 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

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

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

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

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)(

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.

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

Observation of threshold enhancement in

p

pKJ /

Phys. Rev. Lett. 93, 112002 (2004)   

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

Strong enhancement near the threshold of pm

Phase Space

Observed in both of and pmpm

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

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

Similar enhancement also observed in pK'

Fix the parameters, 4 away from phase space.

• 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

Observation of threshold enhancement in

K

pKJ /

)(GeV/c2KΛM

Eve

nts

/10

MeV

Nx

Nx

Nx

)(GeV/c2 MMM KKΛ

PS, eff. corrected

(Arbitrary normalization)

Near-threshold enhancement in MK

We perform PWA studies on the KΛ mass threshold structure:

The most important we want to study is its production BR

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)

Ln

L

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

Dalitz plot (data)

Dalitz plot (PWA)

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

• 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).

• 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

THANK YOU!

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

• 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

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

• 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

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.

Systematic uncertainties

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

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

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