new observations and multiquark candidates at besii

New observations and Multiquark Candidates at

BESII

Xu-Ai ZHUANG(for BES Collaboration)

Institute of High Energy Physics (IHEP)zhxa@mail.ihep.ac.cn

QCD and Hadronic interactions

La Thuile , Mar. 20, 2006

Outline Multi-quark Candidates at BESII

• A possible bound state: mass threshold enhancement in and new observation of X(1835).

• mass threshold enhancement in

• mass threshold enhancement in

mass threshold enhancement in J/

and observation

ppppJ /

pKJ /K

p pKJ /

pp

Multi-quark State, Glueball and Hybrid

Hadrons consist of 2 or 3 quarks:

Naive Quark Model:

New forms of hadrons:• Multi-quark states :Number of quarks >= 4 • Hybrids : qqg, qqqg …• Glueballs : gg, ggg …

Meson: q q

Baryon: q q q

They have been searched for experimentally for a very long time, but none is established.

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 DM 2 BES I BES I I0

2

4

6

8

10

12

14

MKI MKII MKIII CBAL BESI BESII

A possible ppbar bound state

Observation of an anomalous enhancement near the threshold of mass spectrum at BES II

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 space acceptance

2/dof=56/56

acceptance weighted BW +3 +5

10 25

pp

BES II

Phys. Rev. Lett. 91, 022001 (2003)   

This narrow threshold enhancement is NOT observed in B decays

The structure in B decays is obviously different from the BES observation:

KppB

Belle

BES II

ppJ /

The structure 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.

X(1860) has large BR to ppbar

We (BES) measured:

From Crystal Ball result, we estimate:

So we would have:

(This would be the largest BR to ppbar among all known mesons)

5107~))1860(())1860(/( ppXBRXJBR

3102))1860(/( XJBR

%4))1860(( ppXBR

Considering that decaying into ppbar is only fromthe tail of X(1860) and the phase space is very small, such a BR indicates X(1860) has large coupling to ppbar !

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 possibilityE. 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. Yan et al., hep-ph/0405087B. Loiseau et al., hep-ph/0411218G.J. Ding and M.L. Yan, Phys. Rev. C 72,

015208 (2005)…

Observations of this structure in other decay modes are desirable.

New Observation of X(1835) in

J

PRL 95, 262001 (2005)

Observation of X(1835) in J

'

'

'/J

'

' '/J

X(1835)6.0

X(1835)5.1

phase space

phase space

Mass spectrum fitting

54264obsN

MeVM 7.21.67.1833

MeV7.73.207.67

410)4.04.02.2()()( XBXJB

7.7The +- mass spectrum for decaying into +- and

Comparison of two decay modes

Mass and width from m=1827.48.1MeV/c2 , =54.234.5MeV/c2

Mass and width from m=1836.37.9MeV/c2 , =70.323.1MeV/c2

)( J

)( J

)( J410)7.08.1()()( XBXJB

)( J410)5.03.2()()( XBXJB

The mass, width and branching fractions obtained from two different decay modes are consistent with each other.

Re-fit to J/p pbar including FSI

Include FSI curve from A.Sirbirtsev et al. ( Phys.Rev.D71:054010, 2005 ) in the fit (I=0)

M = 1830.6 6.7 MeV

< 153 MeV @90%C.L.

ppp mM 2

In good agreement with X(1835)

A Possible ppbar Bound State X(1835) could be the same structure as ppbar

mass threshold enhancement.

It could be a ppbar bound state since it dominantly decays to ppbar when its mass is above ppbar mass threshold.

Its spin-parity should be 0-+: this would be an important test.

Observation of mass threshold

enhancement in

p

pKJ /

Phys. Rev. Lett. 93, 112002 (2004)   

Observation of an anomalous enhancement near the threshold of mass spectrum at BES IIp

BES II pKJ /

3-body phase space

For a S-wave BW fit: M = 2075 12 5 MeV Γ = 90 35 9 MeV

Possible Interpretations FSI? Theoretical calculations are needed. Conventional K* or a multiquark resonance?

• Search for its Kπ , Kππ decay modes would help to understand its nature.

• We are now studying J/ KKπ , KKππ

K mass threshold enhancement

)(GeV/c2ΛK

M

PS, eff. corrected

MMM KΛK

Observation of a strong enhancement near the threshold of mass spectrum at BES IIK

(Arbitrary normalization)

BES II pKJ /

NX*

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-

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.

A ΛK resonance predicted by chiral SU(3) quark model

Based on a coupled-channel study of ΛK and ΣK states in the chiral SU(3) quark model, the phase shift shows the existence of a ΛK resonance between ΛK and ΣK mass threshold.

Phys.Rev. C71 (2005)

064001

Ecm – ( MΛ+MK ) (MeV)

The KΛ mass threshold enhancement NX(1610) could be a KΛ bound/resonant state.

Observation of mass threshold enhancement

Submitted to Phys. Rev. Lett., hep-ex/0602031

We studied DOZI process:

J/ + + +-0 K+ K-

Clear and signals

recoiling against

-sideband

Dalitz Plot

A clear mass threshold enhancement is observed

Acceptance

Side-bands do not have mass threshold enhancement

Side-bands

Partial Wave Analysis is performed

0++ is favored over 0-+ and 2++

410)65.027.061.2(

)()/(

XBrXJBr

2

21926

MeV/c 2820105

MeV/c 181812

M

Is it the same 0++ observed in KK mass of J/KK? It is a multiquark state, a glueball, or a hybrid?

hep-ph/0602172, hep-ph/0602190

Further look in , K*K*, …. are desirable !

σ and κ observations

The pole in at BESII

/J

M()

M(+-0)

M(+-

)

0

Averaged pole position:(541 39) (252 42)i MeV

Phys. Lett. B 598 (2004) 149

κ

24872

8173 MeV/c )45309()30841(

i

Phys. Lett. B 633 (2006) 681

Summary (I) A very narrow and strong mass

threshold enhancement is observed in decays at BES II:• It is not observed in B decays.• Its large BR to suggests it be a

bound state.

X(1835) is observed in It could be same structure as the ppbar mass threshold enhancement, i.e., it could be a ppbar bound state.

pp

ppJ /

pppp

J

mass threshold enhancement was observed in

Evidence of NX(1610) was observed near KΛ mass threshold, suggesting a KΛ bound or resonant state.

An mass threshold enhancement f0(1810) was observed in J/

and are observed at BESII

Summary (II)

p pKJ /

Thank You!

KKJ /

00/ ss KKJ

)1710(0f

)1710(0fMeV 166

MeV 41740155108

1025

M

PWA analysis shows one scalar in 1.7 GeV region

Phys. Rev. D 68 (2003) 052003

f0(1710)

Crystal Ball results on inclusive photon spectrum of J/psi decays

3109.0~)/( JBR

3103.4~)'/( JBR

3103~))1440(/(

50~

JBR

MeV

3102)/(

)18601830(

XJBR

X

The large BR to ppbar suggest it could be an unconventional meson

For a conventional qqbar meson, the BRs decaying into mesons are usually at least one order higher than decaying into baryons.• There are many examples in PDG. E.g.

So the large BR to ppbar (with limited phase space from the tail of X(1830)) seems very hard to be explained by a conventional qqbar meson.

310)4.03.1()(

)%9.06.2()(

ppBR

BR

c

c

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 experimentally.

4/)(4)(4

)12)(12()12(

22

2

22

2

21

rescm

outin

pcmres mE

BBmEc

SSJ

pp

J/ decays do not suffer large t-channel “background” as ppbar collision.

>>

p p p p

p

p p p p

p

/J

In ppbar collision, the background is much larger (I)

A.Sibirtsev, J. Haidenbauer, S. Krewald, Ulf-G. Meißner, A.W. Thomas, Phys.Rev.D71:054010, 2005

P-waveI=0 S-wave

I=1 S-wave

In ppbar elastic scattering, I=1 S-wave dominant,while in J/ radiative decays I=0 S-wave dominant.

ppbar elastic cross section near threshold

In ppbar collision, the background is much larger (II)

So, the mechanism in ppbar collision is quite different from J/ decays and the background is much smaller in J/ decays

It would be very difficult to observe an I=0 S-wave ppbar bound state in ppbar collisions if it exists.

J/ decays (in e+e- collider) have much cleaner environment: “JP, I” filter

Pure FSI disfavored (I)

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

FSI Factors

Most reliable full FSI factors are from A.Sirbirtsev et al. ( Phys.Rev.D71:054010, 2005 ) ， which fit ppbar elastic cross section near threshold quite well.

ppbar elastic cross section near threshold

I=1 S-wave

I=0 S-waveP-wave

pmppM 2)(

Pure FSI disfavored (II)

I=0 S-wave FSI CANNOT fit the BES data.

ppp mM 2

58/192../2 fod

So, pure FSI is disfavored.

However, we do not exclude the contribution from FSI.

pp near threshold enhancement is very likely due to some broad sub-threshold 0-+ resonance(s) plus FSI.

From B.S. Zou, Exotics 05:

From A. Sirbirtsev :FSI factors should be included in BW fit.

Re-fit to J/p pbar including FSI

Include FSI curve from A.Sirbirtsev et al. ( Phys.Rev.D71:054010, 2005 ) in the fit (I=0)

M = 1830.6 6.7 MeV

= 0 93 MeV

ppp mM 2

What do we expect from J/psigamma ppbar results?

The baryonium interpretation of the ppbar mass threshold enhancement predicts a new particle around 1.85 GeV which should be observed in other decay mode with full BW resonant structure.

Observation of X(1835) in

The +- mass spectrum for decaying into +- and

Statistical Significance 7.7

J

J

Similar enhancement also observed in pK'

4 away from phase space.

This enhancement is NOT observed in process at SAPHIR Kp

pKJBES/

Non-observation of NX in suggests an evidence of new baryon (personal view):

It is unlikely to be N*(1535). If NX were N*(1535), it should be observed

in process, since: •

• From PDG, for the N* in the mass range 1535~1750 MeV, N*(1535) has the largest , and from previous estimation, NX would also have almost the largest BR to KΛ.

Also, the EM transition rate of NXto proton is very low.

Kp

Kp)*()*()*( KNBRpNBRKNp

)*( pNBR

Discussion on KΛ mass threshold enhancement NX(1610)

NX(1610) has strong coupling to KΛ:• From (S&D-wave d

ecay) and is a P-wave decay, we can estimate

• From BESII,

• The phase space of NX to KΛ is very small, so such a big BR shows NX has very strong coupling to KΛ, indicating it has a big hidden ssbar component. (5-quark system)

3102)/( ppJBR )1600(/ XpNJ

3100.1)/( XpNJBR

4102~)()/( KNBRpNJBR XX

%20)( KNBR X

0

/

J

M(+-0)(GeV/c2) M(+-0)(GeV/c2) M(+-0)(GeV/c2)

M(

+ -

0 )(G

eV/c

2 )

sideband

BES II Preliminary

signal after best candidate selection (best masses)

signal with multiple entries

Eff. curvePhase Space

Side-band

M()

BES II Preliminary

(1760) f0(1710)f0(1790)f0(1810)

PWA Fit results

M()

Total

f2(1910) f2(160) BG

BES II Preliminary

Being submitted to PRD.

Using observed mass and width for f0(1810)in J/

f0(1710)

dominated by (1760) a 0++ is needed (6.5 )

f2(1910) 5.8

f2(1640) 5.5

BG

(1760) > 10

f0 6.5