1. New observations and Multiquark Candidates at BESII
Xu-Ai ZHUANG
(for BES Collaboration)
Institute of High Energy Physics (IHEP)
QCD and Hadronic interactions
La Thuile , Mar. 20, 2006

2. Outline Multi-quark Candidates at BESII  and  observation
A possible bound state: mass threshold enhancement in and new observation of X(1835).
mass threshold enhancement in
 mass threshold enhancement in J/  
 and  observation

3. 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.

4. BESII VC: xy = 100 m TOF: T = 180 ps  counter: r= 3 cm
MDC: xy = m BSC: E/E= 22 % z = 5.5 cm
dE/dx= 8.5 %  = 7.9 mr B field: T
p/p=1.7%(1+p2) z = 2.3 cm

5. World J/ and (2S) Samples (106)
Largest from BES
J/
(2S)
2001
2002

6. A possible ppbar bound state

7. Observation of an anomalous enhancement near the threshold of mass spectrum at BES II
J/ygpp
acceptance weighted BW
M= MeV/c2
G < 30 MeV/c2 (90% CL)
c2/dof=56/56
0.1
0.2
0.3
Phys. Rev. Lett. 91, (2003)   
M(pp)-2mp (GeV)
3-body phase space
acceptance

8. This narrow threshold enhancement is NOT observed in B decays
The structure in B decays is obviously different from the BES observation:
Belle
The structure in B decays is much wider and is not really at threshold.
It can be explained by fragmentation mechanism.
BES II
Threshold enhancement in J/ decays
is obviously much more narrow and
just at threshold, and it cannot be
explained by fragmentation mechanism.

9. 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)
Considering that decaying into ppbar is only from
the tail of X(1860) and the phase space is very small,
such a BR indicates X(1860) has large coupling to ppbar !

10. pp bound state (baryonium)?
There is lots & lots of literature about this possibility
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)
Datta, P.J. O’Donnell, PLB 567, 273 (2003)]
M.L. Yan et al., hep-ph/
B. Loiseau et al., hep-ph/
G.J. Ding and M.L. Yan, Phys. Rev. C 72, (2005)
deuteron:
baryonium:
attractive nuclear force
attractive force? + n + -
loosely bound 3-q 3-q color singlets with Md = 2mp- e
loosely bound
3-q 3-q color singlets with Mb = 2mp-d ?
Observations of this structure in other decay modes are desirable.

11. New Observation of X(1835) in
PRL 95, (2005)

12. Observation of X(1835) in X(1835) 5.1  X(1835) 6.0  phase space

13. Mass spectrum fitting 7.7
The +- mass spectrum for  decaying into +- and  
7.7

14. Comparison of two decay modes
Mass and width from
m=1827.48.1MeV/c2 , =54.234.5MeV/c2
m=1836.37.9MeV/c2 , =70.323.1MeV/c2
The mass, width and branching fractions obtained from two different decay modes are consistent with each other.

15. 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 =  6.7 MeV
 < 153
In good agreement with X(1835)

16. 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.

17. Observation of mass threshold enhancement in
Phys. Rev. Lett. 93, (2004)   

18. Observation of an anomalous enhancement near the threshold of mass spectrum at BES II
3-body phase space
For a S-wave BW fit: M = 2075 12  5 MeV
Γ = 90  35  9 MeV

19. 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

20. K mass threshold enhancement

21. Observation of a strong enhancement near the threshold of mass spectrum at BES II
NX*
BES II
PS, eff. corrected
(Arbitrary normalization)

22. 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 ~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.

23. 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)
Ecm – ( MΛ+MK ) (MeV)

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

25. Observation of  mass threshold enhancement
Submitted to Phys. Rev. Lett., hep-ex/

26. We studied DOZI process:
J/    +  
 
+-0 K+ K-

27. Clear  and  signals   
-sideband
recoiling against 

28. Dalitz Plot

29. A clear mass threshold enhancement is observed
Acceptance

30. Side-bands do not have mass threshold enhancement

31. 0++ is favored over 0-+ and 2++
Partial Wave Analysis is performed
0++ is favored over 0-+ and 2++

32. Further look in , K*K*,  …. are desirable !
Is it the same 0++ observed in KK mass of J/KK? It is a multiquark state, a glueball, or a hybrid?
Further look in , K*K*,  …. are desirable !
hep-ph/ , hep-ph/

33. σ and κ observations

34. The  pole in at BESII   Averaged pole position: MeV M() M(+-)0  
Averaged pole position:
MeV
Phys. Lett. B 598 (2004) 149

35. κ
Phys. Lett. B 633 (2006) 681

36. 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.

37. Summary (II) 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

38. Thank You!

39. f0(1710) PWA analysis shows one scalar in 1.7 GeV region
Phys. Rev. D 68 (2003)

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

41. 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.

42. NO strong dynamical threshold enhancement in cross sections (at LEAR)
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 !
|M|2
|M|2
BES
BES
Both arbitrary normalization
Both arbitrary normalization

43. 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.

44. In ppbar collision, the background is much larger (I)
J/ decays do not suffer large t-channel “background” as ppbar collision.
>>

45. In ppbar collision, the background is much larger (II)
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
I=1 S-wave
P-wave
I=0 S-wave
A.Sibirtsev, J. Haidenbauer, S. Krewald, Ulf-G. Meißner, A.W. Thomas, Phys.Rev.D71:054010, 2005

46. 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

47. Pure FSI disfavored (I)
Theoretical calculation (Zou and Chiang, PRD (2003)) shows: The enhancement caused by one-pion-exchange (OPE) FSI is too small to explain the BES structure.
The enhancement caused by Coulomb interaction is even smaller than one-pion-exchange FSI.
|M|2
|M|2
BES
BES
Both arbitrary normalization
Both arbitrary normalization
one-pion-exchange FSI
Coulomb interaction

48. 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
P-wave
I=0 S-wave

49. Pure FSI disfavored (II)
I=0 S-wave FSI CANNOT fit the BES data.

50. So, pure FSI is disfavored
So, pure FSI is disfavored. However, we do not exclude the contribution from FSI.

51. From B.S. Zou, Exotics 05: From A. Sirbirtsev :
`pp near threshold enhancement is very likely
due to some broad sub-threshold 0-+ resonance(s)
plus FSI.
From A. Sirbirtsev :
FSI factors should be included in BW fit.

52. 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 =  6.7 MeV
 = 0  93 MeV

53. What do we expect from J/psigamma 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.

54. Observation of X(1835) in
Statistical Significance 7.7 
The +- mass spectrum for  decaying into +- and  

55. Similar enhancement also observed in
4 away from phase space.

56. This enhancement is NOT observed in process at SAPHIR

57. It is unlikely to be N*(1535).
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.

58. Discussion on KΛ mass threshold enhancement NX(1610)
NX(1610) has strong coupling to KΛ:
From (S&D-wave decay) 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)

60.    sideband w signal with multiple entries w signal after best
BES II Preliminary  
M(+-0)(GeV/c2)
M(+-0)(GeV/c2)
M(+-0)(GeV/c2)
M(+-0)(GeV/c2)
 sideband
w signal with
multiple entries
w signal after best
candidate selection
(best w masses)

61. BES II Preliminary
M()
Eff. curve
Side-band
Phase Space

62. PWA Fit results Total f2(1910) f2(160) M() dominated by (1760)
BES II Preliminary
Total
(1760)
> 10  f0
6.5 
f0(1710)
f0(1790)
f0(1810)
(1760)
f0(1710)
Using observed
mass and width
for f0(1810)
in J/y  g w
f2(1910)
f2(1910)
5.8 
f2(160)
f2(1640)
5.5  BG BG
M()
dominated by (1760)
a 0++ is needed (6.5 )
Being submitted to PRD.