1. New Observations and Multiquark Candidates at BESII
HongXun Yang
(for BES Collaboration)
Institute of High Energy Physics (IHEP)
CPS2006
BEIJING, Sept. 16, 2006

2. J/ decays are an ideal factory to search for and study light exotic hadrons:
The production cross section of J/ is high.
The production BR of hadrons in J/ decays are one order higher than ’ decays (“12% rule”).
The phase space to 1-3 GeV hadrons in J/ decays are larger than  decays.
Exotic hadrons are naively expected to have larger or similar production BR to conventional hadrons in J/ decays.
Clean background environment compared with hadron collision experiments, e.g., “JP, I” filter.

3. World J/ Sample (106)
Largest from BES
2001

4. Outline
A possible bound state: mass threshold enhancement in and new observation of X(1835).
mass threshold enhancement in
 mass threshold enhancement in J/  
New observation of a broad 1- - resonance in J/  K+K- 0

5. A possible ppbar bound state

6. 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

7. This narrow threshold enhancement is NOT observed in at CLEO
This result cannot be explained by pure FSI effect, since FSI is a universal effect.
FSI interpretation of the narrow and strong ppbar threshold enhancement is disfavored.
This indicates that X(1860) has a production property similar to ’ meson.
c.f.:
No enhancement
near threshold

8. This narrow threshold enhancement is NOT observed in at BESII
This again disfavors FSI and indicates that X(1860) has a production property similar to ’ meson.
c.f.:
This also indicates X(1860) may have strong coupling to gluons as ’ meson.
No narrow strong enhancement
near threshold

9. Pure FSI disfavored I=0 S-wave FSI CANNOT fit the BES data.
FSI curve from A.Sirbirtsev et al. ( Phys.Rev.D71:054010, 2005 ) in the fit (I=0)
FSI * PS * eff + bck

10. X(1860) has large BR to ppbar
We (BES) measured:
From Crystal Ball result, we etimate:
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 !

11. So far, it is only observed in J/ radiative decays:
Summary of the properties of the strong ppbar mass threshold enhancement X(1860)
So far, it is only observed in J/ radiative decays:
It has production properties similar to ’ meson.
It could have strong coupling to gluons as ’ meson.
It could have the largest decay BR to ppbar among all PDG particles:
It has strong coupling to ppbar.

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

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

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

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

18. Observation of an anomalous enhancement near the threshold of mass spectrum at BES II
3-body phase space
Phys. Rev. Lett. 93, (2004)   
For a S-wave BW fit: M = 2075 12  5 MeV
Γ = 90  35  9 MeV

19. K mass threshold enhancement

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

21. 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.

22. 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.
( F. Huang, Z.Y. Zhang et al.
Phys. Rev. C71: , 2005 )
Ecm – ( MΛ+MK ) (MeV)

23. The KΛ mass threshold enhancement NX(1610) could be a KΛ bound/resonant state
(5-quark system with hidden ssbar components).

24. Observation of  mass threshold enhancement

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

26. Clear  and  signals   
recoiling against 

27. Dalitz Plot

28. A clear mass threshold enhancement is observed
Acceptance
Phys. Rev. Lett. 96, (2006)   

29. The radiative decay of J/ has been observed in the 58M J/ data.
A significant structure of  has been found near the mass threshold.
PWA shows the structure favors 0++, with a mass , width 1052028 MeV, and the corresponding branch ration is (2.610.270.65)x10-4.
It could be a multiquark/hybrid/glueball state.
Its relation with f0(1710),f0(1790)?

30. Is the STRONG threshold enhancement universal/naïve in J/ decays
Is the STRONG threshold enhancement universal/naïve in J/ decays ? —— NO !
Actually in many other cases we do NOT see STRONG threshold enhancements !
For example: In J/ decays at BES II

31. New observation of a broad 1- - resonance in J/  K+K- 0
hep-ex/ , Submitted to Phys. Rev. Lett.

32. J/  K+K- 0
very clean 0 signal

33. J/  K+K- 0 PID and kinematic fit can significantly reduce
Background
PID and kinematic fit can significantly reduce
the dominant background from J/  + - 0.

34. Partial Wave Analysis of J/  K+K- 0 events
Parity conservations in J/  K+K- 0 requires that
spin-parity of K+K- should be 1--,3--,…
PWA fit with and
phase space (PS) gives ( preliminary ):
( can be ruled out by much worse likelihood )
X pole position
big destructive interference among and PS

35. Broad X cannot be fit with known mesons or their interference
It is unlikely to be (1450), because:
The parameters of the X is incompatible with (1450).
(1450) has very small fraction to KK. From PDG:
It cannot be fit with the interference of (770) , (1900) and (2150):
The log-likelihood value worsens by 85 (2=170).

36. How to understand broad X(1580)?
Search of a similar structure in J/  KSK  will help to determine its isospin.
X(1580) could have different nature from conventional mesons:
There are already many 1- - mesons nearby.
Width is much broader than other mesons.
Broad width is expected for a multiquark state.

37. Summary (I)
BES II has observed several strong mass threshold enhancements in J/ decays.
Why strong mass threshold structures are important?
Multiquark states may be only observable near mass thresholds with limited decay phase space.
 Otherwise, it might be too wide to be observed as a resonance since it can easily fall apart into two or more mesons.
I can see
f0(980)
I can see broad  under other peaks
broad
resonance or phase space?
any broad
resonance
under other peaks?

38. Summary (II)
A very narrow and strong mass threshold enhancement is uniquely observed in decays at BES II:
It is NOT observed in Y(1S) decays, nor in J/psi hadronic decays. FSI is strongly disfavored.
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.

39. Summary (III) J/ψ decay is an ideal place to study exotic structures.
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 was observed in J/  .
A very broad 1- - resonance X(1580) is observed in J/  K+K- 0 .
J/ψ decay is an ideal place to study exotic structures.

40. 谢 谢！ Thank You!

41. 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）
How quarks/gluons form a hadron is far from being well understood.

42. Multi-quark states, glueballs and hybrids have been searched for experimentally for a very long time, but none is established. However, during the past two years, a lot of surprising experimental evidences showed the existence of hadrons that cannot (easily) be explained in the conventional quark model Most of them are multi-quark candidates. Searching for multi-quark states becomes one of the hottest topics in the hadron spectroscopy.

43. 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.

44. 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

45. 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

46. 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.

47. 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.

48. 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

49. 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

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

51. In ppbar collision, the background is much lager (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

52. 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

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

54. 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.

55. 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
FSI * BW * PS * eff + bck

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

57. Discussion on I=1 S-wave FSI

58. Pure FSI disfavored (III) — I = 1
Pure I=1 S-wave FSI is disfavored by more than 3 .
FSI + BW
Pure FSI
M =  21 MeV
 = 0  191 MeV

59. I=0 dominant in J/  radiative decays
Most I = 0 states have been observed in J/  radiative decays with big production rate ( especially for 0-+ mesons ) such as , ’, (1440), (1760), f2(1270), f2(1525), f0(1500), f0(1710).
The only observed I=1 meson in J/  radiative decays is 0 with low production rate 4*10– 5, e.g., no evidence for (1800) in J/   3  process.
It is unlikely to be from (1800) .
I=1 S-wave FSI seems unlikely.

60. ppbar bound state in NNbar potential
Paris NNbar potential: ( Paris 93, B. Loiseau et al., hep-ph/ , )
For 11S0 , there is a bound state:
E = i 26.3 MeV
quite close to the BES observation.
However, Julich NNbar model: ( A. Sibirtsev et al., hep-ph/ )
For 11S0 : E = i 413 MeV
seems quite far away from BES observation.
They both predict an 11S0 ppbar bound state, although they are quantitatively different.

61. BES II Preliminary
No (1800)

62. 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

63. The large BR to ppbar suggest it could be an unconventional meson
For a conventional qqbar meson, the BRs decaying into baryons are usually at least one order lower than decaying into mesons.
There are many examples in PDG.
E.g.
So the large BR to ppbar (with limited phase space from the tail of X(1860)) seems very hard to be explained by a conventional qqbar meson.

64. 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/
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.

65. Analysis of
X(1835)
5.1 

66. Analysis of
X(1835)
6.0 

67. 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.

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

69. This enhancement is NOT observed in process at SAPHIR

70. 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)

71. Non-observation of NX in suggests an evidence of new baryon :
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.

72. Side-bands do not have mass threshold enhancement

74. Partial Wave Analysis of J/  K+K- 0 events
Four decay modes are included :
Amplitudes are defined by
Covariant tensor formalism
B.S. Zhou and D.V. Bugg, Eur. Phys. J. A16, 537(2003)
BW with energy-dependent width
J.H. Kuhn, A. Satamaria, Z. Phys. C48, 445 (1990).

75. Angular distributions for events with
from PWA fit
Figures on the right:
(a),(c),(e) are polar angles
in lab. reference frame
(b),(d),(f) are polar angles
in CM frames of
respectively

76. Features of the enhancement near the threshold of mass spectrum at BES II
J/ygpp
Peak position:
~ 0 MeV above threshold
“Width”:
~ 60M eV
Strong (“Height”):
(S+B)/B ~ 2
The above features may
help us easily to judge
whether it is observed in
other processes.
0.1
0.2
0.3
M(pp)-2mp (GeV)