1. Study of excited baryons at BESII
HongXun Yang
Representing BES Collaboration
IHEP
January 25-26, 2007,

2. Outline Introduction FDC system for PWA N* in the decay of
X(2075) and Nx in the decay of
Summary

3. The Beijing Electron Positron Collider
L ~ ~51030 /cm2s at J/ peak
Ecm~2-5 GeV

4. BESII Detector 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=1.78%(1+p2) z = 3.1 cm

5. Study of Excited Baryon States
Motivation
Probe the internal structure of light quark baryons
Search for “missing” baryons predicted by quark model
Obtain a better understanding of the strong interaction force in the non-perturbative regime
Examples for FDC application

6. relatively large branching ratios (PDG2004)
J/ decays
relatively large branching ratios (PDG2004)
processes
branching ratios(10-3)
N* decays
1.10.1
6.00.5
2.00.1
2.10.2
0.90.4
1.30.3

7. Pure isospin 1/2 For and , N
Feynman diagram of the production of
For and , N
and N systems are limited to be pure isospin 1/2.

8. FDC system

9. Physics model
Feynman rules
Counterms
physical parameters

10. Physical process Generate Feynman Diagram Manipulate Amplitude and
generate fortran source
Manipulate Kinematics and
Compile files and link sources

11. Application of FDC Loglikelyhood MC integration
Minimize lnL to get the parameters of each partial wave

12. from BES II data Events selection Background < 8%
2 good charged tracks
Q1+Q2 = 0
|cos| < 0.8
PID: TOF and dE/dx
Mp > 1.15 GeV
0.88<Mmiss<1.0 GeV
Background < 8%

13. N* in ? L=0 limits it to be 3/2+ or 1/2+ N*(1520) N*(1650) N*(1535)

14. N* in Dalitz Plot: Acceptance and other reasons make the plot
asymmetry.

15. PWA with FDC
Try to determine the JPC of the resonance around 2.0 GeV/c2
Following resonances are considered N(939), P11(1440), D13(1520), S11(1535), S11(1650), D15(1675), F15(1680), X
For the background: phase space and sideband

16. Results of PWA (I)
data
fit

17. Results of PWA (II)

18. Results of PWA (III)
L = 0 is preferred due to the suppression of the centrifugal barrier factor for L  1
For L = 0, JP is limited to be ½+ and 3/2+
S decrease more than 400 if either ½+ or 3/2+ is included in the PWA fit
S decrease more than 60 if one of them are added while another has been included

19. conclusion
The peak around 2.0 GeV/c2 cannot be reproduced by reflections of well-established N* resonances
Jp=1/2+,3/2+ is preferred by PWA

20. Events selection
4 good charged tracks
PID: kaon and at least 1 proton ID
2 (4C)<20
Bg rate:1~2%

21. N* & * in (I) Two clear peaks at 1520, 1690 MeV/c2 in pK mass
*(1520)
*(1690)
Two clear peaks at 1520, 1690 MeV/c2 in pK mass
N* in K mass

22. X(2075) in Bands for excited baryon states in Dalitz plot N*(1535)?
*(1690)
*(1520)
Phase Space
Bands for excited baryon states in Dalitz plot

23. Possible N* and Λ* in PWA
mass(JP)
N*: (1/2-), 1650(1/2-), 1710(1/2+), 1720(3/2+),
1900(3/2+ or 3/2-), 2050(1/2+ or 3/2+)
Λ*: (1/2-), 1520(3/2-), 1600(1/2+), 1670(1/2-),
1690(3/2-), 1800(1/2-), 1810(1/2+), 1890(3/2+)

24. Mathematical fit W/o any constrains, PWA with N* and Λ* can fit data:
However, it needs many unexpected big BRs and many large destructive interferences to cancel these big BRs.

25. Big BRs Fraction of Ndata Nevent N*(1900) 3/2+ 108% 5900
Λ*(1890) 3/ %
Λ*(1810) 1/ %
Λ*(1800) 1/ %
(1/2- is P-wave decay, which should be suppressed )

26. Estimated Nevent in Ndata
Nevent/2 (each decay mode, not include c.c.)
N*(1900) 3/ ~
N*(2050) 3/ ~
Λ*(1890) 3/ ~
Λ*(1810) 1/ ~
Λ*(1800) 1/ ~
(1/2- is P-wave decay, which should be strongly
suppressed near threshold)

27. PWA with tight constrains and w/o X(2075)
Constrain the Nevent of near threshold states ~
The PWA fit cannot reproduce the enhancement near pΛ threshold (S=-900).

28. PWA with tight constrains and with X(2075)
Adding X(2075), PWA fit can reproduce the enhancement near pΛ threshold.
Significance of X(2075) >> 5 sigma. (S=-952)

29. PWA with looser constrains and w/o X(2075)
Constrain the Nevent of near threshold states ~
(already too big at least for Λ*(1810), Λ*(1890) as limited in )
The PWA fit can hardly reproduce the enhancement near pΛ threshold (S=-940).

30. PWA with looser constrains and with X(2075)
Adding X(2075), PWA fit can reproduce the enhancement near pΛ threshold.
Significance of X(2075) > 5 sigma. (S=-962)

31. conclusion of pΛ enhancement
Reproducing the pΛ threshold enhancement with pure N* and Λ* interferences needs many unexpected big BRs and large destructive interferences.
PWA fits with pure N* and Λ* and with constrains can hardly reproduce the enhancement.
PWA fit with X(2075) can easily reproduce the enhancement (independent of constrains) with high significance.

32. A strong enhancement near the threshold of mass spectrum of
NX*
BES II
PS, eff. corrected
(Arbitrary normalization)

33. We perform PWA studies on the KΛ mass threshold structure: The most important we want to study is its production BR

34. PWA is performed to possible N* and *states listed in PDG are fitted
N(1720), N(1900), (1520), (1690), …
many different combinations are tried
different JP of Nx is tried
also tried N(1535) to fit Nx

35. An example of PWA fit Mass and Width scan NXN(1535)
Mass scan(GeV/c2)
Mass and Width scan
Total fit (S=-952)
Nevent:
Fraction Nevent
NX % 799
N(1720) 17.1%
N(1900) 13.2% 717
(1520) 4.8% 261
(1570) 21.8% 1184
(1690) 14.4% 782
(1890) 13.8% 750
X(2075) 11.3%
NXN(1535)
Total fit (S=-932)
Nevent:
Fraction Nevent
N(1535) 26.0% 1413
N(1720) 9.7%
N(1900) 11.4% 619
(1520) 4.8% 261
(1570) 22.2% 1026
(1690) 3.6% 739
(1890) 18.3% 994
X(2075) 11.2%
Width scan(GeV/c2)

36. Events/10MeV Crosses: data Hist.: PWA fit projection Dalitz plot (PWA)
Dalitz plot (data)

37. JP check with various combinations
JP ½ ½ / /2+ non A B C D E F G

38. Fit results 1.50 ~1.65 70 ~110 >14.7 >800 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
17.1
929
-880 e
1.57 90
20.6
1119
-957 f
70 ~ 90
19.9
1081
-970 g
1.58 80
15.6
845
-954
1.50 ~1.65
70 ~110
>14.7
>800

39. 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-
 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).

40. Summary FDC applied in the analysis of and
Clear signals of excited baryons observed
Seemed to be “missing” states observed in
Possible multi-quark states X(2075) and Nx in
PWA on Nx, N* and * in
is still going on…

41. Thank you!