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Observation of near-threshold enhancement at BES

Observation of near-threshold enhancement at BES. HongXun Yang Representing BES Collaboration IHEP yanghx@mail.ihep.ac.cn September 26- 30, 2004. Outline. Introduction threshold enhancement in threshold enhancement in

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Observation of near-threshold enhancement at BES

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  1. Observation of near-threshold enhancement at BES HongXun Yang Representing BES Collaboration IHEP yanghx@mail.ihep.ac.cn September 26- 30, 2004

  2. Outline • Introduction • threshold enhancement in • threshold enhancement in • threshold enhancement in • Summary

  3. Introduction BESII Detector Data Sample

  4. 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

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

  6. Observation of threshold enhancement in pp bound state (baryonium)? Phys. Rev. Lett., 91 (2003) 022001

  7. Near pp threshold enhancement in enhancement c

  8. Fit Result fitted peak location J/ygpp acceptance weighted BW +3 +5 -10 -25 M=1859 MeV/c2 G < 30 MeV/c2 (90% CL) c2/dof=56/56 0 0.1 0.2 0.3 M(pp)-2mp (GeV) 3-body phase space acceptance

  9. MARK-III & DM2 Results MARK-III Threshold enhancement DM2 Claimed in Phys. Rep. 174(1989) 67-227 Too small statistics to draw any conclusion on the threshold enhancement, e.g., cannot exclude known particles such as h(1760)

  10. 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

  11. 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.

  12. 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 ! >>

  13. Final State Interaction ?—— Not favored • 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. • 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

  14. 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 explain the BES structure. |M|2 BES Both arbitrary normalization elastic scattering

  15. 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. Belle “Threshold” enhancement 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.

  16. 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. Yang et al., hep-ph/0405087 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.

  17. Phys. Rev. Lett. 93, 112002 (2004)    Observation of threshold enhancement in

  18. High Purity of Signal after Selection • It can be shown by the clean Λ signal • MC background study: only 1~2% Dominantly from  Data Data/MC

  19. Strong enhancement near the threshold of Phase Space

  20. Observed in both ofand

  21. 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

  22. Similar enhancement also observed in Fix the parameters, 4 away from phase space.

  23. The distribution is consistent with S-Wave • MC (phase space) also show non-uniform and asymmetric distribution of . • The enhancement is consistent with S wave. Err: Data His: MC

  24. Observation of threshold enhancement in

  25. Near-threshold enhancement inMK Nx Nx Events/10MeV (Arbitrary normalization) PS, eff. corrected Nx

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

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

  28. N(1720), N(1900), (1520), (1690) …. included in the PWA fit An example: • 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) Ln L Width scan(GeV/c2)

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

  30. JP check with various combinations  • JP½- 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

  31. Fit results

  32. 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 Width70~110MeV 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).

  33. Summary • 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)

  34. THANK YOU!

  35. BES-I Result Threshold enhancement But NOT claimed in Phys. Rev. Lett. 76(1996) 3502 Too small statistics to draw any conclusion on the threshold enhancement

  36. NO strong threshold enhancement in collision (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

  37. S-wave BW fit results P-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 • 2/d.o.f = 31.1/26 • About 7σ statistical significance high L hypotheses fail The systematic errors are carefully studied in S-wave case.

  38. Why Dalitz plot not uniform for events DATA MC • This is due to acceptance • It can be shown in distribution, where is the decay angle of p in

  39. 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.

  40. Systematic uncertainties

  41. 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

  42. 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

  43. Conclusion

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