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Study of excited baryons at BESII

Study of excited baryons at BESII. HongXun Yang Representing BES Collaboration IHEP yanghx@mail.ihep.ac.cn January 25-26, 2007,. Outline. Introduction FDC system for PWA N* in the decay of X(2075) and Nx in the decay of Summary.

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Study of excited baryons at BESII

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  1. Study of excited baryonsat BESII HongXun Yang Representing BES Collaboration IHEP yanghx@mail.ihep.ac.cn 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 = 220 m BSC: E/E= 22 % z = 5.5 cm dE/dx= 8.5 %  = 7.9 mr B field: 0.4 T p/p=1.78%(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 quarkmodel • Obtain a better understanding of the stronginteraction force in the non-perturbative regime • Examples for FDC application

  6. J/ decays • relatively large branching ratios (PDG2004)

  7. Pure isospin 1/2 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 generate fortran source • Compile files and link sources

  11. Application of FDC • Loglikelyhood • MC integration • Minimize lnLto get the parameters of each partial wave

  12. from BES II data • Events selection • 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 N*(1520) N*(1535) N*(1650) N*(1675) N*(1680) N*(1440) ? L=0 limits it to be 3/2+ or 1/2+

  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 consideredN(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*(1535)? N*(1650)? *(1520) *(1690) N* & * in (I) • Two clear peaks at 1520, 1690 MeV/c2 in pK mass • N* in K mass

  22. X(2075)in N*(1535)? N*(1650)? X(2075) *(1690) *(1520) Phase Space • Bands for excited baryon statesin Dalitz plot

  23. Possible N* and Λ* in PWA • mass(JP) N*: 1535 (1/2-), 1650(1/2-), 1710(1/2+), 1720(3/2+), 1900(3/2+ or 3/2-), 2050(1/2+ or 3/2+) Λ*: 1405(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 withN* and Λ* can fit data: (S= -997) • However, it needs many unexpected big BRs and many large destructive interferences to cancel these big BRs.

  25. Big BRs Fraction of NdataNevent N*(1900) 3/2+ 108% 5900 N*(2050) 3/2+ 33% 1800 Λ*(1890) 3/2+ 21% 1100 Λ*(1810) 1/2+ 9% 500 Λ*(1800) 1/2- 34% 1900 (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) 3/2+ ~ 300 -400 N*(2050) 3/2+ ~ 100 -150 Λ*(1890) 3/2+ ~ 100 -150 Λ*(1810) 1/2+ ~ 150 -200 Λ*(1800) 1/2- ~ 20 - 30 (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 Neventof near threshold states ~ 100-200 • 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 Neventof near threshold states ~ 500-600 (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 pΛenhancement • 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), … • manydifferent combinations are tried • different JP of Nx is tried • also tried N(1535) to fit Nx

  35. Mass and Width scan Total fit (S=-952) Nevent: Fraction Nevent NX 14.7% 799 N(1720)17.1% 929 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%614 An example of PWA fit Mass scan(GeV/c2) • NXN(1535) • Total fit (S=-932) • Nevent: • Fraction Nevent • N(1535) 26.0% 1413 • N(1720)9.7% 527 • 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%608 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  • 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

  38. Fit results

  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 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).

  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!

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