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Observation of a threshold enhancement in the p  invariant mass spectrum

Observation of a threshold enhancement near the Λp̄ mass threshold in the invariant mass spectrum. The study confirms the existence of possible multi-quark mesons and highlights the importance of searching for baryon-antibaryon resonances or bound states.

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Observation of a threshold enhancement in the p  invariant mass spectrum

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  1. Phys. Rev. Lett. 93, 112002 (2004)Cited 28 times Observation of a threshold enhancement in the p invariant mass spectrum H.X. Yang (BES Collaboration) IHEP yanghx@mail.ihep.ac.cn Oct.28 - Nov.01, 2006, Guilin

  2. Outline • Introduction • Features of signal and event selection • Reliability of the result • Summary

  3. Introduction

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

  5. 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

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

  7. Motivation • The enhancement near the ppbar mass threshold observed by BESII in could be interpreted as a ppbar bound state (multi-quark meson). • The new observation of BESII as well as new resonances observed by other experiments makes the searches for multiquark states become a very hot topic of high energy physics field. • It is of special importance to search for possible baryon-antibaryon resonances or bound states.

  8. Strong enhancement near the threshold of Data MC (Phase Space) 11 Data Data

  9. S-wave BW fit results • M = (2075 +/- 12) MeV Γ = (90 +/- 35) MeV BR = (1.18+/- 0.28)*10**(-4) • About 7σ statistical significance

  10. Features of signal and event selection

  11. Features of signal (I) Signal means • Λ signal (very strong constraint) After selection, the clear Λ signal in data shows high purity of signal. Data MC

  12. Features of signal (II) • Four charged tracks, no photon, no pi0 • Three heavy particles: p, pbar, Kaon • So kinematics (with correct mass assignment) provides very strong constraint in the event selection, and in thekinematic fit can reject most background events.

  13. Key points in event selection Keep • High purity of signal ( Λ mass as a control) • Efficiency as high as possible • Systematic uncertainties from selection criteria as small as possible Low momentum feature should be considered

  14. Event Selection • Four good charged tracks ( Pxy>50 MeV) • PID: Kaon-ID and at least one proton-ID • 4-C kinematic fit PID information is used in the mass assignment • Remove events with combined information of Emiss and Mpk_miss no requirement on Λ vertex reconstruction

  15. 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

  16. Strong enhancement near the threshold of Phase Space

  17. Observed in both ofand

  18. S-wave BW fit results P-wave BW fit results • M = (2075 12  5) MeV • Γ = (90  35  9) MeV • BR = (1.18  0.28  0.4) 10-4 • M = (2044 17) MeV • Γ = (20  45) MeV • 2/d.o.f = 32.5/26 • 2/d.o.f = 31.1/26 • ~ 7σ statistical significance high L hypotheses fail The systematic errors are carefully studied in S-wave case.

  19. Reliability of the result Extensive and careful checks!

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

  21. Is the enhancement from N* or Λ*? • Hard to understand why N* or Λ* only contribute to very small phase space

  22. Other checks • Can the enhancement be interpreted as known PDG particles? NO • MC Input/Output test: check systematic uncertainties of fitting bias – is small. • Other fits (L=1, 2, 3): P-wave fine, other failed

  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. 重要科学意义 • 这一研究成果与新发现的质子－反质子奇异结构具有同样重要意义：有可能是一个多夸克态粒子（六夸克态）。 • 自然界已发现的由夸克组成的粒子中的夸克数均为两个或三个，因此发现（多于三个夸克组成的）多夸克态粒子对检验和发展量子色动力学QCD 理论具有非常重要的意义。也将是新的物质存在形式。 • 两个 PRL Referee 均对该文章给予好评。（文章被直接接收） • 被高能所作为重要科研成果对外宣传

  25. PRL Referee 给予好评 • Referee A: … 由于 BES 等实验的多个新发现的共振峰，以及关于五夸克态粒子的争论，使得存在重子－反重子束缚态及多夸克态粒子的问题再次引起极大关注。… • Referee B: 这篇文章研究了关于重子－反重子质量阈值共振结构的重要课题…。BES 是第一个报道质子－反质子阈值增长结构的实验。而这篇文章观测到类似的 质量阈值增长结构，支持了前面的发现结果。…

  26. Summary • We observed an enhancement at 2.08 GeV in the invariant mass spectrum of • Various checks show that the result is robust. • It has been fitted with S-wave B-W function successfully: mass = 2075+/-12+/-5 MeV width=90+/35+/-9 MeV Br=(1.18+/0.28+/-0.40)x10E-4 • It is very unlikely that the enhancement is from pure N* and Λ* interference, although they contribute to Dalitz plot. • A possible multi-quark state candidate

  27. THANK YOU!

  28. 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

  29. 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. • Keep in mind: Ndata=5433

  30. 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 )

  31. Physics constrains (I) • Typically, BR(J/psibaryon anti-baryon) ~ 2x10-3 e.g., BR(J/psi  p pbar) =2.2x10-3 • So, if there is no suppression, the typical number of observed events for one decay mode is: BR(J/psi ΛΛ *)BR(Λ *pK) BR(Λpπ) ε N J/psi = 2x10-3 x0.1 x0.1 x0.6 x5.8x107 ~ 700 events

  32. Physics constrains (II) – near threshold suppression • Phase space ~ p • Angular momentum barrier ~ (p / M J/psi) 2L+1 • If the width is large, some fraction of N* or Λ* cannot be produced. • Global efficiency may be lower.

  33. Physics constrains (III) – from other channels • E.g., Since Λ* decays both to pK and Σπ, and most BRs of them are of the same order, so this channel can provide constrains on Λ*(1520), Λ*(1600), Λ*(1690) production rate. • No clear evidence for strong production of Λ*(1800), Λ*(1810), Λ*(1890). Near threshold suppression is a reasonable estimation.

  34. 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 suppressed near threshold)

  35. PWA with 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).

  36. PWA with 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)

  37. 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.

  38. Fit the enhancement in • Fit the similar enhancement in with the same parameters of X(2075) as in J/psi. • The significance of X(2075) in psi’ data is ~4.0 σ. • The similar enhancements in the psi’ data and in the Belle data ( in BpΛπ process ) CANNOT be from N*(2050), Λ*(1890) … interference.

  39. Summary of PWA studies • 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. • The similar enhancements in the psi’ data and in the Belle data ( in BpΛπ process ) CANNOT be from N*(2050), Λ*(1890) … interference.

  40. Systematic uncertainties

  41. 被高能所作为重要科研成果对外宣传

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