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轻强子谱的实验研究

轻强子谱的实验研究. 沈肖雁 中国科学院高能物理研究所 shenxy@ihep.ac.cn 2013 年 7 月 9 日 （中国科技大学）. 目录. 引言 介子谱 常规介子谱 胶子球谱 （ glueball) 混杂态 (hybrid) 多夸克态（ Multi-quark state ） 重子谱 实验结果. 量子色动力学 (QCD)-- 描述强相互作用的基本理论： 在高能（ >10 GeV) 下预言的 “ 渐近自由 ” 现象已被大量实验所证实。 “ 渐近自由 ” 的发现获得 2004 年 Nobel 奖。

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轻强子谱的实验研究

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  1. 轻强子谱的实验研究 沈肖雁 中国科学院高能物理研究所 shenxy@ihep.ac.cn 2013年7月9日 （中国科技大学）

  2. 目录 • 引言 • 介子谱 • 常规介子谱 • 胶子球谱 （glueball) • 混杂态 (hybrid) • 多夸克态（Multi-quark state） • 重子谱 • 实验结果

  3. 量子色动力学(QCD)-- 描述强相互作用的基本理论： • 在高能（>10 GeV) 下预言的“渐近自由”现象已被大量实验所证实。“渐近自由” 的发现获得 2004 年 Nobel 奖。 • 然而，低能下(<3GeV) QCD 理论尚有待进一步实验检验，尤其是有许多重大问题亟待实验回答，例如： 自然界是否存在由新型强子构成的新的物质形态？

  4. 谱学研究是人类探索与认识微观世界自然规律的重要手段谱学研究是人类探索与认识微观世界自然规律的重要手段 格点QCD理论预言的胶子球谱 • 通过对原子光谱的研究获得原子结构的知识，从而奠定了原子物理基础,推动量子力学的建立和发展。 • 对强子谱的研究则提供了大量强子结构的信息，推动了夸克模型的建立和量子色动力学的发展。 Y. Chen et al., PRD 73 (2006) 014516 4

  5. Heavy Quarkonia Spectra （重夸克偶素谱） cc bb Rich spectroscopy, various production schemes, interesting decay scenarios

  6. 夸克模型 在夸克模型中： 介子由（qq）构成 重子由（q q q）构成 one up quark (charge +2/3) one down quark (charge -1/3) one neutrino (no charge, “no” mass) 质子proton (uud) composite particles nucleons 中子neutron(udd)

  7. 新型强子态 • 强子由 2个或 3个夸克组成： Naive Quark Model： • QCD 预言了新型强子的存在: • Multi-quark states：Number of quarks >＝ 4 • (多夸克态） • Hybrids（混杂态）： qqg，qqqg … • Glueballs（胶子球）： gg， ggg … Meson（ qq ） Baryon（q q q）

  8. Multi-quark states, glueballs and hybrids have been searched for experimentally for a very long time, but none is established. The observation of the new forms of hadrons will be a direct test of QCD. This has been one of the important physics goals for many experiments.

  9. Light hadron spectroscopy（轻强子谱） • Meson spectrum（qq) （介子谱） • New forms of hadrons（glueballs,hybrid states, multi-quark states） • Baryon spectrum (qqq) （重子谱） Charmonium decays provide good lab.

  10. Conventional meson spectrum In the quark model framework, the Hamiltonian for a color-singlet qq system can be written as: With an explicit form of V(r), qq spectrum can be produced. Spin-dependant forces between quarks result in fine and hyperfine structures in the hadron spectroscopy.

  11. However, CQM is only a phenomenological model. It's not derived from the underlying theory of the strong interaction---Quantum Chromodynamics (QCD). Hence the CQM spectrum is not necessarily the same as the physical spectrum in QCD.

  12. Glueball spectrum（胶子球谱） • Glueballs are bound states of at least 2 or 3 gluons in a color - singlet due to the non-Abelian property of QCD • Early phenomenologies find rather light masses for the scalar glueball in a potential model. • Other QCD-based approaches produce larger masses for the scalar such as M=1.52 GeV in a flux-tube model and M=1.5 GeV in QCD sum rule calculations. • In the last twenty years, extensive numerical studies have been carried out to calculate the glueball spectrum in LQCD. Although earliest LQCD predictions for the glueball masses vary significantly, nowadays, the predictions for several lightest glueballs converge to similar mass region despite of different approaches being used.

  13. Glueball spectrum from LQCD • LQCD predicts the lowest glueball • state is 0++. The mass is around • 1.5 GeV – 1.7 GeV. • LQCD predicts the next lightest • glueball is 2++. The mass is • around 2.4 GeV. • The mix of glueball with ordinary • qq meson makes the situation • more difficult. • The spectrum is from unquenched • LQCD calculations • Glueball candidates: f0(1500), f0(1700), fJ(2220), ... Y. Chen et al., PRD 73 (2006) 014516

  14. Glueball signatures • no place in qq nonet -- Scalar nonet (JPC = 0++) The scalar nonet should lie in the mass range of 1-2 GeV. a0(1450) and K0*(1430) in this mass region can be naturally assigned as the I=1 and I=1/2 multiplets. For I=0, there are more than two states f0(1370),f0(1500), f0(1710), f0(1790) and f0(1810). There also exists another scalar nonet below 1 GeV: f0(980), a0(980), (500), and (800).

  15. -- Pseudoscalars (JPC = 0-+) • In Particle Data Book, five 0-+ states above 1 GeV are included in the particle lists: (1295), (1405), (1475), (1760), and (2225). The former three are well established by various experimental observations, while the latter two states need further confirmation. • possible assignment: (1295): the radial excitation of ’(958) (1475): ss state (1405): 0-+ glueball

  16. Glueball signatures • Flavor-blindness of glueball decays flavor singlet glueball decay width:

  17. Glueball signatures • Enhanced production in gluon rich processes such as pp central production, J/ radiative decays and pp annihilation.

  18. Glueball signatures • Reduced couplings Gluons are charge neutral, glueball production in  collision and glueball decays into  are suppressed. Glueballs have large stickiness. • Decay branching fractions incompatible with SU(3) predictions for states Stickiness:

  19. The above criteria cannot individually provide indisputable evidence for a glueball candidate with conventional quantum numbers, especially for the scalar glueball. However, putting all together the above expectations and criteria for a glueball candidate, one might still be able to place a bound on the glueball and qq contents of a state, and gain some insights into the complex issue of strong QCD.

  20. Hybrid mesons • Hybrid mesons are composed of a pair of qq and one explicit gluon g -- qqg • From theoretical estimation, the production cross section of hybrid mesons is expected to be roughly the same as that of ordinary mesons. • Hybrid mesons and ordinary mesons mix freely if they carry the same quantum numbers. • The identification of hybrid mesons is very difficult unless they have exotic quantum numbers. 0+-, 0--, 1-+, 2+-, 3-+,……, exotic states …

  21. Theoretical prediction on the decay modes of exotics

  22. The lowest-lying exotic hybrid meson is predicted to be with the quantum number JPC=1-+ and a mass around 1.9 GeV/c2 • Flux-tube model and LQCD give consistent prediction on the mass of 1-+. • Comparison of decay width predictions

  23. Multi-quark states • Multi-quark states: number of quarks  4 • The multi-quark states are expected to have very broad width since they can easily fall apart into mesons and/or baryons if their masses are above the sum of the masses of the hadrons in the final states. • The multi-quark states may be only experimentally observable near the mass thresholds: either below or just above the mass thresholds, otherwise, the widths of multi-quark states might be too wide to be observed experimentally.

  24. Where to find? • Hunting for multi-quark states in πJ/ψ, πψ(2S), πcJ, … • Decays to charmonium thus has a cc pair! • With electric charge thus has two more light quarks!  Nquark 4 !

  25. Baryon spectrum • Baryons: qqq • Up to now all established baryons are ascribed to 3-quark (qqq) configurations • The non-relativistic constituent quark model (NRCQM) provides an explicit classification for light baryons in terms of group symmetry. • The classical simple 3q constituent quark model has been very successful in explaining the static properties, such as mass and magnetic moment, of the spatial ground states of the flavor SU(3) octet and decuplet baryons. Ex.theoretical calculation: m- 1670 MeV (sss) m- 1672.45 0.29 MeV

  26. Two problems in qqq model

  27. Missing N* Resonances (uud, udd) PDG2012 Theory predicts much more baryons than what observed  missing baryons (**) not well-established

  28. Order of masses for lowest states • the lowest spatial excited baryon is expected to be a N*(uud) state with one quark in orbital angular momentum L=1, and hence should have negative P PDG: N*(1535) (1/2-) (?) N*(1440) (1/2+) (uud) (should be heavier than N*(1535)) *(1405) (1/2-) (uds) (should be 130MeV heavier than N*(1535) ) (N*(1535) partner)

  29. The baryon spectrum reflects the underlying degrees of freedom and the effective forces between them. • Validate theoretical models Baryon-meson system

  30. QCD N*, Δ* LQCD Models N-N* Reaction Theory Amplitude analysis Data

  31. Charmonium decays – ideal place for baryon spectroscopy study

  32. 高能物理实验的三大主要工作方向 探测器设计、建造、取数 （硬件） 数据重建、模拟 （软件） 物理分析 理论研究 高能物理研究团队性强，也需要各方面人才

  33. 北京谱仪探测器（BESIII)

  34. 北京谱议探测器 • 大型通用探测器，综合提供以下信息： 粒子鉴别；带电径迹的动量、位置；沉积能量 这些信息被用于物理分析的事例选择中 • 能够探测和鉴别的粒子 带电径迹：e， ， π，K，p （质子最容易被鉴别） 中性径迹：γ 最终在物理分析中重建出原始产生事例

  35. 什么是 Monte Carlo? Monte Carlo （MC）模拟 Monte Carlo is a name of casino in Monaco. gambling probability Monte Carlo is a technique of simulation based on probability using known theory/model/knowledge.

  36. 高能物理中的Monte-Carlo模拟 • 产生子 Theoretical model simulation • 探测器模拟

  37. 高能物理实验中，MC 模拟软件至关重要 探测结果 ＝ 理论模型 ×探测效率 • 为了得到与理论模型可比较的物理结果，必须进行探测效率修正。 • 通常，探测效率只能由 MC 模拟得出。 • 因此，MC 模拟软件能否正确模拟实际数据，对能否获得正确的物理结果至关重要！

  38. 什么是共振态（resonance） • 寿命极短的粒子被称作为共振态。 已观测到的强子大多是共振态。（例：J/粒子） • 当粒子寿命 时，很难在探测器中留下径迹，即不能直接被探测到而只能通过其衰变产物的反应截面来观测。通常反应截面有类似共振现象的增强，因而被称为共振态。 （例：J/粒子的发现） • 共振态的（衰变）宽度Γ： 其质量不确定范围，与寿命成反比： （h为普朗克常数） • 共振态的宽度是理解其结构组成的重要基本性质(参数）。 相互作用越强 → 寿命越短 → 宽度越大 典型的强作用衰变宽度：≥100～200 MeV

  39. 共振态的观测与描述 • 衰变末态不变质量谱是观测共振态（反应截面）的重要手段。 例：A → B + C • 共振态的数学描述 (Breit-Wigner 函数) 质量宽度

  40. 事例选择 • 物理分析时，利用探测器提供的各种信息，通过事例选择去除本底，保留纯度较高的信号。 • 合理的事例选择应当尽量压低本底的同时，又尽量提高信号的选择效率。 • 好的事例选择条件是得到高质量物理结果的最关键的基础！ 尤其对新发现：因为新发现的信号通常较小 因而通常也是不同实验分析竞争最激烈之处。

  41. Light Scalar Mesons Below 1 GeV: σ, κ, f0(980) Above 1 GeV: f0(1370), f0(1500), f0(1710), f0(1790), f0(1810) Above 1 GeV, only 2 scalars in Quark Model.

  42. Why light scalar mesons are interesting? • There have been hot debates on the existence of σ and κ . • σ, κ and f0(980) are also possible mutiquark states. They are all near threshold. • Lattice QCD predicts the 0++ scalar glueball mass ~ 1.6 GeV. f0(1500) and f0(1710) are good candidates.

  43. The study of  • evidence for a low mass pole in the early DM2 and BESI data on J/   . • huge event concentration in the I=0 S-wave  channel seen in M~ 500 – 600 MeV in the pp central production exp. • to explain  scattering phase shift data， should be introduced in chiral perturbative theory. • FNAL E761 exp. D++-+ data

  44. The  pole in at BESII   M(+-) Different parameterizations of BW are used in PWA. Averaged pole: BES, PLB 598 (2004) 149

  45. observation of  in ’+-J/ • Measure the universal pole position (552 - i232 MeV) • World largest  signal (with ~ 40,000 tagged events) Phys. Lett. B 645 (2007) 19

  46. The study of  • A possible  pole is controversial. • Some analyses of LASS K scattering data needs (800), some don’t. • Scadron et al. favors a nonet made up of , (800), f0(980) and a0(980). • Julich group used t-channel exchanges to explain K scattering data. • evidence of  in FNAL E791 data on D+ K-++ • slightly lower statistics of CLEO D0 K-+0 data find no evidence of  • FOCUS data on K+K-++ require K*0 interfere with either a constant amplitude or a broad 0+ resonance in K

  47. BES observed  in J/K*KKK A possible  pole is controversial. PWA result:  is needed in the fit. Pole position of : BES II 58 M J/ Phys. Lett. B 633 (2006) 681

  48. f0(980) at BES BES II Preliminary • Important parameters from PWA fit: • Large coupling with KK indicates big component in f0(980) f0(980) f0(980)

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