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Test of QCD Symmetries via Measurements on Light Pseudoscalar Mesons

Test of QCD Symmetries via Measurements on Light Pseudoscalar Mesons. Liping Gan University of North Carolina Wilmington. Contents. Physics Motivation Symmetries of QCD Properties of π 0 , η and η ’ PrimEx experimental program at Jlab 12 GeV Primakoff experiments

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Test of QCD Symmetries via Measurements on Light Pseudoscalar Mesons

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  1. Test of QCD Symmetries via Measurements on Light Pseudoscalar Mesons Liping Gan University of North Carolina Wilmington Liping Gan, UNCW

  2. Contents • Physics Motivation • Symmetries of QCD • Properties of π0, η and η’ • PrimEx experimental program at Jlab 12 GeV • Primakoff experiments • Rare decays of η and η’ Liping Gan, UNCW

  3. Noether’s theorem Symmetry Conservation Law What is symmetry? Symmetry is an invariance of a physical system to a set of changes . Symmetry and symmetry breaking are fundamental in the laws of physics Liping Gan, UNCW

  4. Continuous Symmetrys of QCD in the Chiral Limit chiral limit:is the limit of vanishing quark masses mq→ 0. QCD Lagrangian with quark masses set to zero: Large global symmetry group: Liping Gan, UNCW

  5. Fate of Symmetrys Liping Gan, UNCW

  6. Discrete Symmetries of QCD Charge: C Parity: P Time-Reversal: T Combinations: CP, CT, PT, CPT Liping Gan, UNCW

  7. Lightest pseudoscalar mesons • Chiral SUL(3)XSUR(3) spontaneously broken Goldstone mesons π0, η8 • Chiral anomalies Mass of η0P→ ( P: π0, η, η׳) • Quark flavor SU(3) breaking The mixing of π0, η and η׳ LipingGan, UNCW

  8. Some Interesting η Rare Decay Channels The π0, η and η’ system provides a rich laboratory to study the symmetry structure of QCD. Liping Gan, UNCW

  9. PrimEx Program at Jlab (1) Primakoff experiments to measure: • Two-Photon Decay Widths: Γ(π0 →) @ 6 GeVΓ(η →),Γ(η׳ →) • Transition Form Factor Fγγ*P of π0, η and η׳ at low Q2 (0.001--0.5 GeV2/c2) (2) Measure the branching ratios of η and η’rare decays * LipingGan, UNCW

  10. Experimental Status Decay width Transition Form Factor Liping Gan, UNCW

  11. Determine the quark masse ratio Γ(η→3)=Γ(→)×B.R. Liping Gan, UNCW

  12. Mixing angles of η-η׳ • Mixing angles: • Decayconstants: Γ(η/η’→)widths are crucial inputs for obtaining fundamental mixing parameters. Liping Gan, UNCW

  13. Two-Photon Decay Widths Test chiral anomaly predictions: • Features of anomaly: • Unique property of the quantum theory. • Calculable exactly to all orders in the chiral limit Liping Gan, UNCW

  14. Number of colors in QCD • In the past 30 years, many textbooks stated that Γ(π0 →γγ) was the best probe to determine the number of quark colors at low energy • Recent calculations pointed out that Γ(π0→γγ) is less sensitive to Nc due to partial cancellations of the WZW term with a Goldstone-Wilczek term • The decay amplitude of the single field (η0) depends strongly on Nc and yield under the inclusion of mixing also a strong Nc dependence for the ηdecay • Both the Γ(η→γγ) and Γ(η’→γγ) decays are suited to confirm the number of colors Liping Gan, UNCW

  15. Transition Form Factors at Low Q2 • Direct measurement of slopes • Interaction radii:Fγγ*P(Q2)≈1-1/6▪<r2>PQ2 • ChPT for large Nc predicts relation between the three slopes. Extraction of Ο(p6) low-energy constant in the chiral Lagrangian • Input for light-by-light scattering for muon (g-2) calculation • Test of future lattice calculations Liping Gan, UNCW

  16. ρ,ω Primakoff Process Challenge: Extract the Primakoff amplitude with unprecedented accuracy Liping Gan, UNCW

  17. Cross Section • Features of Primakoff cross section: • Beam energy sensitive • Peaked at very small forward angle • Coherent process Liping Gan, UNCW

  18. PrimExExperiment on 0 at 6 GeV • JLab Hall B high resolution, high intensity photon tagging facility • New pair spectrometer for photon flux control at high intensities • New high resolution hybrid multi-channel calorimeter Liping Gan, UNCW

  19. PrimEx-I Experiment: Γ(0) Decay Width • Nuclear targets: 12C and 208Pb; • 6 GeV Hall B tagged beam; • experiment performed in 2004 12C 208Pb Liping Gan, UNCW

  20. PrimEx-I Result • 0 = 7.82eV ± 2.2%stat. ± 2.1%syst. (± 3.0% total) PrimEx Liping Gan, UNCW

  21. PrimEx-II run @ 6 GeV Projected PrimEx-II Liping Gan, UNCW

  22. 12 GeV Experimental Setup • New high energy photon tagger • Improved PrimEx calorimeter HYCAL with all PbWO4 • Choose the light targets 4He and 1H Liping Gan, UNCW

  23. Proposed Experiment on Γ(η→)with GlueX Setup Counting House • General characteristics of proposed experiment: • Incoherent bremsstrahlung photon beam • Eγ =10.5 – 11.7 GeV • (~10-4r.l. Au radiator, 5.0 mm beam collimator) • High resolution, high segmentation HyCal • Calorimetor • 30 cm LH2 target (~3.6 r.l.) 75 m Liping Gan, UNCW 23

  24. Advantages of Hydrogen target • no inelastic hadronic contribution; • no nuclear final state interactions; • proton form factor is well known; • better separation between Primakoff and nuclear processes; • new theoretical developments of Regge description of hadronic processes. LipingGan, UNCW

  25. Statistics and Beam Time Request Target: 30 cm (3.46% r.l.) LH2, Np=1.28x1024 p/cm2 Photon intensity: 7.6x106γ/sec in Eγ= 10.5–11.7 GeV Total cross section on P for θη=0 - 3.50, Δσ = 1.1x10-5 mb (10% is Primakoff). • N(evts) = Np x Nγ x Δσ x ε(eff.)x(Br. Ratio) = 1.28x1024x7.6x106x1.1x10-32x0.6x0.4 = 2.6 x 10-2events/sec = 2200 events/day = 220 Primakoff events/day Beam time request: Statistics: 45 days of run on LH2: 1% stat. error Liping Gan, UNCW

  26. Estimated Error Budget on Γ(η→) • Systematical errors: • Total estimated error: Liping Gan, UNCW

  27. Some η Rare Decay Channels Liping Gan, UNCW

  28. Study ofη→0 0 Reaction • The Origin of CP violation is still a mystery • CP violation is described in SM by the phase in the Cabibbo-Kobayashi-Maskawa quark mixing matrix. A recent SM calculation predicts BR(η→0 0)<3x10-17 • The η→0 0 is one of a few available flavor-conserving reactions listed in PDG to test CP violation. • Unique test of P and PC symmetries, and search for new physics beyond SM Liping Gan, UNCW

  29. History of the η→0 Measurements After 1980 A long standing “η” puzzle is still un-settled. Liping Gan, UNCW

  30. High Energy η Production(GAMS Experiment on η→0 at Serpukhov) • Experimental result was first published in 1981 • The η’s were produced with 30 GeV/c- beam in the -p→ηn reaction • Decay ’s were detected by lead-glass calorimeter Final result (D. Alde et al.) ~40 of η→0 events BR(η→0γγ)=(7.1±1.4)x10-4 (η→0γγ)=0.84±0.17 eV • Major Background • -p→ 00n • η →000 Liping Gan, UNCW

  31. Low energy η production (CB experiment on η→0 at AGS, by S. Prakhovet al. ) η →000 -p→ 00n • The η’s were produced with 720MeV/c - beam through the -p→ηn reaction • Decay ’s energy range: 50-500 MeV • Final result • 1600 of η→0 events • (η→0γγ)=0.45±0.12 eV Liping Gan, UNCW

  32. What can be improved at 12 GeVJlab? • High energy tagged photon beam to reduce the background from η→ 30 • Lower relative threshold for -ray detection • Improve calorimeter resolution • Tag η by measuring recoiled particles to reduce non-resonance 00background • High resolution PWO Calorimeter • Higher energy resolution → improve 0γγ invariance mass • Higher granularity→ better position resolution and less overlap clusters • Large statistics to provide a precision measurement of Dalitz plot Liping Gan, UNCW

  33. Suggested Experiment in Hall D at Jlab FCAL Photon Tagger GlueX • η produced on LH2 target with 11 GeV tagged photon beam γ+p → η+p • Tag η by measuring recoil p with GlueX detector • Forward calorimeter with PWO insertion to detect multi-photons from the η decay Counting House Simultaneously measure the η→0, η →00: 75 m LipingGan, UNCW

  34. S/N Ratio vs. Calorimeter Granularity PWO dmin=4cm S/N=1.4 Pb Glass dmin=8cm S/N=0.024 Liping Gan, UNCW

  35. Summary • (1) Primakoff experiments to measure: • Two-Photon Decay Widths: Γ(0 →),Γ(η→),Γ(η׳ →) • Transition Form Factor Fγγ*P of π0, η and η׳ at low Q2 (0.001-0.5 GeV2/c2) • (2) Measure the branching ratios for η and η’ rare decays • Fundamental input to Physics: • Determination of quark mass ratio • Mixing parameters ⇒ decay constants and mixing angles of η―η׳ • Test chiral anomaly predictions • Confirm number of colors Nc • Ο(p6) low-energy-constant in ChiralLagrangian • Test P, CP and C symmetries, and search for new physics beyond Standard Model LipingGan, UNCW

  36. Invariant Mass Resolution σ=6.9 MeV σ=3.2 MeV PWO M0 M σ=6.6 MeV σ=15 MeV Pb glass M M0 Liping Gan, UNCW

  37. Why do we need 12 GeV beam? • Increase Primakoff cross section: • Better separation of Primakoff reaction from nuclear processes: • Momentum transfer to the nuclei becomes less reduce the incoherent background • Unique CEBAF beam quality Liping Gan, UNCW

  38. Strong force Color confinement:the potential energy between (in this case) a quark and an antiquark increases while increasing the distance between them. Solution:Understanding symmetries of nature Challenges in Modern Physics • What are the building blocks of matter? The properties of strong force at large distance represents one of the biggest intellectual challenges in physics. Strong force obeys the rules of Quantum Chromodynamics (QCD) • What happens if one tries to separate two quarks? Liping Gan, UNCW

  39. Experimental Resolutions: Prod. Angle • Precision Primakoff measurement requires high resolutions in: • Production angle (fit); • Invariant mass (background) • Energy (elasticity) Liping Gan, UNCW 39

  40. Experimental Resolutions (contd.) • γγinvariant mass • Energy conservation (elasticity) • High resolution, high granularity calorimeter is critical in: • event selection; • extraction of Primakoff from hadronic processes Liping Gan, UNCW 40

  41. PrimEx 12 GeV Project History • This program had been reviewed by 3 special high energy PACs: • PAC18 (2000) • PAC23 (2003) • PAC27 (2005) • It is included in the CEBAF 12 GeV CDR With the following statement in the Abstract: • “… Precision measurements of the two-photon decay widths and transition form factors of the three neutral pseudoscalar mesons via the Primakoff effect will lead to a significant improvement on our knowledge of chiral symmetry in QCD, in particular on the ratios of quark masses and on chiral anomalies.” Liping Gan, UNCW

  42. Low Energy η Production Continue(KLOE, by B. Micco et al., Acta Phys. Slov. 56 (2006) 403) • Produce Φ through e+e- collision at √s~1020 MeV • The decay η→0γγproceeds through: Φ→η, η→0γγ, 0→γγ • Final result • 68±23 of η→0 events • BR(η→0γγ)=(8.4±2.7±1.4)x10-5 • (η→0γγ)=0.109±0.035±0.018 eV Liping Gan, UNCW

  43. Determine the quark masse ratio • There are two ways to determine the quark mass ratio: • Γ(η→3π) is the best observable for determining the quark mass ratio, which is obtained from Γ(η→γγ) and known branching ratios: • The quark mass ratio can also be given by a ratio of meson masses: Liping Gan, UNCW

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