Testing QCD Symmetries via Precision Measurements of Light Pseudoscalar Mesons

1. Testing QCD Symmetries via Precision Measurements of Light Pseudoscalar Mesons Liping Gan University of North Carolina Wilmington Wilmington, NC, USA

2. Contents • Physics Motivation • Symmetries of QCD • QCD symmetries and Properties of π0, η and η’ • Experiments at Jlab • Part I: Primakoff experiments • Part II: Rare decays of η and η’ • Summary

3. Noether’s theorem Symmetry Conservation Law What is symmetry? Symmetry is an invariance of a physical system to a set of changes . Symmetries and symmetry breakings are fundamental in physics

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:

5. Fate of QCD symmetries

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

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׳

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

9. Precision measurements of electromagnetic properties of 0, , via Primakoff effect. Two-Photon Decay Widths: Γ(0→) @ 6 GeV Γ(→) Γ(’→) Part I: Primakoff Program at Jlab 6 & 12 GeV Transition Form Factors at low Q2 (0.001-0.5 GeV2/c2):F(*→0), F(* →), F(* →) • Input to Physics: • precision tests of Chiral • symmetry and anomalies • determination of light quark • mass ratio • -’ mixing angle • Input to Physics: • 0, and ’ electromagnetic • interaction radii • is the ’ an approximate • Goldstone boson?

10. Status of Primakoff Experiments at JLab • PrimEx-I for Γ(0) was performed in 2004 with 6 GeV beam. Published • in PRL 106, 162303 (2011) • PrimEx-II for Γ(0) was carried out in 2010. Analysis is in progress. • The 12 GeV part of program has was reviewed by 3 special high energy • PACs. Included in Jlab 12 GeV upgrade White paper and CDR. • PAC18 (2000) • PAC23 (2003) • PAC27 (2005) • The first 12 GeV Primakoff experiment on Γ(→) in Hall D was approved by PAC35 in Jan 2010.

11. Γ(0→) Experiments

12. k1 π Axial Anomaly Determines π0Lifetime • 0→ decay proceeds primarily via the chiral anomaly in QCD. • The chiral anomaly prediction is exact for massless quarks: k2 • Γ(0)is one of the few quantities in confinement region that QCD can calculate precisely to higher orders! • Corrections to the chiral anomaly prediction: Calculations in NLO ChPT: • Γ(0) = 8.10eV ±1.0% (J. Goity, et al. Phys. Rev. D66:076014, 2002) • Γ(0) = 8.06eV ±1.0% (B. Ananthanarayan et al. JHEP 05:052, 2002) Calculations in NNLO SU(2) ChPT: • Γ(0) = 8.09eV ±1.3% (K. Kampf et al. Phys. Rev. D79:076005, 2009) 0→ • Calculations in QCD sum rule: • Γ(0) = 7.93eV ± 1.5% (B.L. Ioffe, et al. Phys. Lett. B647, p. 389, 2007) • Precision measurements of (0→) at the percent levelwill provide a stringent test of a fundamental prediction of QCD.

13. ρ,ω Primakoff Method 12C target Primakoff Nucl. Coherent Nucl. Incoh. Interference Challenge: Extract the Primakoff amplitude • Requirement: • Photon flux • Beam energy • 0production Angular resolution

14. PrimEx-I (2004) • JLab Hall B high resolution, • high intensity photon tagging • facility • New pair spectrometer for photon flux control at high beam intensities 1% accuracy has been achieved • New high resolution hybrid multi-channel calorimeter (HyCal)

15. 0 Event selection • We measure: • incident photon energy: E and time • energies of decay photons: • E1, E2 and time • X,Y positions of decay photons • Kinematical constraints: • Conservation of energy; • Conservation of momentum; • m invariant mass

16. Fit Differential Cross Sections to Extract Γ(0) Theoretical angular distributions smeared with experimental resolutions are fit to the data on two nuclear targets:

17. Verification of Overall Systematical Uncertainties •  + e  +eCompton • cross section measurement • e+e-pair-production cross • section measurement: Systematic uncertainties on cross sections are controlled at 1.3% level.

18. PrimEx-I Result PRL 106, 162303 (2011) (0) = 7.820.14(stat)0.17(syst) eV 2.8% total uncertainty

19. Goal for PrimEx-II • PrimEx-I has achieved 2.8% precision (total) on (0) : 7.82 eV 1.8% (stat) 2.2% (syst.) PrimEx-II projected 1.4% PrimEx-I 7.82eV2.8% • Task for PrimEx-II is to obtain 1.4% precision: Projected uncertainties: 0.5% (stat.) 1.3% (syst.)

20. Estimated Systematic Uncertainties • Improve Background Subtraction: • Add timing information • in HyCal (~500 chan.) • Improve photon beam • line • Improve PID in HyCal • (add horizontal veto ) • More empty target • data • Measure HyCal • detection efficiency

21. PrimEx-II Status • Experiment was performed from Sep. 27 to Nov. 10 in 2010. • Physics data collected: • π0 production run on two nuclear targets: 28Si (0.6% statistics) and 12C (1.1% statistics). • Good statistics for two well-known QED processes to verify the systematic uncertainties: Compton scattering and e+e- pair production.

22. PrimEx-II Experimental Yield (preliminary) ( E = 4.4-5.3 GeV) Primakoff Primakoff 12C 28Si ~8K Primakoff events ~20K Primakoff events

23. Γ(η→) Experiment

24. Physics Outcome from New  Experiment Resolve long standing discrepancy between collider and Primakoff measurements: Determine Light quark mass ratio: Γ(→3π) ∝ |A|2∝ Q-4 Extract -’mixing angle: H. Leutwyler Phys. Lett., B378, 313 (1996)

25. Measurement of Γ(→) in Hall D at 12 GeV CompCal Counting House 75 m FCAL Photon tagger • Incoherent tagged photons • Pair spectrometer and a TAC detector for the photon flux control • 30 cm liquid Hydrogen and 4He targets (~3.6% r.l.) • Forward Calorimeter (FCAL) for → decay photons • CompCal and FCAL to measure well-known Compton scattering for control • of overall systematic uncertainties. • Solenoid detectors and forward tracking detectors (for background rejection) 25

26. Advantages of the Proposed Light Targets • Precision measurements require low A targets to control: • coherency • contributions from nuclear processes • Hydrogen: • 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 J.M. Laget, Phys. Rev. C72, (2005) A. Sibirtsev, et al. arXiv:1001.0646, (2010) • 4He: • higher Primakoff cross section • the most compact nucleus • form factor well known • new theoretical developments for FSI S. Gevorkyan et al., Phys. Rev. C 80, (2009) 26

27. Approved Beam Time

28. Estimated Error Budget • Systematical uncertainties (added quadratically): • Total uncertainty (added quadratically):

29. 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

30. Part II: Rare decays of η and η’

31. Why Are η Decays Interesting for New Physics? • The heaviest member in the octet • pseudoscalar mesons (547.9 MeV/c2) • Due to the conservation of G and P in the strong interaction, the η • decay width Γη=1.3 KeV is extremely narrow (comparing to • Γρ= 149 MeV) • η decays have the dominant strong interaction filtered out, enhancing the branching ratios for other interactions by a factor of ~100,000. η decays provide a unique, flavor-conserving laboratory to search for new sources of C, P, and CP violation in the regime of EM and suppressed strong processes.

32. Selection Rule Summary Table:η Decay to π’s and γ’s Key: Gamma Column implicitly includes γ*e+e- C and P allowed, observed C and P allowed, upper limits only L = 0 L = 1 L = even or odd (no parity constraint) . . . . . C, CP C violating, CP conserving, etc. Forbidden by energy and momentum conservation.

33. Some Interesting η Rare Decay Channels

34. 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

35. Study of theη→0Decay • A stringent test of the χPTh prediction at Ο(p6) level • Tree level amplitudes (both Ο(p2) and Ο(p4)) vanish; • Ο(p4) loop terms involving kaons are suppressed by large mass of kaon • Ο(p4) loop terms involving pions are suppressed by G parity • The first sizable contribution comes at Ο(p6) level • A long history that experimental results have large discrepancies with theoretic predictions. • Current experimental value in PDG is BR(η→0 )=(2.7±0.5)x10-4

36. Theoretical Status on η→0 By E. Oset et al. Average of χPTh 0.42

37. History of the η→0 Measurements After 1980 A long standing “η” puzzle is still un-settled.

38. High Energy η ProductionGAMS Experiment at SerpukhovD. Alde et al., Yad. Fiz 40, 1447 (1984) • 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: (η→0 ) = 0.84±0.17 eV 40 η→0  events • Major Background • -p→ 00n • η →000

39. Low energy η production CB experiment at AGSS. Prakhov et al. Phy.Rev.,C78,015206 (2008) NaI(T1) η →000 • The η’s were produced with 720 MeV/c - beam through the -p→ηn reaction • Decay ’s energy range: 50-500 MeV • Final result: • (η→0 ) = 0.285±0.031±0.061 eV • 92±23 η→0  events

40. Major background in CB-AGS experiment MC data MC MC

41. CB Data Analysis II N. Knechtet al. phys. Lett., B589 (2004) 14 • Final result • (η→0γγ)=0.32±0.15 eV

42. Low Energy η Production Continue KLOE experiment B. Micco et al., Acta Phys. Slov. 56 (2006) 403 • Produce Φ through e+e- collision at √s~1020 MeV • The decay η→0γγ proceeds through: Φ→η, η→0γγ, 0→γγ • Final result: • (η→0γγ)=0.109±0.035±0.018 eV • η→0  events

43. 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 recoiled particles to reduce non-resonance p→00p background • High resolution, high granularity PbWO4 Calorimeter • Higher energy resolution → improve 0   invariance mass • Higher granularity→ better position resolution and less overlap clusters to reduce background from η→ 30 • Fast decay time → less pile-up clusters • Large statistics to provide a precision measurement of Dalitz plot

44. Suggested Experiments in Hall D at Jlab GlueX FCAL Counting House Simultaneously measure the η→0, η →00: • η produced on LH2 target with 11 GeV tagged photon beam γ+p → η+p • Tag η by measuring recoil p with GlueX detector to reduce the p→ 00p background • Forward calorimeter (upgraded with high resolution , high granularity PbWO4) to detect multi-photons from the η decays 75 m 44

45. Kinematics of Recoil Proton Recoil θp vsθη (Deg) Angle θη(Deg) Recoil θp (Deg) Recoil Pp (GeV) • Polar angle ~55o-80o • Momentum ~200-1200 MeV/c Recoil Pp (GeV) vs θp (Deg)

46. Detection of recoil p by GlueX

47. Reconstructed missing mass and efficiency

48. 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

49. S/N Ratio vs. Calorimeter Granularity PWO dmin=4cm S/N=1.4 Pb Glass dmin=8cm S/N=0.024 η η Background is from MC only

50. Rate Estimation • The +p→η+p cross section ~70 nb (θη=1-6o) • Photon beam intensity Nγ~1.5x107 Hz • The η→0 detection rate: BR(η→0 )~3x10-4 , detection efficiency ~24% In order to get 10% precision on BR measurement, we need ~12 days beam time