1. Test of QCD Symmetries via Measurements on Light Pseudoscalar Mesons
Liping Gan
University of North Carolina Wilmington
China, 2009
Liping Gan, UNCW

2. Contents Physics Motivation PrimEx experimental program at Jlab 12 GeV
Symmetries of QCD
Properties of π0, η and η’
PrimEx experimental program at Jlab 12 GeV
Primakoff experiments
Rare decays of η and η’
China, 2009
Liping Gan, UNCW

3. Symmetry Conservation Law
What is symmetry?
Symmetry is an invariance of a physical system to a set of changes .
Noether’s theorem
Symmetry Conservation Law
Symmetry and symmetry breaking are fundamental in the laws of physics
China, 2009
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:
China, 2009
Liping Gan, UNCW

5. Fate of Symmetrys
China, 2009
Liping Gan, UNCW

6. Discrete Symmetries of QCD
Charge: C
Parity: P
Time-Reversal: T
Combinations: CP, CT, PT, CPT
China, 2009
Liping Gan, UNCW

7. Lightest pseudoscalar mesons
Chiral SUL(3)XSUR(3) spontaneously broken Goldstone mesons π0, η8
Chiral anomalies Mass of η P→ ( P: π0, η, η׳)
Quark flavor SU(3) breaking
The mixing of π0, η and η׳
China, 2009
Liping Gan, UNCW

8. Some Interesting η Rare Decay Channels
Mode
Branching Ratio
Physics Highlight
π0 π0
<3.5 × 10 − 4
CP, P
π0 2γ
( 2.7 ± 0.5 ) × 10 − 4
χPTh, Ο(p6)
π+ π−
<1.3 × 10 − 5
π0 π0 γ
<5 × 10 − 4 C 3γ
<1.6 × 10 − 5
π0 π0 π0 γ
<6 × 10 − 5
π0 e+ e−
<4 × 10 − 5
4π0
<6.9 × 10 − 7
The π0, η and η’ system provides a rich laboratory to study the symmetry structure of QCD.
China, 2009
Liping Gan, UNCW

9. PrimEx Program at Jlab (1) Primakoff experiments to measure:
Two-Photon Decay Widths:
Γ(π0 6 GeV Γ(η →), Γ(η׳ →)
Transition Form Factor Fγγ*P of π0, η and η׳ at low Q2 ( GeV2/c2)
(2) Measure the branching ratios of η and η’ rare decays *
China, 2009
Liping Gan, UNCW

10. Transition Form Factor
Experimental Status
Decay width
Transition Form Factor
China, 2009
Liping Gan, UNCW

11. Determine the quark masse ratio
Γ(η→3)=Γ(→)×B.R.
China, 2009
Liping Gan, UNCW

12. Mixing angles of η-η׳ Mixing angles: Decay constants:
Γ(η/η’ →) widths are crucial inputs for obtaining fundamental mixing parameters.
China, 2009
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
China, 2009
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 η decay
Both the Γ(η→γγ) and Γ(η’→γγ) decays are suited to confirm the number of colors
China, 2009
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
China, 2009
Liping Gan, UNCW

16. Primakoff Process ρ,ω Challenge: Extract the Primakoff amplitude
with unprecedented accuracy
China, 2009
Liping Gan, UNCW

17. Features of Primakoff cross section:
Beam energy sensitive
Peaked at very small forward angle
Coherent process
China, 2009
Liping Gan, UNCW

18. PrimEx Experiment 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
China, 2009
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
China, 2009
Liping Gan, UNCW

20. 0 = 7.82eV ± 2.2%stat. ± 2.1%syst. (± 3.0% total)
PrimEx-I Result
0 = 7.82eV ± 2.2%stat. ± 2.1%syst. (± 3.0% total)
PrimEx
China, 2009
Liping Gan, UNCW

21. PrimEx-II 6 GeV
Projected PrimEx-II
China, 2009
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
China, 2009
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
(~10-4 r.l. Au radiator, 5.0 mm beam collimator)
High resolution, high segmentation HyCal
Calorimetor
30 cm LH2 target (~3.6 r.l.)
China, 2009
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.
China, 2009
Liping Gan, UNCW

25. Statistics and Beam Time Request
Target: 30 cm (3.46% r.l.) LH2, Np=1.28x1024 p/cm2
Photon intensity: 7.6x106 γ/sec in Eγ = 10.5–11.7 GeV
Total cross section on P for θη= ,
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 10-2 events/sec
= 2200 events/day
= Primakoff events/day
Beam time request:
LH2 target run
45 days
Empty target run
5 days
Tagger efficiency, TAC
3 days
Setup calibration and checkout
7 days
Total
60 days
Statistics: 45 days of run on LH2:
1% stat. error
China, 2009
Liping Gan, UNCW

26. Estimated Error Budget on Γ(η →)
Systematical errors:
Contributions
Estimated Error
Photon flux
1.0%
Target number
0.5%
Background subtraction
Event selection
0.8%
Acceptance, misalign.
Beam energy
0.2%
Branching ratio (PDG)
0.66%
Total Systematic
1.9%
Total estimated error:
Statistical error
1.0%
Systematic error
1.9%
Total Error
2.2%
China, 2009
Liping Gan, UNCW

27. Some η Rare Decay Channels
Mode
Branching Ratio
Physics Highlight
π0 π0
<3.5 × 10 − 4
CP, P
π0 2γ
( 2.7 ± 0.5 ) × 10 − 4
χPTh, Ο(p6)
π+ π−
<1.3 × 10 − 5
π0 π0 γ
<5 × 10 − 4 C 3γ
<1.6 × 10 − 5
π0 π0 π0 γ
<6 × 10 − 5
π0 e+ e−
<4 × 10 − 5
4π0
<6.9 × 10 − 7
China, 2009
Liping Gan, UNCW

28. Study of η→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
China, 2009
Liping Gan, UNCW

29. History of the η→0 Measurements
After 1980
A long standing “η” puzzle is still un-settled.
China, 2009
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
China, 2009
Liping Gan, UNCW

31. Low energy η production (CB experiment on η→0 at AGS, by S
Low energy η production (CB experiment on η→0 at AGS, by S. Prakhov et al. )
η →000
-p→ 00n
The η’s were produced with 720 MeV/c - beam through the -p→ηn reaction
Decay ’s energy range: MeV
Final result
1600 of η→0 events
(η→0γγ)=0.45±0.12 eV
China, 2009
Liping Gan, UNCW

32. What can be improved at 12 GeV Jlab?
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0 background
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
July, 2009
Liping Gan, UNCW

33. Suggested Experiment in Hall D at Jlab
Counting
House
Photon Tagger
GlueX
FCAL
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
Forward calorimeter with PWO insertion to detect multi-photons from the η decay
China, 2009
Liping Gan, UNCW

34. S/N Ratio vs. Calorimeter Granularity
PWO
dmin=4cm
S/N=1.4
Pb Glass
dmin=8cm
S/N=0.024
July, 2009
Liping Gan, UNCW

35. Summary Fundamental input to Physics:
(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 ( 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 Chiral Lagrangian
Test P, CP and C symmetries, and search for new physics beyond Standard Model
China, 2009
Liping Gan, 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
Oct 31, 2008
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
China, 2009
Liping Gan, UNCW

38. Challenges in Modern Physics
What are the building blocks of matter?
Strong force
Strong force obeys the rules of Quantum Chromodynamics (QCD)
What happens if one tries to separate two quarks?
Color confinement: the potential energy between (in this case) a quark and an antiquark increases while increasing the distance between them.
The properties of strong force at large distance represents one of the biggest intellectual challenges in physics.
Solution: Understanding symmetries of nature
China, 2009
Liping Gan, UNCW

39. Experimental Resolutions: Prod. Angle
Precision Primakoff measurement
requires high resolutions in:
Production angle (fit);
Invariant mass (background)
Energy (elasticity)
China, 2009
Liping Gan, UNCW 39
A. Gasparian, Hall D, May 9, 2008 39

40. Experimental Resolutions (contd.)
γγ invariant mass
Energy conservation (elasticity)
High resolution, high granularity calorimeter is critical in:
event selection;
extraction of Primakoff from hadronic processes
China, 2009
Liping Gan, UNCW 40
A. Gasparian, Hall D, May 9, 2008 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.”
China, 2009
Liping Gan, UNCW

42. Low Energy η Production Continue (KLOE, by B. Micco et al. , Acta Phys
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
China, 2009
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:
China, 2009
Liping Gan, UNCW