1. 1A. GasparianHall D, March 7, 20081 Primakoff Experiments at 12 GeV with GlueX A. Gasparian NC A&T State University, Greensboro, NC For the PrimEx Collaboration Outline  The project and physics motivation:  The first experiment @ 6 GeV:  0 lifetime  Development of precision technique  Results for  0 lifetime  Experiments @ 12 GeV with GlueX  Summary

2. 2A. GasparianHall D, March 7, 20082 The PrimEx Project at JLab Experimental program Precision measurements of:  Two-Photon Decay Widths: Γ(  0 →  ), Γ(  →  ), Γ(  ’ →  )  Transition Form Factors at low Q 2 (0.001-0.5 GeV 2 /c 2 ): F(  * →  0 ), F(  * →  ), F(  * →  ) Test of Chiral Symmetry and Anomalies via the Primakoff Effect

3. 3A. GasparianHall D, March 7, 20083 Physics Motivation Fundamental input to Physics:  precision test of chiral anomaly predictions  determination of quark mass ratio   -  ’ mixing angle   0,  and  ’ interaction electromagnetic radius  is the  ’ an approximate Goldstone boson?

4. 4 First experiment:  0  decay width   0 →  decay proceeds primarily via the chiral anomaly in QCD.  The chiral anomaly prediction is exact for massless quarks:  Corrections to the chiral anomaly prediction: (u-d quark masses and mass differences) Calculations in NLO ChPT: (J. Goity, at al. Phys. Rev. D66:076014, 2002) Γ(  0  ) = 8.10eV ± 1.0% ~4% higher than LO, uncertainty: less than 1%  Precision measurements of  (  0 →  ) at the percent level will provide a stringent test of a fundamental prediction of QCD.  0 →   Recent calculations in QCD sum rule: (B.L. Ioffe, et al. Phys. Lett. B647, p. 389, 2007)  Γ(  ) is only input parameter   0 -  mixing included Γ(  0  ) = 7.93eV ± 1.5%

5. 5 Decay Length Measurements (Direct Method)     1x10 -16 sec  too small to measure solution: Create energetic  0 ‘s, L = v   E  /m  But, for E= 1000 GeV, L mean  100 μm very challenging experiment  Measure  0 decay length 1984 CERN experiment: P=450 GeV proton beam Two variable separation (5-250  m) foils Result:  (  0  ) = 7.34eV  3.1% (total)  Major limitations of method  unknown P  0 spectrum  needs higher energies for improvement  0 → 

6. 6 e + e - Collider Experiment  e + e -  e + e -  *  *  e + e -  0  e + e -   e +, e - scattered at small angles (not detected)  only  detected  DORIS II @ DESY  Results: Γ(  0  ) = 7.7 ± 0.5 ± 0.5 eV ( ± 10.0%)  Not included in PDG average  Major limitations of method  knowledge of luminosity  unknown q 2 for  *  *  0 → 

7. 7A. GasparianHall D, March 7, 20087 Primakoff Method ρ,ωρ,ω Challenge: Extract the Primakoff amplitude 12 C target Primakoff Nucl. Coherent Interference Nucl. Incoh.

8. 8 Previous Primakoff Experiments  DESY (1970)  bremsstrahlung  beam, E  =1.5 and 2.5 GeV  Targets C, Zn, Al, Pb  Result:  (  0  )=(11.7  1.2) eV  10.%  Cornell (1974)  bremsstrahlung  beam E  =4 and 6 GeV  targets: Be, Al, Cu, Ag, U  Result:  (  0  )=(7.92  0.42) eV  5.3%  All previous experiments used:  Untagged bremsstrahlung  beam  Conventional Pb-glass calorimetry

9. 9 PrimEx Experiment at Hall B  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 (HYCAL)  Requirements of Setup:  high angular resolution (~0.5 mrad)  high resolutions in calorimeter  small beam spot size (‹1mm)  Background:  tagging system needed  Particle ID for (  -charged part.)  veto detectors needed

10. 10 Electromagnetic Calorimeter: HYCAL  Energy resolution  Position resolution  Good photon detection efficiency @ 0.1 – 5 GeV;  Large geometrical acceptance PbWO4 crystals resolution Pb-glass budget HYCAL only Kinematical constraint

11. 11 Fit to Extract  0  Decay Width: Γ(  0  )  7.93 eV  2.1%(stat.)  2.0% (syst)

12. 12 PrimEx Current Result  (  ) = 7.93eV  2.1%  2.0%  0  Decay width (eV) ±1.%

13. 13 Estimated Systematic Errors Type of Errors Errors in current dataExpected errors from 2 nd run Photon flux1.0% Target number<0.1% Background subtraction1.0%0.4% Event selection0.5%0.35% HYCAL response function0.5%0.2% Beam parameters0.4% Acceptance0.3% Model errors (theory)1.0%0.25% Physics background0.25% Branching ratio0.03% Total2.0%1.3%

14. 14A. GasparianHall D, March 7, 200814 PrimEx @ 12 GeV Precision Measurement of  →  decay width  All  decay widths are calculated from  decay width and experimental Branching Ratios (B.R.): Γ(η → decay) = Γ(  →  ) × B.R.  Any improvement in Γ(  →  ) will change the whole will change the whole  - sector in PDB  - sector in PDB

15. 15A. GasparianHall D, March 7, 200815 Physics Outcome from  Experiment Γ(η → 3  )=Γ(  →  )×B.R.   -  ’ mixing angle  light quark mass ratio

16. 16A. GasparianHall D, March 7, 200816 Primakoff Method ρ,ωρ,ω Challenge: Extract the Primakoff amplitude 12 C target Primakoff Nucl. Coherent Interference Nucl. Incoh.

17. 17  Increase Primakoff cross section:  Better separation of Primakoff reaction from nuclear processes:  Momentum transfer to the nuclei becomes less reduce the incoherent background Why do we need 12 GeV?

18. 18A. GasparianHall D, March 7, 200818  Experiment with GlueX Advantages:  High energy tagged photon beam Eγ=10 – 11.5 GeV  High acceptance electromagnetic calorimeter (FCAL)  Solenoid detector to veto charged particles, and reduce background on FCAL  Targets (~1-5% R.L.):  LH2,  LHe4,  solid 12 C Challenges:  Photon flux stability and control: possible solutions:  e + e - pair spectrometer;  Compton scattering;  High resolution FCAL needed for precision experiments: possible solution:  Pb-glass + PbWO 4 crystals

19. 19A. GasparianHall D, March 7, 200819 FCAL Geometrical Coverage  Forward Calorimeter FCAL (~2800 Pb-glass blocks  Radius = 120 cm:  Central beam hole3x3 blocks removed (12x12 cm2)  FCAL has a good coverage for the forward  →  production

20. 20A. GasparianHall D, March 7, 200820 Geometrical Acceptances  Forward Calorimeter FCAL (~2800 Pb-glass blocks  Radius = 120 cm:  Central beam hole: 3x3 blocks removed (12x12 cm2)  A good geometrical acceptance can be reached for L = 6-9 m for η forward production angles needed for the experiment

21. 21A. GasparianHall D, March 7, 200821 Experimental Resolutions (prod. angle)  Precision cross section measurement requires high resolutions in:  luminosity (flux + target)  production angle (for fit);  invariant mass (background)  … FCAL with all Pb-glass FCAL with Pb-glass and PbWO4 crystal insertion (75x75 blocks (150x150 cm 2 )

22. 22A. GasparianHall D, March 7, 200822 Experimental Resolutions (inv. mass) FCAL with all Pb-glass FCAL with Pb-glass and PbWO4 crystal insertion (75x75 blocks (150x150 cm 2 )

23. 23A. GasparianHall D, March 7, 200823 Experimental Resolutions (production angle vs. beam spot size) Photon beam size up to 5 mm, as it is designed, seams reasonable

24. 24A. GasparianHall D, March 7, 200824 Experimental Resolutions (production angle vs. target length)  Reaction vertex can not be reconstructed in this experiment (recoil energies are too small T< 1 MeV)  Large size of the FCAL calorimeter provides longer target to FCAL distance  That makes less sensitivity from the target length up to designed 30 cm liquid targets

25. 25 Luminosity Control: Pair Spectrometer Scint. Det. Measured in experiment:  absolute tagging ratios:  TAC measurements at low intensities  Uncertainty in photon flux at the level of 1% has been reached  Verified by known cross sections of EM processes  Compton scattering  e + e - pair production  relative tagging ratios:  pair spectrometer at low and high intensities

26. 26 Luminosity Control: Pair Production Cross Section Theoretical Inputs to Calculation:  Bethe-Heitler (modified by nuclear form factor)  Virtual Compton scattering  Radiative effects  Atomic screening  Electron field pair production Experiment/Theory = 1.0004

27. 27 Verification of Overall Systematics: Compton Cross Section  Average stat. error: 0.6%  Average syst. error: 1.2%  Total: 1.3% Δσ/ΔΩ (mb/6.9 msrad) Data with radiative corrections

28. 28 15 Days Beam Time and Statistics  Target: L=20 cm, LHe4 N He = 4x10 23 atoms/cm 2 N γ = 1x10 7 photon/sec (10-11.5 GeV part) = 1.6x10 -5 mb N(  ) = N He xN γ x xεx(BR) = 4x1023x 1x107x 1.6x10-32x0.7x0.4 = 64 events/hour = 1500 events/day = 45,000 events/30 days  Will provide sub-percent systematic error

29. 29 Statistical Target thickness Photon flux Acceptance misalignment Background subtraction Beam energy Distorted form factor Nuclear coherent contr. Branching ratio Total 0.5% 1.0% 0.5% 0.4% 0.2% 0.3% 0.5% 0.8% (PDG) 2.0% Estimated Error Budget

30. 30A. GasparianHall D, March 7, 200830 Summary  A state-of-the-art high resolution experimental setup has been designed, developed and constructed for the 6 GeV run.  New proposal for the 1.4% accuracy in Γ(  0  ) has been approved for the second 6 GeV run.  PrimEx collaboration has developed an experimental program to perform precision test of chiral symmetry and anomaly effects in the light pseudoscalar meson sector.  The first experiment, the  0 lifetime measurement has been successfully performed in Hall B in 2004. Preliminary result: Γ(  0  )  7.93 eV  2.10%(stat.)  2.0%(syst.)  Reach experimental program for η, η’ widths measurements has been developed and approved by high energy PACs. These precision experiments can be performed with the upgraded GlueX experimental setup at 12 GeV.

31. 31A. GasparianHall D, March 7, 200831 The End

32. 32A. GasparianHall D, March 7, 200832 The Primakoff Effect ρ, ω Challenge: Extract the Primakoff amplitude

33. 33A. GasparianHall D, March 7, 200833 PbWO 4 Energy Resolution 6 x 6 crystals  E /E = 1.3 % 1 x 1 3 x 3

34. 34A. GasparianHall D, March 7, 200834 PbWO 4 Position Resolution  x = 1.3 mm

35. 35A. GasparianHall D, March 7, 200835 Experimental Setup Development: Pair Spectrometer  Pair spectrometer was designed for relative photon flux monitoring at high beam intensities:  Combination of:  16 KGxm dipole magnet  2 telescopes of 2x8 scintillating detectors Dipole  Precision cross section measurements need control of photon flux at 1% level e-e- e+e+ HYCAL Photon beam Scint. Det.

36. 36A. GasparianHall D, March 7, 200836  (  0 →  ) World Data   0 is lightest quark-antiquark hadron  The lifetime:  = B.R.(  0 → γγ )/  (  0 → γγ )  0.8 x 10 -16 second  Branching ratio : B.R. (  0 → γγ ) = (98.8±0.032)%  0 →  ±1%