1. An Updated High Precision Measurement of the Neutral Pion Lifetime via the Primakoff Effect A. Gasparian NC A&T State University, Greensboro, NC for the PrimEx Collaboration Outline  Physics Motivation  Different methods of lifetime measurements  The PrimEx experiment and our first results  Control of systematic errors  Summary

2. A. GasparianPAC33, January 15, 20082  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%

3. A. GasparianPAC33, January 15, 20083 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 → 

4. A. GasparianPAC33, January 15, 20084 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 → 

5. A. GasparianPAC33, January 15, 20085 Primakoff Method ρ,ωρ,ω Challenge: Extract the Primakoff amplitude 12 C target Primakoff Nucl. Coherent Interference Nucl. Incoh.

6. A. GasparianPAC33, January 15, 20086 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

7. A. GasparianPAC33, January 15, 20087 PrimEx Experiment  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

8. A. GasparianPAC33, January 15, 20088 PrimEx Milestones  Proposal approved in 1999 by PAC15, re-approved by PAC22 (E02-103) in 2002 with A rating.  Full support of JLab (Engineering group, machine-shop, installation, etc.).  In 2000, NSF awarded a collaborative MRI grant of $1 M to develop the experimental setup.  In 4 years the experimental setup, including procurement of all hardware, was designed, constructed and tested.  Commissioning and data taking was performed in Sept-November 2004 run.  First publication is expected in Spring, 2008.  Preliminary results released at the April, 2007 APS meeting with AIP press conference.

9. A. GasparianPAC33, January 15, 20089 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

10. A. GasparianPAC33, January 15, 200810 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. A. GasparianPAC33, January 15, 200811  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 Three groups analyzed the data independently PrimEx Online Event Display

12. A. GasparianPAC33, January 15, 200812 Differential Cross section Experimental Yield per   GEANT:  acceptances;  efficiencies;  resolutions; Diff. cross section

13. A. GasparianPAC33, January 15, 200813 Fit to Extracted  0  Decay Width  Combined average from three groups: Γ(  0  )  7.93 eV  2.1%(stat.)  2.0% (syst)  Theoretical angular distributions smeared with experimental resolutions are fit to the data

14. A. GasparianPAC33, January 15, 200814 Fit to Extracted  0  Decay Width: 208 Pb Target  Theoretical angular distributions smeared with experimental resolutions are fit to the data

15. A. GasparianPAC33, January 15, 200815 PrimEx Current Result  (  ) = 7.93eV  2.1%  2.0%  0  Decay width (eV) ±1.%

16. A. GasparianPAC33, January 15, 200816 Estimated Systematic Errors Type of Errors Errors in current dataErrors in this proposal 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%

17. A. GasparianPAC33, January 15, 200817  0 Forward Photoproduction off Complex Nuclei (theoretical models)  Coherent Production  A →  0  A PrimakoffNuclear coherent  0 rescattering Photon shadowing Leading order processes: (with absorption) Next-to-leading order: (with absorption)

18. A. GasparianPAC33, January 15, 200818 Γ(  0  ) Model Sensitivity  Incoherent Production  A →  0  A´  Two independent approaches:  Glauber theory  Cascade Model (Monte Carlo)  Photon shadowing effect Deviation in Γ(  0  ) less than 0.2%  Overall model error in Γ(  0  ) extraction is controlled at 0.25%

19. A. GasparianPAC33, January 15, 200819 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

20. A. GasparianPAC33, January 15, 200820 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

21. A. GasparianPAC33, January 15, 200821 Beam Time Request 12 C target7 days 208 Pb target6 days Compton and pair prod.4 days Empty target2 days Calibrations/checkout6 days HYCAL config. change2 days Tagging efficiency1 days Total28 days  Response to TAC comment on statistical error: 2% statistical error on current result Primakoff stat. (1.46%) + fit error  0.44% error from proposal requires (1.46/0.44) 2 = 11 times more events:  Increase DAQ rate from 1 kHz to 5 kHz  Implement more stringent trigger, (factor of 2-3)  Will provide 0.44% Primakoff statistics on each target

22. A. GasparianPAC33, January 15, 200822 Summary  A high resolution experimental setup including an EM calorimeter and pair spectrometer has been designed, developed, constructed and commissioned with first physics run in Fall, 2004.  Our first result: Γ(  0  )  7.93 eV  2.1% (stat.)  2.0% (syst.)  The  0 lifetime is one of the few precision predictions of QCD.  Percent level measurement is a stringent test of QCD at these energies.  Compton and pair-production cross section measurements demonstrate that the systematic errors are controlled at the 1.3% level.  The experimental setup is capable of percent level cross section measurements. Collaboration has developed required expertise.  High resolution, high intensity tagging facility together with recent developments in calorimetry make the Primakoff method the best way to reach the required accuracy in  0  decay width.  Control of model error in  0 lifetime at 0.25% level has been reached.  Requesting 28 days of beam time to reach the goal of 1.4% on  0 life time.

23. A. GasparianPAC33, January 15, 200823  “… There is nothing new to be discovered in physics now. All that remains is more and more precise measurement.” William Thomson (Lord Kelvin), 1900

24. A. GasparianPAC33, January 15, 200824 The End

25. A. GasparianPAC33, January 15, 200825  0 Event selection (cont.) Three groups analyzed the data independently

26. A. GasparianPAC33, January 15, 200826 Model dependence of Γ(  0  ) Extraction Model error in Γ(  0  ) Extraction can be controlled at < 0.25%

27. A. GasparianPAC33, January 15, 200827 Some results on Coherent Production  A →  0  A Electromagnetic form factors Strong form factors 12 C E  =5.2 GeV 208 Pb 208 Pb E  =5.2 GeV Without shadowing With shadowing

28. A. GasparianPAC33, January 15, 200828 Estimated Systematic Errors on Compton (preliminary) Photon flux1.0% Target thickness (+impurity)0.05% Coincidence timing0.03% Coplanarity0.065% Radiative tail cut0.098% Geometric cuts stability0.65% Background subtraction0.40% Yield fit stability0.063% Total1.27%

29. A. GasparianPAC33, January 15, 200829 PrimEx Collaboration North Carolina A&T State University University of Massachusetts Idaho State University University of North Carolina Wilmington Jefferson Lab MIT Catholic University of America Arizona State University CIAE Beijing, China Norfolk State University Beijing University, China Lanzhou University, China ITEP Moscow, Russia IHEP Protvino, Russia Duke University Kharkov Inst. of Physics and Tech. Ukraine Northwestern University IHEP, China University of Sao Paulo, Brazil Yerevan Physics Institute, Armenia RIKEN, Japan JINR Dubna, Russia USTC, China Hampton University George Washington University

30. A. GasparianPAC33, January 15, 200830 Compton as Stability Control (maybe to question section) σ (mb)

31. A. GasparianPAC33, January 15, 200831 Primakoff Method ρ, ω Challenge: Extract the Primakoff amplitude

32. A. GasparianPAC33, January 15, 200832 Compton Cross section

33. A. GasparianPAC33, January 15, 200833 Compton Cross section: Theory Pure QED process: Calculable at the percent level  Leading Order: Klein-Nishina  Corrections to LO:  Rad. correction (virtual/soft)  Double Compton (hard emiss.)  Klein-Nishina + full rad. Corr. (Monte Carlo Method)  Klein-Nishina + full rad. Corr. (Numerical Integration Method)

34. A. GasparianPAC33, January 15, 200834 Trigger Improvement

35. A. GasparianPAC33, January 15, 200835 Impact of Giant Excitation of Nucleus on  0 Primakoff production With nuclear collective excitation, the longitudinal momentum transfer in  0 photo-production is Δ in = Δ+E av, where the average excitation energy E av for 12 C is ~20-25 MeV. The ratio of the cross section of the  0 photo-production in the Coulomb field with nuclear excitation to “elastic” electromagnetic production can be estimated as: Nuclear Giant Excitation effect for lead is small as well.