1. Status of the PrimEx Experiment: A Precision Measurement of the Neutral Pion Lifetime 7 th European Conference on “Electromagnetic Interactions with Nucleons and Nuclei” Milos Island, Greece September 12-15, 2007 Dan Dale Idaho State University

2. Participating Institutions North Carolina A&T State University University of Massachusetts University of Kentucky Idaho State University Jefferson Lab Hampton University MIT University of North Carolina at Wilmington Catholic University of America Arizona State University University of Virginia ITEP, Russia IHEP, Russia Norfolk State University University of Illinois Kharkov Inst. of Physics and Technology Chinese Institute of Atomic Energy IHEP, Chinese Academy of Sciences George Washington University Yerevan Physics Institute, Armenia Southern University at New Orleans Tomsk Polytechnical University, Russia University of Sao Paulo

3. A High Precision Measurement of    Physics motivation The Primakoff effect Experimental setup Towards high precision Current analysis event selection radiative width extraction Summary

4. The axial anomaly Leading order corrections (Donoghue, 1989) Corrections O(p 4 )

5. Current experimental and theoretical status Expected PrimEx error bar, arbitrarily projected to the leading order chiral anomaly    decay width (eV) Next to Leading Order, +- 1%

6.  Sum rule approach to axial-vector-vector (AVV) form factor.  f   – f   caused by strong interaction shown to be small.  Eta width only input parameter.   o   mixing included.  o   eV Recent Theoretical Advances (B.L. Ioffe and A.G. Oganesian, Phys. Lett. B647, p. 389, 2007)

9. The Primakoff Effect Radiative width Known, measured quantities

10. Angular distribution enables separation of amplitudes

11. For high precision: Measure angular distribution well to separate various amplitudes  new state of the art  o detector Measure on a variety of targets to test validity of background extraction (  Primakoff    Tight photon energy (  peak  4  and flux control  photon tagging Measure QED processes (Compton and pair production) to validate setup and analysis techniques.

12. incident electrons Tagged photon beam

13. Bremsstrahlung photon tagging Electron beam Post-brems electrons

14. Tagging Ratios

15. Photon Calorimetry – the HYCAL calorimeter 1152 PbWO 4 and 576 lead glass modules 1.6% energy resolution 1-2 mm position resolution

16. In-beam calibration

18. Effect of periodicity of detector modules.

19. Energy resolution – lead tungstate

21. Luminosity analysis efforts Electron counting (absolute flux) Pair spectrometer (relative tagging ratios) TAC (absolute tagging ratios) Target thickness Known cross sections (pair production, Compton)

22. Online Flux Monitoring e + - e - pair spectrometer dipole magnet plastic scintillators

23. Stability of relative tagging ratios Monitored during production data taking.

24. Stability of relative tagging ratios Stable to 1%.

25. Relative tagging ratios versus rate Independent of rate!

26. Pair production cross section (1)Bethe-Heitler (modified by nuclear form factor). (2)Virtual Compton scattering. (3)Radiative effects. (4)Atomic screening. (5)Electron field pair production.

27. Compton Scattering--Stability of Setup

28. Angular distribution for Carbon Primakoff Peak

29. UMass Analysis  HYCAL – Tagger timing  Invariant Mass  Elasticity Total Probability = Timing x Mass x Elasticity Event selection: Normalized probability distributions for

30. Elasticity versus invariant mass Hybrid mass

31. UMass Analysis: Radiative Width for Carbon Primakoff peak Nuclear coherent and Primakoff – NC interference

32. MIT/JLab Analysis: Event Selection Invariant mass HYCAL – Tagger timing Elasticity

33. Angular distributions and background Carbon Lead

34. MIT/JLab Analysis: Radiative Widths for Carbon and Lead Carbon:  = 8.20 ± 0.15 eV ± 0.20 eV Lead:  = 8.11 ± 0.16 eV ± 0.20 eV

35. ITEP/NCA&T Analysis: Radiative Width for Carbon  = 7.98 ± 0.18eV  All possible cluster combinations.  Elasticity imposed.  HYCAL – tagger timing < 4 nsec.

36. ITEP/NCA&T Analysis: Radiative Width for Lead  = 7.94 ± 0.18eV

37. ITEP/NCA&T Analysis: Radiative Width Combined Fit for Carbon and Lead  = 7.95 ± 0.13eV Carbon Lead

39. Summary I UMass Analysis Carbon  8.41 eV ± 0.15 eV ± 1.8% MIT/JLab Analysis: Carbon:  = 8.20 eV ± 0.15 eV ± 0.20 eV Lead:  = 8.11 eV ± 0.16 eV ± 0.20 eV ITEP/NCA&T Analysis: Carbon:  = 7.98 eV ± 0.18 eV Lead:  = 7.94 eV ± 0.18 eV Combined fit, Carbon and Lead:  = 7.95 eV ± 0.13 eV

40. Summary The neutral pion lifetime is a stringent test of confinement scale QCD. High precision of the experiment has pushed the limits of photon calorimetery, luminosity monitoring, and QED calculations. Experimental systematic errors controlled by pair production and Compton scattering. High quality data taken in late 2004. Three quasi-independent analyses. Our preliminary result is:  = 7.93 eV ± 2.1% (stat) ± 2.0% (stat) In good agreement with ChPT predictions. Further studies of backgrounds underway.

41. Extra Slides

42. PDF1: HYCAL – Tagger timing

43. PDF 3: Invariant mass

44. PDF 2: Elasticity

45. Previous Experiment: The Direct Method (Phys. Lett., vol 158B, no. 1, 81, 1985)  o `s produced by 450 GeV protons in tungsten foils.  o decays observed by detection of 150 GeV/c positrons produced by decay  rays converting in foils. Y(d) = N[A + B(1-e -d/ )] Dalitz decay,  conversion in first foil