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An Updated High Precision Measurement of the Neutral Pion Lifetime via the Primakoff Effect

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

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An Updated High Precision Measurement of the Neutral Pion Lifetime via the Primakoff Effect

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  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. 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% • 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% 0→ • Precision measurements of (0→) at the percent levelwill provide a stringent test of a fundamental prediction of QCD. PAC33, January 15, 2008

  3. Decay Length Measurements (Direct Method) Measure 0decay length 1x10-16 sec too small to measure solution: Create energetic 0 ‘s, L = vE/m But, for E= 1000 GeV, Lmean 100 μm very challenging experiment 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→ PAC33, January 15, 2008

  4. e+e- Collider Experiment DORIS II @ DESY e+e-e+e-**e+e-0e+e- e+,e- scattered at small angles (not detected) only  detected Results: Γ(0) = 7.7 ± 0.5 ± 0.5 eV ( ± 10.0%) 0→ • Not included in PDG average • Major limitations of method • knowledge of luminosity • unknown q2 for ** PAC33, January 15, 2008

  5. ρ,ω Primakoff Method 12C target Primakoff Nucl. Coherent Nucl. Incoh. Interference Challenge: Extract the Primakoff amplitude PAC33, January 15, 2008

  6. 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 PAC33, January 15, 2008

  7. PrimEx Experiment • 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 • 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) PAC33, January 15, 2008

  8. PrimEx Milestones • Proposal approved in 1999 by PAC15, re-approved by PAC22 (E02-103) in 2002 with A rating. • In 2000, NSF awarded a collaborative MRI grant of $1 M to develop the experimental setup. • Full support of JLab (Engineering group, machine-shop, installation, etc.). • 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. • Preliminary results released at the April, 2007 APS meeting with AIP press conference. • First publication is expected in Spring, 2008. PAC33, January 15, 2008

  9. Luminosity Control: Pair Spectrometer Measured in experiment: • absolute tagging ratios: • TAC measurements at low intensities • relative tagging ratios: • pair spectrometer at low and high intensities Scint. Det. • 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 PAC33, January 15, 2008

  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 Kinematical constraint HYCAL only PAC33, January 15, 2008

  11. 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 PAC33, January 15, 2008

  12. Differential Cross section GEANT: • acceptances; • efficiencies; • resolutions; Experimental Yield per  Diff. cross section PAC33, January 15, 2008

  13. Fit to Extracted 0 Decay Width • Theoretical angular distributions smeared with experimental resolutions are fit to the data • Combined average from three groups: Γ(0)  7.93 eV  2.1%(stat.)  2.0% (syst) PAC33, January 15, 2008

  14. Fit to Extracted 0 Decay Width:208Pb Target • Theoretical angular distributions smeared with experimental resolutions are fit to the data PAC33, January 15, 2008

  15. PrimEx Current Result () = 7.93eV2.1%2.0% 0 Decay width (eV) ±1.% PAC33, January 15, 2008

  16. Estimated Systematic Errors PAC33, January 15, 2008

  17. 0 Forward Photoproduction off Complex Nuclei (theoretical models) • Coherent Production A→0A Leading order processes: (with absorption) Primakoff Nuclear coherent Next-to-leading order: (with absorption) 0 rescattering Photon shadowing PAC33, January 15, 2008

  18. Γ(0)Model Sensitivity • Two independent approaches: • Glauber theory • Cascade Model (Monte Carlo) • Photon shadowing effect • Incoherent Production A→0A´ Deviation in Γ(0) less than 0.2% Overall model error in Γ(0) extraction is controlled at 0.25% PAC33, January 15, 2008

  19. 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 PAC33, January 15, 2008

  20. Verification of Overall Systematics: Compton Cross Section Δσ/ΔΩ (mb/6.9 msrad) Data with radiative corrections Average stat. error: 0.6% Average syst. error: 1.2% Total: 1.3% PAC33, January 15, 2008

  21. Beam Time Request • Will provide 0.44% Primakoff statistics on each target • 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) PAC33, January 15, 2008

  22. Summary • The 0 lifetime is one of the few precision predictions of QCD. • Percent level measurement is a stringent test of QCD at these energies. • 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. • 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. • Compton and pair-production cross section measurementsdemonstrate 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. • Control of model error in 0 lifetime at 0.25% level has been reached. • Our first result: Γ(0)  7.93 eV  2.1% (stat.)  2.0% (syst.) • Requesting 28 days of beam time to reach the goal of 1.4% on 0 life time. PAC33, January 15, 2008

  23. The End PAC33, January 15, 2008

  24. 0 Event selection (cont.) Three groups analyzed the data independently PAC33, January 15, 2008

  25. Model dependence of Γ(0) Extraction Model error in Γ(0) Extraction can be controlled at < 0.25% PAC33, January 15, 2008

  26. Some results on Coherent Production A→0A • Electromagnetic form factors 12C E=5.2 GeV 208Pb • Strong form factors 208PbE=5.2 GeV Without shadowing With shadowing PAC33, January 15, 2008

  27. Estimated Systematic Errors on Compton (preliminary) PAC33, January 15, 2008

  28. 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 PAC33, January 15, 2008

  29. Compton as Stability Control(maybe to question section) σ (mb) PAC33, January 15, 2008

  30. ρ, ω Primakoff Method Challenge: Extract the Primakoff amplitude PAC33, January 15, 2008

  31. Compton Cross section PAC33, January 15, 2008

  32. 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) PAC33, January 15, 2008

  33. Trigger Improvement PAC33, January 15, 2008

  34. Impact of Giant Excitation of Nucleus on 0 Primakoff production • With nuclear collective excitation, the longitudinal momentum transfer in 0 photo-production is Δin= Δ+Eav, where the average excitation energy Eav for 12C 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. PAC33, January 15, 2008

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