A Precision Measurement of the Neutral Pion Lifetime: New Results from the PRIMEX Experiment Rory Miskimen University of Massachusetts, Amherst for the PRIMEX collaboration A short review of  0 →  The PRIMEX experiment at JLab Electromagnetic calibration reactions i.Pair production ii.Double arm Compton scattering Preliminary results for the  0 lifetime PRIMEX at 12 GeV

A short review of  0 →  : pre-QCD  0 discovered by Bjorklund, Crandall, Moyer and York at Berkeley Cyclotron in The soft-pion limit of PCAC predicts A  →  = 0 Adler, Bell, Jackiw and Bardeen discover triangle diagrams that alter PCAC predictions for  o decay p k1k1 k2k2

Wess, Zumino and Witten construct anomalous O(p 4 ) lagrangian that permits transitions between even and odd numbers of pseudo-scalar mesons The chiral anomaly has special status in QCD: there are no low energy constants in lagrangian A short review of  0 →  : post-QCD

Corrections due to (i) u- d- quark masses (ii) isospin breaking ( , , and  ´ mixing) proportional to quark mass differences Recent calculations in NLO ChPT gives ≈ 5% higher than LO, with uncertainty of less than 1% A short review of  0 →  : era of ChPT

  →  Decay width (eV) NLO, 1% error

Primakoff Reaction Primakoff Coherent Interference Incoherent

Arizona State University, Tempe, AZ, Catholic University of America, Washington, DC, Chinese Institute of Atomic Energy, Beijing, China, Eastern Kentucky University, Richmond, KY, George Washington University, Washington, DC, Hampton University, Hampton, VA, Institute for High Energy Physics, Chinese Academy of Sciences, Beijing, China, Institute for High Energy Physics, Protvino, Moscow region, Russia, Institute for Theoretical and Experimental Physics, Moscow, Russia, Kharkov Institute of Physics and Technology, Kharkov, Ukraine, Massachusetts Institute of Technology, Cambridge, MA, Norfolk State University, Norfolk, VA, North Carolina A&T State University, Greensboro, NC, North Carolina Central University, Durham, NC, Thomas Jefferson National Accelerator Facility, Newport News, VA, Tomsk Polytechnical University, Tomsk, Russia, Idaho State University, Pocatello, ID, University of Illinois, Urbana, IL, University of Kentucky, Lexington, KY, University of Massachusetts, Amherst, MA, University of North Carolina at Wilmington, Wilmington, NC, University of Virginia, Charlottesville, VA, Yerevan Physics Institute, Yerevan, Armenia PRIMEX Collaboration

EXPERIMENTAL SETUP  DATA TAKING FALL 2004  C, Pb TARGETS  JLAB HALL B PHOTON TAGGING FACILITY – TAGGING FACILITY – HIGH INTENSITY, HIGH INTENSITY, HIGH RESOLUTION DEVICE HIGH RESOLUTION DEVICE  STATE-OF-THE-ART CALORIMETRY – HyCal CALORIMETRY – HyCal  PAIR SPECTROMETER

 /T=0.3% PRIMEX Targets The effective number of carbon atoms/cm 2 in the carbon target is known to precision of ±0.05% Lead target

Pair Spectrometer

PrimEx Hybrid Calorimeter - HyCal A highly segmented hybrid calorimeter –576 lead-glass detectors (4.00×4.00×40 cm 3 ) –1152 lead-tungstate detectors (2.125×2.125×21.5 cm 3 ) –energy resolution –position resolution

Electromagnetic calibration reactions: 1.Pair production cross section on 12 C 2.Double arm Compton scattering on 12 C

Pair Production in PrimEx * *analysis results from A. Teymurazyan

Pair Production Events

Experimental results: differential cross sections Theory by Alexandr Korchin

Systematic Error Estimation Photon Flux2.5 % Target Thickness0.05% Background Estimation0.5% Calibrations2.8 % Total3.8 % Summary of pair analysis Agreement with theory at the level of ~3.8 % Work in progress to reduce the systematic errors to 1 – 2 % level

COMPTON SCATTERING IN PRIMEX* * analysis results from by P. Ambrozewicz, L. Gan, Y. Prok

COMPTON SCATTERING EVENTS  MINIMAL OPENING ANGLE/ MINIMAL SEPARATION CUT MINIMAL SEPARATION CUT  ADDITIONAL CUTS SERVE TO MINIMIZE TO MINIMIZE SIGNAL-TO-NOISE RATIO SIGNAL-TO-NOISE RATIO

RADIATIVE EFFECTS IN THE DATA

Radiative corrections: I. virtual loops, and soft double Compton scattering, Brown and Feynman; II. hard double Compton scattering Mork, and Mandl and Skyrme

SYSTEMATIC ERROR ESTIMATION  PHOTON FLUX 2.0 %  TARGET 0.05 %  SIGNAL/BACKGROUND SEPARATION < 1.5 %  RADIATIVE EFFECTS IN ACCEPTANCE CALC. 2.5 %  TOTAL 3.6 % Summary of Compton analysis Agreement with theory at the level of few % Work in progress to reduce the systematic errors to 1 – 2 % level

New results for the  0 lifetime* * analysis results from E. Clinton, I. Larin, and D. McNulty

Extracting Elastic Pion Yields versus  

Analysis 1* Find pion yield in bin centered on Elasticity = 1 Subtract inelastic pions: find pion yield as a function of elasticity * D. McNulty

Analysis 2* Kinematic fitting to the condition Elasticity = 1 Fit peak in mass distribution * I. Larin

Analysis 3* Project events onto axis perpendicular to the kinematic correlation between Elasticity and M  for elastic events Fit peak in mass distribution * E. Clinton

  yield (analysis 1, PbWO only)

Fitting the radiative width Analysis 1 and 2: Data are corrected using (1) geometric acceptance, or (2) full GEANT simulation. Angular resolution smeared theoretical distributions are fit to the data Analysis 3: Theoretical distributions are thrown in full GEANT simulation Resulting angular distributions include beam energy, resolution, acceptance, and reconstruction effects, and are fitted to the uncorrected experimental angular distributions

Fitted distribution (analysis 2, PbWO only) Primakoff Coherent Interference Incoherent

Fitted distribution (analysis 3, PbWO and lead glass)

Average = 7.93 eV ± 2.1%(stat) ± 2.5%(sys) PRELIMINARY RESULTS

Systematic Errors (major contributors) Photon Flux1.1 % Lineshape uncertainty1.0 % Integration cutoffs1.1 % Cluster position finding0.33 % Veto counter conversion0.25 % Incoherent background shape*1.5 % Total2.4 % Estimated systematic errors from analysis 3 * Analysis I estimate. Calculations by T. Rodrigues in progress

7.93 eV ± 2.1%(stat) ± 2.5%(sys) PRELIMINARY RESULTS   →  Decay width (eV) NLO, 1% error

PrimEx Program at 12 GeV JLAB We propose to measure: –Two-Photon Decay Widths: Γ(η―> γγ ), Γ(η ׳ ―> γγ ) – Transition Form Factor F γγ* P of π 0, η and η ׳ at low Q 2 ( GeV 2 /c 2 ) via the Primakoff effect. PrimEx Program at 12 GeV JLAB 11 GeV  photoproduction on proton

Fundamental input to physics:  Determination of quark mass ratio   -  ´ mixing  Interaction radius of  0,  and  ´  Is the  ´ an approximate Goldstone boson?

Summary and Conclusions  Pair production and Compton cross sections have been extracted with 4 % total errors. The goal is to obtain errors at the level of 1-2%  We are in the process of finalizing the π o lifetime measurement, and the systematic errors. Our preliminary result is:  Our result is in agreement with LO and NLO ChPT  Look forward to additional running to reduce our statistical and systematic errors  A Primakoff program at 12 GeV holds great promise for studies of the pseudo-scalar mesons