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

A. GasparianPAC33, January 15,  0  decay width   0 →  decay proceeds primarily via chiral anomaly in QCD.  The prediction of chiral anomaly is exact for massless quarks:  Corrections to 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 percent level will provide a stringent test of fundamental predictions of QCD.  0 →   Recent calculations in QCD sum rule: (B.L. Ioffe, at al. Phys. Lett. B647, p. 389, 2007)  Γ(  ) is only input parameter   0 -  mixing included Γ(  0  ) = 7.93eV ± 1.5%

A. GasparianPAC33, January 15, Decay Length Measurements (Direct Method)     1x 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 An experiment had been done at CERN, in 1984, P=450 GeV proton beam 2 variable (5-250  m) foils Result:  (  0  ) = 7.34eV  3.1%(total) Dominant systematic error: Uncertainty in P  0 (  1.5%)  Limitations of method  unknown P  0 spectrum  foil position dependent exptl. bgnd.

A. GasparianPAC33, January 15, e + e - Collider Experiment  e + e -  e + e -  *  *  e + e -  0  e + e -   e +, e - scattered at small angles (not detected)  only  detected  experiment: DORIS DESY  Results: Γ(  0  ) = (7.7 ± 0.5 ± 0.5 eV ( ± 10.0%)  dominant systematic errors:  luminosity (~6%)  beam-residual gas interaction  Not included in PDG average  Limitations of method  luminosity  unknown q 2 for  *  *

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

A. GasparianPAC33, January 15, 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)  bresstrahlung  beam E  =4 and 6 GeV  targets: Be, Al, Cu, Ag, U  Result:  (  0  )=(7.92  0.42) eV  5.3%  dominant systematic errors:  N  (  4%) and quantameter (  2%)  All previous experiments used:  Bremsstrahlung (untagged)  beam  Conventional Pb-glass calorimetry

A. GasparianPAC33, January 15, 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 to 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

A. GasparianPAC33, January 15, 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 August-November 2004 run.  First publication is expected in spring,  Preliminary results had been released at APS April, 2007 meeting with AIP press conference.

A. GasparianPAC33, January 15, Luminosity Control: Pair Spectrometer Dipole  Precision cross section measurement:  photon flux at 1% level required e-e- e+e+ HYCAL Photon beam Scint. Det.  absolute tagging ratios:  TAC measurements at low intensities  Checked by cross sections of known EM processes at the 1% level:  Compton scattering  e + e - pair production  relative tagging ratios:  pair spectrometer at low and high intensities

A. GasparianPAC33, January 15, Electromagnetic Calorimeter: HYCAL  1152 PbWO 4 crystal detectors  576 Pb-glass Cherenkov detectors  Energy resolution  Position resolution  Good photon detection 0.1 – 5 GeV;  Large geometrical acceptance PbWO4 crystals resolution Pb-glass budget Design concept hybrid calorimeter

A. GasparianPAC33, January 15,  -  Invariant Mass Resolution

A. GasparianPAC33, January 15,  0 Event selection We measure:  initial photon energy: E  and time  energies of decayed photons: E  1, E  2 and time  X,Y positions of decayed photons Kinematical constrains:  Conservation of energy;  Conservation of momentum;  m  invariant mass Three groups analyzed the data independently

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

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

A. GasparianPAC33, January 15,  0 Forward Photoproduction off Complex Nuclei (theoretical models)  Incoherent Production  A →  0  A´  Two independent approaches:  Glauber theory  Cascade Model (Monte Carlo) Deviation in Γ(  0  ) Extraction: less than <0.2%

A. GasparianPAC33, January 15, Fit to Extract  0  Decay Width  Combined average from three groups: Γ(  0  )  7.93 eV  2.10%(stat.)  2.0% (syst)  Theoretical angular distributions smeared with experimental resolutions are fit to the data

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

A. GasparianPAC33, January 15, Estimated Systematic Errors Type of Systematic Errors Estimated contributions in first run Estimated contributions for current 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%

A. GasparianPAC33, January 15, Compton Cross section: Theory Pure QED process: Should be calculable on 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)

A. GasparianPAC33, January 15, Compton Cross section: Experiment  Average stat. error: 0.6%  Average syst. error: 1.2%  Total: 1.3% Δσ/ΔΩ (mb/6.9 msrad)

A. GasparianPAC33, January 15, Summary  A state-of-the-art high resolution experimental setup including a high precision EM calorimeter and pair spectrometer has been designed, developed, constructed and commissioned with first physics run in fall,  Our first result: Γ(  0  )  7.93 eV  2.10% (stat.)  2.0% (syst.)  The  0 lifetime is one of the few parameter-free predictions in QCD reflecting effects of fundamental symmetry and axial anomaly.  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 1.3% level.  The experimental setup is capable for a percent level cross section measurement.  Availability of high resolution and high intensity tagging facility together with recent developments in calorimetry made the Primakoff method the viable way to reach the projected percent level 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.

A. GasparianPAC33, January 15, The End

A. GasparianPAC33, January 15, Stability of relative tagging ratios  Monitored by PS during production data taking. PS+tagger Tagger

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

A. GasparianPAC33, January 15, Theoretical Study of  0 Forward Photoproduction off Complex Nuclei Coherent Production  A→  0  A: Primakoff Nuclear coherent  0 rescattering Photon shadowing Absorption of  0

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

A. GasparianPAC33, January 15, 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

A. GasparianPAC33, January 15, Incoherent Production  A →  0  A´ Two independent approaches: Glauber theory Cascade Model Deviation in Γ(  0  ) Extraction is <0.2%

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

A. GasparianPAC33, January 15, New from Ilya, (animation)  Combined average from three groups: Γ(  0  )  7.93 eV  2.10%(stat.)  Theoretical angular distributions smeared with experimental resolutions are fit to the data

A. GasparianPAC33, January 15, Control of Systematic Errors: Compton Events Selection  Energy conservation  3-momentume conservation (including co-planarity) We measure:  Initial photon energy: E  and time  Energies of scattered particles: E , Ee and time  X,Y positions on HYCAL

A. GasparianPAC33, January 15, 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%

A. GasparianPAC33, January 15, 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

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

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

A. GasparianPAC33, January 15, Compton Cross section

A. GasparianPAC33, January 15, Trigger Improvement

A. GasparianPAC33, January 15, e + e - Pair Production in PrimEx  Agreement with theory at the level of 2.5%  Work in progress to reduce the systematic errors to 1-2% level

A. GasparianPAC33, January 15, An Example: 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

A. GasparianPAC33, January 15, Compton Cross section

A. GasparianPAC33, January 15, PbWO 4 Development: Optical Properties Optical transparency

A. GasparianPAC33, January 15, PbWO 4 Development Specified Size: 20.5x20.5x180 mm 3 Tolerances: in trans in long.  Collaboration managed to double the number of crystals: to from 650 to 1250  Critical for the experiment

A. GasparianPAC33, January 15,  0  decay width: recent theoretical advances  QCD sum rule approach:  f  0 - f  + caused by strong interaction shown to be small   0 -  mixing included Γ(  0  ) = 7.93eV ± 1.5%  error is dominated by Γ(  ) decay width  Precision measurements of (  0 →  ) at percent level will provide ultimate test of fundamental predictions of QCD.  0 → 

A. GasparianPAC33, January 15,  (  0 →  ) World Data (do not need)   0 is lightest quark-antiquark hadron  The lifetime:  = B.R.(  0 → γγ )/  (  0 → γγ )  0.8 x second  Branching ratio : B.R. (  0 → γγ ) = (98.8±0.032)%  0 →  ±1%

A. GasparianPAC33, January 15, 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.

Outline  Physics Motivation  Different methods of lifetime measurements  The PrimEx experiment and our first results  Control of systematic errors  Summary