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π 0 Lifetime from the PrimEx Experiments

This outline discusses the properties of π0, the chiral anomaly, and the PrimEx experiments aiming to measure the lifetime of π0 to test QCD predictions. It includes the PrimEx-I result and the status of PrimEx-II.

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π 0 Lifetime from the PrimEx Experiments

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  1. π0 Lifetime from the PrimEx Experiments Outline • 0 and QCD symmetries • PrimEx-I result • PrimEx-II status • Summary Liping Gan University of North Carolina Wilmington (for the PrimEx Collaboration)

  2. Properties of 0 • 0 is the lightest hadron: m = 135 MeV • 0 is unstable. 0→ γγ B.R.(0 →γγ)=(98.8±0.032)% • Lifetime and Radiative Decay width:  = B.R.( 0 →γγ)/(0 →γγ)  0.8 x 10-16 second

  3. Spontaneous Chiral Symmetry Breaking Gives Rise to π0 In massless quark limit Massless Goldstone Bosons Corrections to theory: • Non-zero quark masses generate meson masses • Quark mass differences cause mixing among the mesons Since π0 is the lightest quark-antiquark system in nature, the corrections are small.

  4. k1 p Axial Anomaly Determines π0Lifetime • 0→ decay proceeds primarily via the chiral anomaly in QCD. • The chiral anomaly prediction is exact for massless quarks: k2 • Γ(0)is one of the few quantities in confinement region that QCD can calculate precisely to higher orders! • Corrections to the chiral anomaly prediction: Calculations in NLO ChPT: • Γ(0) = 8.10eV ±1.0% (J. Goity, et al. Phys. Rev. D66:076014, 2002) • Γ(0) = 8.06eV ±1.0% (B. Ananthanarayan et al. JHEP 05:052, 2002) Calculations in NNLO SU(2) ChPT: • Γ(0) = 8.09eV ±1.3% (K. Kampf et al. Phys. Rev. D79:076005, 2009) 0→ • Calculations in QCD sum rule: • Γ(0) = 7.93eV ± 1.5% (B.L. Ioffe, et al. Phys. Lett. B647, p. 389, 2007) • Precision measurements of (0→) at the percent levelwill provide a stringent test of a fundamental prediction of QCD.

  5. ρ,ω Primakoff Method 12C target Primakoff Nucl. Coherent Nucl. Incoh. Interference Challenge: Extract the Primakoff amplitude Requirement: • Photon flux • Beam energy • 0production Angular resolution

  6. Pri PrimEx-I (2004) • JLab Hall B high resolution, high intensity photon tagging facility • New pair spectrometer for photon flux control at high beam intensities 1% accuracy has been achieved • New high resolution hybrid multi-channel calorimeter (HyCal)

  7. PrimEx Hybrid Calorimeter - HyCal • 1152 PbWO4 crystal detectors • 576 Pb-glass Cherenkov detectors HYCAL only Kinematical constraint x = 1.3 mm

  8. 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 PrimEx Online Event Display

  9. 0 Event Selection (contd.)

  10. Fit Differential Cross Sections to Extract Γ(0) Theoretical angular distributions smeared with experimental resolutions are fit to the data on two nuclear targets:

  11. Verification of Overall Systematical Uncertainties •  + e  +eCompton cross section measurement • e+e-pair-production cross section measurement Systematic uncertainties on cross sections are controlled at 1.3% level.

  12. PrimEx-I Result

  13. PrimEx-I Result (contd.) (0) = 7.820.14(stat)0.17(syst) eV 2.8% total uncertainty

  14. Goal for PrimEx-II • PrimEx-I has achieved 2.8% precision (total): (0) = 7.82 eV 1.8% (stat) 2.2% (syst.) PrimEx-II projected 1.4% PrimEx-I 7.82eV2.8% • Task for PrimEx-II is to obtain 1.4% precision Projected uncertainties: 0.5% (stat.) 1.3% (syst.)

  15. Estimated Systematic Uncertainties

  16. Improvements for PrimEx-II 1.4 % Total 1.3 % Syst. 0.5 % Stat. • Better control of Background: • Add timing information in HyCal (~500 chan.) • Improve photon beam line • Improve PID in HyCal (add horizontal veto counters to have both x and y detectors) • More empty target data • Measure HyCal detection efficiency • Double target thickness (factor of 2 gain) • Hall B DAQ with 5 kHz rate, (factor of 5 gain) • Double photon beam energy interval in the trigger

  17. Improvements in PrimEx-II Photon Beam Line Monte Carlo Simulations • Make the primary collimator “tapered”. • Triple the Permanent Magnet • Reduce the size of the central hole in Pb-shielding wall Total relative gain: PrimEx-I config. 100 % suggested PrimEx-II config. 19 % ~5 times less background events

  18. Add Timing in HyCal~500 channels of TDC’s in HYCAL

  19. Improvement in PID Additional horizontal veto

  20. PrimEx-II Run Status • Experiment was performed from Sep. 27 to Nov. 10 in 2010. • Physics data collected: • π0 production run on two nuclear targets: 28Si(0.6% statistics) and12C(1.1% statistics). • Good statistics for two well-known QED processes to verify the systematic uncertainties: Compton scattering and e+e- pair production.

  21. PrimEx-II Analysis Status HyCal energy calibration Reconstructed 0 distribution vs. the number of iterations

  22. Tagger Timing After calibration Before calibration  ~ 1.5ns  ~ 0.6ns

  23. HyCal Timing HyCal TDC groups scheme: HyCal TDC spectrum:

  24. Preliminary 0 Reconstruction Empty target

  25. PrimEx-II Experimental Yield (preliminary) ( E = 4.4-5.3 GeV) Primakoff Primakoff 12C 28Si ~8K Primakoff events ~20K Primakoff events

  26. Other Multi- Channels in Data Set  → 30  → 0 +   = 6MeV  = 15MeV ʹ→ (→2) + 20 a0 → (→2) + 0  = 10MeV  100MeV

  27. Summary • The 0 lifetime is one of the few precision predictions of low energy QCD • Percent level measurement is a stringent test of QCD. • New generation of Primakoff experiments have been developed in Hall B to provide high precision measurement on Γ(0) • Systematic uncertainties on cross sections are controlled at the 1.3% level, verified by two well-known QED processes: Compton and pair-production. • PrimEx-I result (2.8% total uncertainty): Γ(0)  7.82  0.14(stat.)  0.17(syst.) eV Phys. Rev. Lett., 106, 162302 (2011) • PrimEx-II (fall 2010): high statistical data set has been collected on two nuclear targets, 12C and 28Si. • PrimEx-II analysis is in progress. The 0 lifetime at level of 1.4% precision is expected.

  28. This project is supported by: • NSF MRI (PHY-0079840) • Jlab under DOE contract (DE-AC05-84ER40150) Thank You!

  29. Measurement of Γ(→) in Hall D at 12 GeV • Use GlueX standard setup for this measurement: CompCal Counting House 75 m • Photon beam line -incoherent tagged photons • Pair spectrometer • Solenoid detectors (for background rejection) • 30 cm LH2 and LHe4 targets (~3.6% r.l.) • Forward tracking detectors (for background rejection) • Forward Calorimeter (FCAL) for → decay photons • Additional CompCal detector for overall control of systematic uncertainties. 29

  30. 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

  31. Theoretical Calculation (cont.) • Two independent approaches: • Glauber theory • Cascade Model (Monte Carlo) • Incoherent Production A→0A´ Deviation in Γ(0) is less than 0.2%

  32. Γ(0)Model Sensitivity Variations in absorption parameter sΔΓ <0.06% Variations in energy dependence Parameter n ΔΓ <0.04% Variations in shadowing parameter xΔΓ <0.06% Overall model error in Γ(0) extraction is controlled at 0.25%

  33. Primakoff Experiments before PrimEx • 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

  34. 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→

  35. 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 **

  36. Multi- Reconstructions from HyCal • We observe multi- states produced in He gas close to HyCal (at distance 1 – 2 m) • HyCal shows good invariance mass resolution even with unknown decay vertex position and particle energy He

  37. Luminosity Control: Pair Spectrometer • Combination of: • 16 KGxM dipole magnet • 2 telescopes of 2x8 scintillating detectors Measured in experiment: • absolute tagging ratios: • TAC measurements at low intensities • relative tagging ratios: • pair spectrometer at low and high intensities • Uncertainty in photon flux at the level of 1% has been reached • Verified by known cross sections of QED processes • Compton scattering • e+e- pair production User's meeting, 6/7/2011

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