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π 0 Lifetime from the PrimEx Experiments

π 0 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). Properties of  0.  0 is the lightest hadron: m  = 135 MeV.

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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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