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High Precision Measurement of the π 0 Radiative Decay Width

This paper discusses the physics motivation and challenges in measuring the π0 radiative decay width. It explores symmetries in nature, chiral symmetry breaking, and the axial anomaly. The paper presents various calculations and experimental methods used to measure the decay width, with a focus on the PrimEx experiments at JLab. The goal of PrimEx-II is to achieve even higher precision in the measurement.

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High Precision Measurement of the π 0 Radiative Decay Width

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  1. High Precision Measurement of the π0 Radiative Decay Width Liping Gan University of North Carolina Wilmington For the PrimEx Collaboration

  2. Contents • Physics Motivation • A big challenge in physics • What is symmetry? • 0 and QCD symmetries • PrimEx experiments at Jlab • PrimEx-I • PrimEx-II • Summary

  3. Strong force Strong force obeys the rules of Quantum Chromodynamics (QCD) Color confinement:the potential energy between (in this case) a quark and an antiquark increases while increasing the distance between them. Solution: Understanding symmetries of nature Challenges in Modern Physics • What are the building blocks of matter? The properties of strong force at large distance represents one of the biggest intellectual challenges in physics. • What happens if one tries to separate two quarks?

  4. Noether’s theorem Symmetry Conservation Law What is symmetry? Symmetry is an invariance of a physical system to a set of changes . Symmetries and symmetry breakings are fundamental in physics

  5. 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

  6. Spontaneous Chiral Symmetry Breaking Gives Rise to π0 In massless quark limit Massless Goldstone Bosons Corrections to theory: • Chiral symmetry explicitly breaking due to non-zero quark masses meson masses • SU(3) symmetry breaking due to quark mass differences mixing among the mesons Since π0 is the lightest quark-antiquark system in nature, the corrections are small

  7. 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.

  8. 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

  9. 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)

  10. 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

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

  12. 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

  13. 0 Event Selection (contd.)

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

  15. 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.

  16. PrimEx-I Result PRL 106, 162303 (2011) (0) = 7.820.14(stat)0.17(syst) eV 2.8% total uncertainty

  17. 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.)

  18. Estimated Systematic Uncertainties

  19. 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

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

  21. Improvement in PID Additional horizontal veto

  22. Preliminary 0 Reconstruction Empty target

  23. 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.

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

  25. Summary • A study of dynamical and anomalous chiral symmetry breakings provides a promising framework to understand QCD confinement. • 0 arises from dynamical chiral symmetry breaking and decays through chiral anomaly. Its 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 at Jlab 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 : high statistical data set has been collected on two nuclear targets, 12C and 28Si. Data analysis is in progress. The 0 lifetime at level of 1.4% precision is expected.

  26. This project is supported by: • NSF grant (PHY-0855578) • NSF MRI (PHY-0079840) • Jlab under DOE contract (DE-AC05-84ER40150) Thank You!

  27. 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

  28. 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%

  29. 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%

  30. 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

  31. 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→

  32. 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 **

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

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

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

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