High Precision Measurement of the π 0 Radiative Decay Width Liping Gan University of North Carolina Wilmington For the PrimEx Collaboration

Contents 1.Physics Motivation  A big challenge in physics  What is symmetry?   0 and QCD symmetries 2. PrimEx experiments at Jlab  PrimEx-I  PrimEx-II 3.Summary 2

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

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

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 second 5

Spontaneous Chiral Symmetry Breaking Gives Rise to π 0 In massless quark limit Corrections to theory: Corrections to theory:  Chiral symmetry explicitly breaking due to non-zero quark masses meson masses meson masses  SU(3) symmetry breaking due to quark mass differences mixing among the mesons Massless Goldstone Bosons 6 Since π 0 Since π 0 is the lightest quark-antiquark system in nature, the corrections are small

Axial Anomaly Determines π 0 Lifetime   0 →  decay proceeds primarily via the chiral anomaly in QCD.  The chiral anomaly prediction is exact for massless quarks:  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)  Precision measurements of  (  0 →  ) at the percent level will provide a stringent test of a fundamental prediction of QCD.  0 →   Calculations in QCD sum rule:  Γ(  0  ) = 7.93eV ± 1.5% (B.L. Ioffe, et al. Phys. Lett. B647, p. 389, 2007) is one of the few quantities in confinement region that QCD can  Γ(  0  ) is one of the few quantities in confinement region that QCD can calculate precisely to higher orders! calculate precisely to higher orders! 7 p k1k1 k2k2

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

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) 9

Luminosity Control: Pair Spectrometer Measured in experiment:  absolute tagging ratios:  TAC measurements at low 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  relative tagging ratios:  pair spectrometer at low and high intensities  Combination of:  16 KGxM dipole magnet  2 telescopes of 2x8 scintillating detectors 10

PrimEx Hybrid Calorimeter - HyCal  1152 PbWO 4 crystal detectors  576 Pb-glass Cherenkov detectors HYCAL only Kinematical constraint 11  x = 1.3 mm

 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 12

 0 Event Selection (contd.) 13

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

Verification of Overall Systematical Uncertainties   + e   +e Compton 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, (2011)  (  0  ) = 7.82  0.14(stat)  0.17(syst) eV 2.8% total uncertainty

Goal for PrimEx-II  PrimEx-I has achieved 2.8% precision (total):  (  0  ) = 7.82 eV  1.8% (stat)  2.2% (syst.)  Task for PrimEx-II is to obtain 1.4% precision Projected uncertainties:  0.5% (stat.)  1.3% (syst.) PrimEx-I 7.82eV  2.8% PrimEx-II projected  1.4% 17

Estimated Systematic Uncertainties Type PrimEx-IPrimEx-II Photon flux1.0% Target number<0.1% Veto efficiency0.4%0.2% HYCAL efficiency0.5%0.3% Event selection1.7%0.4% Beam parameters0.4% Acceptance0.3% Model dependence0.3% Physics background0.25% Branching ratio0.03% Total syst.2.2%1.3% 18

Improvements for PrimEx-II % Total 0.5 % Stat.1.3 % Syst. 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  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

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

Improvement in PID 21 Additional horizontal veto

Empty target 22

PrimEx-II Run Status  Experiment was performed from Sep. 27 to Nov. 10 in  Physics data collected:  π 0 production run on two nuclear targets: 28 Si (0.6% statistics) and 12 C (1.1% statistics).  Good statistics for two well-known QED processes to verify the systematic uncertainties: Compton scattering and e + e - pair production. 23

~8K Primakoff events 24 ~20K Primakoff events ( E  = GeV) Primakoff

Summary  PrimEx-I result (2.8% total uncertainty): Γ(  0  )  7.82  0.14(stat.)  0.17(syst.) eV Phys. Rev. Lett., 106, (2011)   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.  Systematic uncertainties on cross sections are controlled at the 1.3% level, verified by two well-known QED processes: Compton and pair-production.  New generation of Primakoff experiments have been developed at Jlab to provide high precision measurement on Γ(  0  )  PrimEx-II : high statistical data set has been collected on two nuclear targets, 12 C and 28 Si. Data analysis is in progress. The  0 lifetime at level of 1.4% precision is expected. 25  A study of dynamical and anomalous chiral symmetry breakings provides a promising framework to understand QCD confinement.

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

27  0 Forward Photoproduction off Complex Nuclei (theoretical models)  Coherent Production  A →  0  A PrimakoffNuclear coherent  0 rescattering Photon shadowing Leading order processes: (with absorption) Next-to-leading order: (with absorption)

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

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

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 30

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 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 →  31

e + e - Collider Experiment  e + e -  e + e -  *  *  e + e -  0  e + e -   e +, e - scattered at small angles (not detected)  only  detected  DORIS DESY  Results: Γ(  0  ) = 7.7 ± 0.5 ± 0.5 eV ( ± 10.0%)  Not included in PDG average  Major limitations of method  knowledge of luminosity  unknown q 2 for  *  *  0 →  32

33  ~ 1.5ns  ~ 0.6ns Before calibration After calibration

HyCal TDC spectrum: HyCal TDC groups scheme: 34

35  →  0 +   = 15MeV  → 3  0  = 6MeV  ʹ →  (→2  ) + 2  0  = 10MeV a 0 →  (→2  ) +  0  100MeV