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### 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)

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

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.

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.

Primakoff Method

12C target

Primakoff

Nucl. Coherent

Nucl. Incoh.

Interference

Challenge: Extract the Primakoff amplitude

Requirement:

- Photon flux
- Beam energy
- 0production 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)

PrimEx Hybrid Calorimeter - HyCal

- 1152 PbWO4 crystal detectors
- 576 Pb-glass Cherenkov detectors

HYCAL

only

Kinematical

constraint

x = 1.3 mm

0 Event selection

We measure:

- incident photon energy: E and time
- energies of decay photons:

E1, E2 and time

- X,Y positions of decay photons

Kinematical constraints:

- Conservation of energy;
- Conservation of momentum;
- m invariant mass

PrimEx Online Event Display

Fit Differential Cross Sections to Extract Γ(0)

Theoretical angular distributions smeared with experimental resolutions are fit to the data on two nuclear targets:

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.

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

7.82eV2.8%

- Task for PrimEx-II is

to obtain 1.4% precision

Projected uncertainties:

0.5% (stat.) 1.3% (syst.)

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

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

Improvement in PID

Additional horizontal veto

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.

HyCal energy calibration

Reconstructed 0 distribution vs. the number of iterations

Empty target

PrimEx-II Experimental Yield (preliminary)

( E = 4.4-5.3 GeV)

Primakoff

Primakoff

12C

28Si

~8K Primakoff events

~20K Primakoff events

Other Multi- Channels in Data Set

→ 30

→ 0 +

= 6MeV

= 15MeV

ʹ→ (→2) + 20

a0 → (→2) + 0

= 10MeV

100MeV

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.

- NSF MRI (PHY-0079840)
- Jlab under DOE contract (DE-AC05-84ER40150)

Thank You!

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

0 Forward Photoproduction off Complex Nuclei (theoretical models)

- Coherent Production A→0A

Leading order processes:

(with absorption)

Primakoff

Nuclear coherent

Next-to-leading order:

(with absorption)

0 rescattering

Photon shadowing

Theoretical Calculation (cont.)

- Two independent approaches:
- Glauber theory
- Cascade Model (Monte Carlo)

- Incoherent Production A→0A´

Deviation in Γ(0) is less than 0.2%

Γ(0)Model Sensitivity

Variations in absorption

parameter sΔΓ <0.06%

Variations in energy dependence

Parameter n ΔΓ <0.04%

Variations in shadowing

parameter xΔΓ <0.06%

Overall model error in Γ(0) extraction is controlled at 0.25%

Primakoff Experiments before PrimEx

- DESY (1970)
- bremsstrahlung beam,

E=1.5 and 2.5 GeV

- Targets C, Zn, Al, Pb
- Result: (0)=(11.71.2) eV

10.%

- Cornell (1974)
- bremsstrahlung beam

E=4 and 6 GeV

- targets: Be, Al, Cu, Ag, U
- Result: (0)=(7.920.42) eV

5.3%

- All previous experiments used:
- Untagged bremsstrahlung beam
- Conventional Pb-glass calorimetry

Decay Length Measurements (Direct Method)

- Measure 0decay length

- 1x10-16 sec too small to measure

solution: Create energetic 0 ‘s,

L = vE/m

But, for E= 1000 GeV, Lmean 100 μm

very challenging experiment

1984 CERN experiment:

P=450 GeV proton beam

Two variable separation (5-250m) foils

Result:

(0) = 7.34eV3.1% (total)

- Major limitations of method
- unknown P0 spectrum
- needs higher energies for improvement

0→

e+e- Collider Experiment

- DORIS II @ DESY

- e+e-e+e-**e+e-0e+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 **

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

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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