Transcript π0 Lifetime from the PrimEx Experiments Liping Gan University of North Carolina Wilmington (for the PrimEx Collaboration) Outline  0 and QCD symmetries  PrimEx-I result 

π0 Lifetime from the PrimEx Experiments
Liping Gan
University of North Carolina Wilmington
(for the PrimEx Collaboration)
Outline
 0 and QCD symmetries
 PrimEx-I result
 PrimEx-II status
 Summary
Properties of 0
 0
is the lightest hadron:
m = 135 MeV
  (uu  dd ) / 2
0
 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
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Spontaneous Chiral Symmetry Breaking Gives Rise to π0
In massless quark limit
SU L (3)  SU R (3)  SU (3)
Massless Goldstone Bosons
 0 ,   ,   , K  , K  , K 0 , K 0 ,8
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.
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Axial Anomaly Determines π0 Lifetime
 0→ decay proceeds primarily via the chiral anomaly in QCD.
 The chiral anomaly prediction is exact for massless quarks:
 2 N c2 m3
0
    
 7.725 eV
p
576 3 F2
k1
k2
 Γ(0) is one of the few quantities in confinement region that QCD can
calculate precisely to higher orders!
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)

Calculations in QCD sum rule:
0→
 Corrections to the chiral anomaly prediction:
 Γ(0) = 7.93eV ± 1.5%
(B.L. Ioffe, et al. Phys. Lett. B647, p. 389, 2007)
 Precision measurements of (0→) at the percent level will provide
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a stringent test of a fundamental prediction of QCD.
Primakoff Method
ρ,ω
12C
target
Nucl. Coherent
Primakoff
d Pr
8 Z 2  3 E 4
2
2
 
F
(
Q
)
sin

e.m.
3
4
d
m Q
Interference
Nucl. Incoh.
Challenge: Extract the Primakoff amplitude
Requirement:
Photon flux
Beam energy
0 production Angular resolution
 Pr
peak
m2

2E 2
 d Pr 
 E4


 d   peak
2
d


Z
log( E )
Pr

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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)
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PrimEx Hybrid Calorimeter - HyCal
 1152 PbWO4 crystal detectors
 576 Pb-glass Cherenkov detectors
HYCAL
only
Kinematical
constraint
x = 1.3 mm
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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
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0 Event Selection (contd.)
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Fit Differential Cross Sections to Extract Γ(0)
Theoretical angular distributions smeared with experimental
resolutions are fit to the data on two nuclear targets:
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Verification of Overall Systematical Uncertainties
 e+e- pair-production cross section
  + e   +e Compton
cross section measurement
Compton Forward Cross Section
0.085
Klein-Nishina
Primex Compton Data
Uncertainties:
0.080
measurement
Statistical
Systematic
P R E L I M I N A R Y
0.075
0.070
0.065
0.060
0.055
4.9
5.0
5.1
5.2
5.3
5.4
5.5
Energy (GeV)
Systematic uncertainties on cross sections are controlled at 1.3% level.
PrimEx-I Result
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PrimEx-I Result (contd.)
(0) = 7.820.14(stat)0.17(syst) eV
2.8% total uncertainty
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Goal for PrimEx-II
 PrimEx-I has achieved 2.8%
precision (total):
(0) = 7.82 eV 1.8% (stat) 2.2% (syst.)
PrimEx-I
7.82eV2.8%

PrimEx-II
projected 1.4%
Task for PrimEx-II is
to obtain 1.4% precision
Projected uncertainties:
0.5% (stat.) 1.3% (syst.)
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Estimated Systematic Uncertainties
Type
PrimEx-I
PrimEx-II
Photon flux
1.0%
1.0%
Target number
<0.1%
<0.1%
Veto efficiency
0.4%
0.2%
HYCAL efficiency
0.5%
0.3%
Event selection
1.7%
0.4%
Beam parameters
0.4%
0.4%
Acceptance
0.3%
0.3%
Model dependence
0.3%
0.3%
Physics background
0.25%
0.25%
Branching ratio
0.03%
0.03%
Total syst. uncertainties
2.2%
1.3%
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Improvements for PrimEx-II
1.4 % Total
1.3 % Syst.
 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
0.5 % Stat.
 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
 Measure HyCal detection efficiency
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Improvements in PrimEx-II Photon Beam Line
Monte Carlo Simulations
1. Make the primary collimator
“tapered”.
2. Triple the Permanent Magnet
3. 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
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Add Timing in HyCal
~500 channels of TDC’s in HYCAL
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Improvement in PID
Additional horizontal veto
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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:
and 12C (1.1% statistics).
28Si
(0.6% statistics)
 Good statistics for two well-known QED processes to verify the
systematic uncertainties: Compton scattering and e+e- pair
production.
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Before calibration
 ~ 1.5ns
After calibration
 ~ 0.6ns
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HyCal TDC groups scheme:
HyCal TDC spectrum:
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Empty target
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( E = 4.4-5.3 GeV)
Primakoff
~8K Primakoff events
Primakoff
~20K Primakoff events
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 →
0
+
 = 15MeV
 → 30
 = 6MeV
ʹ→ (→2) + 20
 = 10MeV
a0 → (→2) + 0
 100MeV
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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.
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This project is supported by:
 NSF MRI (PHY-0079840)
 Jlab under DOE contract (DE-AC05-84ER40150)
Thank You!
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Measurement of Γ(→) in Hall D at 12 GeV
 Use GlueX standard setup for this measurement:
Counting
House
75 m
CompCal
 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.
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0 Forward Photoproduction off Complex
Nuclei (theoretical models)

Coherent Production A→0A
Leading order processes:
(with absorption)

Primakoff
Next-to-leading
order:
Nuclear coherent

(with absorption)
0 rescattering
Photon shadowing
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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%
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Γ(0) Model Sensitivity
Variations in absorption
parameter  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%
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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
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Decay Length Measurements (Direct Method)
 Measure 0 decay 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
 Major limitations of method
 unknown P0 spectrum
 needs higher energies for improvement
0→
1984 CERN experiment:
P=450 GeV proton beam
Two variable separation (5-250m) foils
Result:
(0) = 7.34eV3.1% (total)
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e+e- Collider Experiment
 DORIS II @ DESY
 e+e-e+e-**e+e-0e+e-
 e+, e- scattered at small angles (not detected)
Results:
Γ(0) = 7.7 ± 0.5 ± 0.5 eV ( ± 10.0%)
 Not included in PDG average
0→
 only  detected
 Major limitations of method
 knowledge of luminosity
 unknown q2 for **
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He
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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
L. Gan
User's meeting, 6/7/2011
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