Results from the PrimEx Experiment:

1
Results from the PrimEx Experiment
A Precision Measurement of the ?0 Lifetime
Pawel Ambrozewicz North Carolina A T State
University for the PrimEx Collaboration
OUTLINE

• Physics motivation
• Primakoff Method
• Experimental setup
• Data Analysis In High Precision Regime
• Results
• Summary

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Physics Motivation

• ?0 is the lightest quark anti-quark system
• m? 135 MeV
• ?0 ? (u?u - d?d)/v2

• ?0 is an unstable particle
• Branching Ratio for ?0 ??? 98.80.032

• Lifetime and Decay width
• ? B.R.(?0 ???)/?(?0 ???)
• ? 8.4 x 10-17 seconds (PDG)

• ?0??? decay proceeds primarily via chiral
  anomaly in QCD.
• The prediction of chiral anomaly is exact for
  massless
• quarks

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Physics Motivation

• Corrections to chiral anomaly prediction

• u and d quark masses
• quark mass differences

Precision measurements of ?(?0???) at percent
level will provide stringent test of
fundamental prediction of QCD
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The Primakoff Method
Targets 12C, 208Pb
Radiative width
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The Primakoff Method
Angular distribution enables separation of
amplitudes
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Previous Primakoff Experiments

• 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
• photon beams
• Conventional Pb-glass
• calorimetry

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PrimEx Experiment Hall B JLab

• Forward production of neutral mesons at 6 GeV,
• in precision measurement regime, requires
• Precise photon flux control
• and monitoring
• High energy resolution
• High position resolution
• Good photon detection efficiency
• _at_ 0.1 5 GeV
• Large geometrical acceptance

• JLab Hall B high resolution, high
• intensity photon tagging facility

• New pair spectrometer for
• photon flux control at high
• intensities

• New, high resolution, hybrid,
• multi-channel calorimeter
• 
  (HyCAL)

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PrimEx Hybrid Calorimeter - HyCal
Crystal only
Kinematical constraint

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

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Luminosity Control Pair Spectrometer

• Combination of
• 1.6 T-m dipole magnet
• 2 arrays 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 (concurrent with data
• 
  taking)

• Uncertainty in photon flux at
• the level of 1 has been reached
• Controlled by known cross
• sections of QED processes
• Compton scattering
• ee- pair production

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?0 Analysis Event selection
Helium

• Measured Quantities
• 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 Analysis Event selection
Three groups analyzed the data independently
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?0 Analysis Differential Cross Section

• Simulation
• acceptances
• efficiencies
• resolutions

• Experimental Yields
• M?? Fit w/ kinematical constraints
• Hybrid Mass Fit
• M?? and (E?1 E?2)/ E? Fits

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Fits to Extract G(?0???) Decay Width
Theoretical angular distributions smeared with
simulated experimental resolutions are fit to
the data
12C
208Pb
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Fits to Extract G(?0???) Decay Width
Combined fit to both targets
12C
Average Result (of all three analyses)
?(????)7.93eV?2.3
208Pb
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Electromagnetic Calibration Reactions

• Since
• ?(?0???) ? d??d? Primakoff

• Well known reactions are needed for the
  verification
• of extracted ?0 cross section

• The data for the following QED processes had
  been
• taken periodically in this experiment
• ee- Pair production on 12 C
• Double arm Compton scattering on 12C

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Systematics Verification ee- Pair Production
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Systematics Verification ee- Pair Production
Cross Section
Theoretical Inputs to Calculation

• Bethe-Heitler
• (modified by nuclear
• form factor)

• Virtual Compton scattering

• Radiative effects

• Atomic screening
• Electron field pair
• production

Experiment / Theory 1.001
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Systematics Verification Compton Scattering

• Events Selection
• Minimal opening angle
• Energy Conservation
• (Tagger Energy HyCAL Total
• 
  Energy)
• Coplanarity

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Systematics Verification Compton Scattering
Cross Section
Theoretical Inputs to Calculation

• Virtual corrections
• Soft photon emission
• Double Compton effect

• Average stat. error
• 0.6
• Average syst. error
• 1.5
• Total error 1.6

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Theoretical Models
Forward ?0 Photoproduction off Complex Nuclei

• Coherent Production ??A??0?A

Leading order processes (with absorption)
Primakoff
Nuclear Coherent
Next-to-leading order (with absorption)
Photon Shadowing
?0 Rescattering
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Model Sensitivity Of The Result

• FA Nuclear Form Factor
• FI Intermediate state
• contribution
• ? - Shadowing parameter

Overall model error does not exceed 0.25
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Theoretical Models
Forward ?0 Photoproduction off Complex Nuclei

• Incoherent Production ??A??0?A

• Two independent approaches
• Glauber theory
• Cascade Model (Monte Carlo)

Deviation in G(?0???) is less than 0.2
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Estimated Systematic Errors
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Current PrimEx Result
?(????)7.93eV?2.3?1.6
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Summary

• A new precision experiment has been developed
  and performed at
• Hall B of Jefferson Lab to test fundamental
  QCD prediction

• It utilized the high performance CEBAF beam,
  high resolution photon tagging facility, and a
  new state-of-the-art calorimeter

• Systematic errors are controlled by Compton and
  pair-production cross
• section measurements at the 1.6 level

• Preliminary result
• G(?0???) 7.93 ? 0.18 (stat.) ? 0.13
  (syst.) eV
• Total error 2.8 (2.5 x smaller than PDG
  value)

• An extension of PrimEx experiment was recently
  approved by Jlab PAC to facilitate achieving 1.4
  precision

This experiment was in part supported by NSF MRI
PHY 0079840 grant