Transcript Primakoff Experiments at 12 GeV with GlueX A. Gasparian NC A&T State University, Greensboro, NC For the PrimEx Collaboration Outline  The project and physics.

Primakoff Experiments at 12 GeV with GlueX
A. Gasparian
NC A&T State University, Greensboro, NC
For the PrimEx Collaboration
Outline
 The project and physics motivation:
 The first experiment @ 6 GeV: 0 lifetime
 Development of precision technique
 Results for 0 lifetime
 Experiments @ 12 GeV with GlueX
 Summary
A. Gasparian
Hall D, March 7, 2008
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The PrimEx Project at JLab
Experimental program
Precision measurements of:
 Two-Photon Decay Widths:
Γ(0→), Γ(→),
Γ(’→)
 Transition Form Factors at
low Q2 (0.001-0.5 GeV2/c2):
F(*→ 0), F(* →),
F(* →)
Test of Chiral Symmetry and Anomalies via the Primakoff Effect
A. Gasparian
Hall D, March 7, 2008
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Physics Motivation
Fundamental input to Physics:
 precision test of chiral
anomaly predictions
 determination of quark
mass ratio
 -’ mixing angle
 0, and ’ interaction
electromagnetic radius
 is the ’ an approximate
Goldstone boson?
A. Gasparian
Hall D, March 7, 2008
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First experiment: 0 decay width
 0→ decay proceeds primarily via the chiral anomaly in QCD.
 The chiral anomaly prediction is exact for massless quarks:
 2 N c2 m3
    
 7.725 eV
576 3 F2
0
 Corrections to the chiral anomaly prediction:
(u-d quark masses and mass differences)
Calculations in NLO ChPT:
(J. Goity, at al. Phys. Rev. D66:076014, 2002)
~4% higher than LO, uncertainty: less than 1%
 Recent calculations in QCD sum rule:
(B.L. Ioffe, et al. Phys. Lett. B647, p. 389, 2007)
0→
Γ(0) = 8.10eV ± 1.0%
 Γ() is only input parameter
 0- mixing included
Γ(0) = 7.93eV ± 1.5%
 Precision measurements of (0→) at the percent level will provide
a stringent test of a fundamental prediction of QCD.
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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
0→
1984 CERN experiment:
P=450 GeV proton beam
Two variable separation (5-250m) foils
Result:
(0) = 7.34eV3.1% (total)
 needs higher energies for improvement
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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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Primakoff Method
12C
ρ,ω
target
Primakoff
Nucl. Coherent
d 3 Pr
8Z 2  3 E 4
2
2
 
F
(
Q
)
sin

e.m.
3
4
d
m Q
 Pr
peak
Interference
m2

2E 2
 d Pr 
 E4


 d  peak
Nucl. Incoh.
Challenge: Extract the Primakoff amplitude
2
d


Z
log(E )
 Pr
A. Gasparian
Hall D, March 7, 2008
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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  beam
 Conventional Pb-glass calorimetry
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PrimEx Experiment at Hall B
 Requirements of Setup:
 high angular resolution (~0.5 mrad)
 high resolutions in calorimeter
 small beam spot size (‹1mm)
 Background:
 tagging system needed
 Particle ID for (-charged part.)
 veto detectors needed
 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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Electromagnetic Calorimeter: HYCAL
 Energy resolution
 Position resolution
 Good photon detection efficiency
@ 0.1 – 5 GeV;
 Large geometrical acceptance
PbWO4 crystals
Pb-glass
resolution
budget
HYCAL
only
Kinematical
constraint
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Fit to Extract 0 Decay Width:
Γ(0)  7.93 eV  2.1%(stat.)  2.0% (syst)
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0 Decay width (eV)
PrimEx Current Result
() = 7.93eV2.1%2.0%
±1.%
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Estimated Systematic Errors
Type of Errors
Errors in current data
Expected errors from
2nd run
Photon flux
1.0%
1.0%
Target number
<0.1%
<0.1%
Background subtraction
1.0%
0.4%
Event selection
0.5%
0.35%
HYCAL response function
0.5%
0.2%
Beam parameters
0.4%
0.4%
Acceptance
0.3%
0.3%
Model errors (theory)
1.0%
0.25%
Physics background
0.25%
0.25%
Branching ratio
0.03%
0.03%
Total
2.0%
1.3%
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PrimEx @ 12 GeV
Precision Measurement of → decay width
 All  decay widths are
calculated from  decay
width and experimental
Branching Ratios (B.R.):
Γ(η→ decay) = Γ(→) × B.R.
 Any improvement in
Γ(→)
will change the whole
- sector in PDB
A. Gasparian
Hall D, March 7, 2008
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Physics Outcome from  Experiment
 light quark mass ratio
  - ’ mixing angle
2
2
m

m
Q 2  s2
,
md  mu2
1
where mˆ  (mu  md )
2
Γ(η→3)=Γ(→)×B.R.
(mK 0  mK  )e.m. Corr.
A. Gasparian
Hall D, March 7, 2008
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Primakoff Method
12C
ρ,ω
target
Primakoff
d 3 Pr
8Z 2  3 E 4
2
2
  3
F
(
Q
)
sin

e.m.
4
d
m Q
Nucl. Coherent
Interference
Nucl. Incoh.
Challenge: Extract the Primakoff amplitude
A. Gasparian
Hall D, March 7, 2008
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Why do we need 12 GeV?
 Increase Primakoff cross section:
 d Pr 
 E4


 d  peak
2
d


Z
log(E )
 Pr
 Better separation of Primakoff reaction from
nuclear processes:
m2
2
 Pr peak  2
 NC 
2E
E  A1/ 3
 Momentum transfer to the nuclei becomes
less
reduce the incoherent background
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 Experiment with GlueX
Advantages:
High energy tagged photon beam
Eγ=10 – 11.5 GeV
 High acceptance electromagnetic calorimeter
(FCAL)
 Solenoid detector to veto charged particles,
and reduce background on FCAL
 Targets (~1-5% R.L.):
 LH2,
 LHe4,
 solid 12C
Challenges:
 Photon flux stability and control:
possible solutions:
 e+e- pair spectrometer;
 Compton scattering;
 High resolution FCAL needed for
precision experiments:
possible solution:
 Pb-glass + PbWO4 crystals
A. Gasparian
Hall D, March 7, 2008
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FCAL Geometrical Coverage
 Forward Calorimeter FCAL (~2800 Pb-glass blocks
 Radius = 120 cm:
 Central beam hole3x3 blocks removed (12x12 cm2)
 FCAL has a good coverage for the forward → production
A. Gasparian
Hall D, March 7, 2008
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Geometrical Acceptances
 Forward Calorimeter FCAL (~2800 Pb-glass blocks
 Radius = 120 cm:
 Central beam hole: 3x3 blocks removed (12x12 cm2)
 A good geometrical acceptance can be reached for L = 6-9 m
for η forward production angles needed for the experiment
A. Gasparian
Hall D, March 7, 2008
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Experimental Resolutions (prod. angle)
 Precision cross section
measurement requires
high resolutions in:
 luminosity (flux + target)
 production angle (for fit);
 invariant mass (background)
…
FCAL with all Pb-glass
FCAL with Pb-glass and
PbWO4 crystal insertion
(75x75 blocks
(150x150 cm2)
A. Gasparian
Hall D, March 7, 2008
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Experimental Resolutions (inv. mass)
FCAL with all Pb-glass
FCAL with Pb-glass and
PbWO4 crystal insertion
(75x75 blocks
(150x150 cm2)
A. Gasparian
Hall D, March 7, 2008
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Experimental Resolutions
(production angle vs. beam spot size)
Photon beam size up to 5 mm,
as it is designed,
seams reasonable
A. Gasparian
Hall D, March 7, 2008
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Experimental Resolutions
(production angle vs. target length)
 Reaction vertex can not be reconstructed
in this experiment
(recoil energies are too small T< 1 MeV)
 Large size of the FCAL calorimeter
provides longer target to FCAL distance
 That makes less sensitivity from the
target length up to designed 30 cm
liquid targets
A. Gasparian
Hall D, March 7, 2008
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Luminosity Control: Pair Spectrometer
Measured in experiment:
 absolute tagging ratios:
 TAC measurements at low intensities
 relative tagging ratios:
 pair spectrometer at low and high
intensities
Scint. Det.
 Uncertainty in photon flux at
the level of 1% has been reached
 Verified by known cross
sections of EM processes
 Compton scattering
 e+e- pair production
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Luminosity Control:
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 = 1.0004
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  e     e
Δσ/ΔΩ (mb/6.9 msrad)
Verification of Overall Systematics:
Compton Cross Section
Compton Forward Cross Section
0.085
Klein-Nishina
Primex Compton Data
Uncertainties:
Statistical
Systematic
0.080
P R E L I M I N A R Y
0.075
0.070
0.065
Data with radiative corrections
0.060
0.055
4.9
5.0
5.1
5.2
5.3
5.4
5.5
Energy (GeV)
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Uncertainties:
No Deviation
Experiment / T heory
Statistical
P R E L I M I N A R Y
5
Deviation (%)
 Average stat. error:
0.6%
 Average syst. error:
1.2%
Experiment To Theory Comparison
0
-5
 Total: 1.3%
-10
4.9
5.0
5.1
5.2
5.3
5.4
5.5
Energy (GeV)
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Beam Time and Statistics
 Target: L=20 cm, LHe4
NHe = 4x1023 atoms/cm2
Nγ = 1x107 photon/sec (10-11.5 GeV part)
<Δσ(prim.)> = 1.6x10-5 mb
N() = NHexNγx<Δσ>xεx(BR)
= 4x1023x 1x107x 1.6x10-32x0.7x0.4
= 64 events/hour
= 1500 events/day
= 45,000 events/30 days
 Will provide sub-percent systematic error
15 Days
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Estimated Error Budget
Statistical
Target thickness
Photon flux
Acceptance misalignment
Background subtraction
Beam energy
Distorted form factor
Nuclear coherent contr.
Branching ratio
Total
0.5%
1.0%
1.0%
0.5%
0.4%
0.2%
0.3%
0.5%
0.8% (PDG)
2.0%
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Summary
 PrimEx collaboration has developed an experimental program
to perform precision test of chiral symmetry and anomaly
effects in the light pseudoscalar meson sector.
 A state-of-the-art high resolution experimental setup has been
designed, developed and constructed for the 6 GeV run.
 The first experiment, the 0 lifetime measurement has been
successfully performed in Hall B in 2004.
Preliminary result: Γ(0)  7.93 eV  2.10%(stat.)  2.0%(syst.)
 New proposal for the 1.4% accuracy in Γ(0) has been
approved for the second 6 GeV run.
 Reach experimental program for η, η’ widths measurements has been
developed and approved by high energy PACs.
These precision experiments can be performed with the upgraded
GlueX experimental setup at 12 GeV.
A. Gasparian
Hall D, March 7, 2008
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The End
A. Gasparian
Hall D, March 7, 2008
31
The Primakoff Effect
ρ, ω
d 3 Pr
8Z 2  3 E 4
2
2
 
F
(
Q
)
sin

e.m.
3
4
d
m Q
Challenge: Extract the Primakoff amplitude
A. Gasparian
Hall D, March 7, 2008
32
PbWO4 Energy Resolution
6 x 6 crystals
E/E = 1.3 %
3x3
1x1
A. Gasparian
Hall D, March 7, 2008
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PbWO4 Position Resolution
x = 1.3 mm
A. Gasparian
Hall D, March 7, 2008
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Experimental Setup Development:
Pair Spectrometer
 Precision cross section
measurements need control of
photon flux at 1% level
Scint. Det.
 Pair spectrometer was designed
for relative photon flux
monitoring at high beam
Dipole
intensities:
 Combination of:
 16 KGxm dipole magnet
 2 telescopes of 2x8
scintillating detectors
A. Gasparian
e+
HYCAL
ePhoton
beam
Hall D, March 7, 2008
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(0→) World Data
 0 is lightest quark-antiquark
hadron
 The lifetime:
 = B.R.( 0 →γγ)/(0 →γγ)
0.8 x 10-16 second
A. Gasparian
0→
 Branching ratio:
B.R. ( 0→γγ)= (98.8±0.032)%
±1%
Hall D, March 7, 2008
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