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An Updated High Precision Measurement of the Neutral Pion Lifetime via the Primakoff Effect. A. Gasparian NC A&T State University, Greensboro, NC for the PrimEx Collaboration. Outline Physics Motivation Different methods of lifetime measurements - PowerPoint PPT Presentation
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An Updated High Precision Measurement of the Neutral Pion Lifetime via the
Primakoff Effect
A. GasparianNC A&T State University, Greensboro, NC
for the PrimEx Collaboration
Outline Physics Motivation Different methods of lifetime measurements The PrimEx experiment and our first results Control of systematic errors Summary
A. Gasparian PAC33, January 15, 2008 2
0 decay width
eVFmNc 725.7
576 23
3220
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: (u-d quark masses and mass differences)
Calculations in NLO ChPT:(J. Goity, at al. Phys. Rev. D66:076014, 2002)Γ(0) = 8.10eV ± 1.0%
~4% higher than LO, uncertainty: less than 1%
Precision measurements of (0→) at the percent level will provide a stringent test of a fundamental prediction of QCD.
0→
Recent calculations in QCD sum rule: (B.L. Ioffe, et al. Phys. Lett. B647, p. 389, 2007)
Γ() is only input parameter 0- mixing includedΓ(0) = 7.93eV ± 1.5%
A. Gasparian PAC33, January 15, 2008 3
Decay Length Measurements (Direct Method)
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
Measure 0 decay length
1984 CERN experiment: P=450 GeV proton beamTwo variable separation (5-250m) foilsResult:(0) = 7.34eV3.1% (total)
Major limitations of method unknown P0 spectrum needs higher energies for improvement
0→
A. Gasparian PAC33, January 15, 2008 4
e+e- Collider Experiment
e+e-e+e-**e+e-0e+e- e+, e- scattered at small angles (not detected)
only detected
DORIS II @ 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 q2 for **
0→
A. Gasparian PAC33, January 15, 2008 5
Primakoff Method
22..4
43
3
2Pr
3
sin)(8 QFQE
mZ
dd
me
ρ,ω
Challenge: Extract the Primakoff amplitude
12C target
Primakoff Nucl. Coherent
Interference Nucl. Incoh.
A. Gasparian PAC33, January 15, 2008 6
Previous Primakoff Experiments 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, UResult: (0)=(7.920.42) eV
5.3%
All previous experiments used: Untagged bremsstrahlung beam Conventional Pb-glass calorimetry
A. Gasparian PAC33, January 15, 2008 7
PrimEx Experiment
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)
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
A. Gasparian PAC33, January 15, 2008 8
PrimEx Milestones Proposal approved in 1999 by PAC15, re-approved by PAC22 (E02-103) in 2002 with A rating.
Full support of JLab (Engineering group, machine-shop, installation, etc.).
In 2000, NSF awarded a collaborative MRI grant of $1 M to develop the experimental setup.
In 4 years the experimental setup, including procurement of all hardware, was designed, constructed and tested.
Commissioning and data taking was performed in Sept-November 2004 run.
First publication is expected in Spring, 2008.
Preliminary results released at the April, 2007 APS meeting with AIP press conference.
A. Gasparian PAC33, January 15, 2008 9
Luminosity Control: Pair Spectrometer
Scint. Det.
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 EM processes
Compton scattering e+e- pair production
relative tagging ratios: pair spectrometer at low and high intensities
A. Gasparian PAC33, January 15, 2008 10
Electromagnetic Calorimeter: HYCAL Energy resolution Position resolution Good photon detection efficiency @ 0.1 – 5 GeV; Large geometrical acceptancePbWO4 crystals resolution
Pb-glass budget
HYCALonly
Kinematicalconstraint
A. Gasparian PAC33, January 15, 2008 11
0 Event selectionWe 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
Three groups analyzed the data independently PrimEx Online Event Display
A. Gasparian PAC33, January 15, 2008 12
Differential Cross section
Experimental Yield per
GEANT: acceptances; efficiencies; resolutions;
Diff. cross section
A. Gasparian PAC33, January 15, 2008 13
Fit to Extracted 0 Decay Width
Combined average from three groups:
Γ(0) 7.93 eV 2.1%(stat.) 2.0% (syst)
Theoretical angular distributions smeared with experimental resolutions are fit to the data
A. Gasparian PAC33, January 15, 2008 14
Fit to Extracted 0 Decay Width:208Pb Target
Theoretical angular distributions smeared with experimental resolutions are fit to the data
A. Gasparian PAC33, January 15, 2008 15
PrimEx Current Result
() = 7.93eV2.1%2.0%
0
D
ecay
wid
th (e
V)
±1.%
A. Gasparian PAC33, January 15, 2008 16
Estimated Systematic Errors Type of Errors Errors in current data Errors in this proposal
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%
A. Gasparian PAC33, January 15, 2008 17
0 Forward Photoproduction off Complex Nuclei (theoretical models)
Coherent Production A→0A
Primakoff Nuclear coherent
0 rescattering Photon shadowing
Leading order processes:(with absorption)
Next-to-leading order:(with absorption)
A. Gasparian PAC33, January 15, 2008 18
Γ(0) Model Sensitivity Incoherent Production A→0A´ Two independent approaches:
Glauber theory Cascade Model (Monte Carlo) Photon shadowing effect
Deviation in Γ(0) less than 0.2%
Overall model error in Γ(0) extraction is controlled at 0.25%
A. Gasparian PAC33, January 15, 2008 19
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 productionExperiment/Theory = 1.0004
A. Gasparian PAC33, January 15, 2008 20
Verification of Overall Systematics: Compton Cross Section
4.9 5.0 5.1 5.2 5.3 5.4 5.5Energy (GeV)
0.055
0.060
0.065
0.070
0.075
0.080
0.085
Systematic Uncertainty
P R E L I M I N A R Y
Uncertainties:StatisticalSystematic
Compton Forward Cross SectionKlein-NishinaPrimex Compton Data
4.9 5.0 5.1 5.2 5.3 5.4 5.5Energy (GeV)
-10
-5
0
5
10
Deviati
on (%)
P R E L I M I N A R Y
Uncertainties:Statistical
Experiment To Theory ComparisonNo DeviationExperiment / Theory Average stat. error:
0.6% Average syst. error: 1.2%
Total: 1.3%
Δσ/Δ
Ω (
mb/
6.9
msr
ad)
e e
Data with radiative corrections
A. Gasparian PAC33, January 15, 2008 21
Beam Time Request12C target 7 days208Pb target 6 daysCompton and pair prod.
4 days
Empty target 2 daysCalibrations/checkout
6 days
HYCAL config. change
2 days
Tagging efficiency 1 daysTotal 28 days
Response to TAC comment on statistical error:2% statistical error on current result Primakoff stat. (1.46%)
+ fit error 0.44% error from proposal requires (1.46/0.44)2 = 11 times more events:
Increase DAQ rate from 1 kHz to 5 kHz Implement more stringent trigger, (factor of 2-3)
Will provide 0.44% Primakoff statistics on each target
A. Gasparian PAC33, January 15, 2008 22
Summary
A high resolution experimental setup including an EM calorimeter and pair spectrometer has been designed, developed, constructed and commissioned with first physics run in Fall, 2004.
Our first result: Γ(0) 7.93 eV 2.1% (stat.) 2.0% (syst.)
The 0 lifetime is one of the few precision predictions of QCD. Percent level measurement is a stringent test of QCD at these energies.
Compton and pair-production cross section measurements demonstrate that the systematic errors are controlled at the 1.3% level. The experimental setup is capable of percent level cross section measurements. Collaboration has developed required expertise.
High resolution, high intensity tagging facility together with recent developments in calorimetry make the Primakoff method the best way to reach the required accuracy in 0 decay width.
Control of model error in 0 lifetime at 0.25% level has been reached.
Requesting 28 days of beam time to reach the goal of 1.4% on 0 life time.
A. Gasparian PAC33, January 15, 2008 23
The End
A. Gasparian PAC33, January 15, 2008 24
0 Event selection (cont.)
Three groups analyzed the data independently
A. Gasparian PAC33, January 15, 2008 25
Model dependence of Γ(0) Extraction
Model error in Γ(0) Extraction can be controlled at < 0.25%
A. Gasparian PAC33, January 15, 2008 26
Some results on Coherent Production A→0A
• Electromagnetic form factors
• Strong form factors
12C E=5.2 GeV
208Pb
208Pb E=5.2 GeV
Without shadowing
With shadowing
A. Gasparian PAC33, January 15, 2008 27
Estimated Systematic Errors on Compton (preliminary)
Photon flux 1.0%Target thickness (+impurity)
0.05%
Coincidence timing 0.03%Coplanarity 0.065%Radiative tail cut 0.098%Geometric cuts stability 0.65%Background subtraction 0.40%Yield fit stability 0.063%Total 1.27%
A. Gasparian PAC33, January 15, 2008 28
PrimEx Collaboration
North Carolina A&T State University University of Massachusetts Idaho State University University of North Carolina WilmingtonJefferson Lab MITCatholic University of America Arizona State University CIAE Beijing, China Norfolk State UniversityBeijing University, China Lanzhou University, ChinaITEP Moscow, Russia IHEP Protvino, Russia Duke University Kharkov Inst. of Physics and Tech. UkraineNorthwestern University IHEP, China
University of Sao Paulo, Brazil Yerevan Physics Institute, ArmeniaRIKEN, Japan JINR Dubna, Russia USTC, China Hampton University George Washington University
A. Gasparian PAC33, January 15, 2008 29
Compton as Stability Control(maybe to question section)
NOV 02
NOV 09
NOV 14
NOV 190.24
0.25
0.26
0.27
0.28
0.29
0.30
0.31
0.32Compton Cross Section Time Stability
P R E L I M I N A R Y
Uncertainties:Statistical
E = 5.220 GeV
2% Band
T Counter: 3
Run Number
TheoryPrimEx Compton Data
σ (m
b)
A. Gasparian PAC33, January 15, 2008 30
Primakoff Method
22..4
43
3
2Pr
3
sin)(8 QFQE
mZ
dd
me
ρ, ω
Challenge: Extract the Primakoff amplitude
A. Gasparian PAC33, January 15, 2008 31
Compton Cross section
4.9 5.0 5.1 5.2 5.3 5.4 5.5Energy (GeV)
0.230.240.250.260.270.280.290.300.310.32
P R E L I M I N A R Y
Uncertainties:StatisticalSystematic
Compton Total Cross SectionTheoryPrimEx Compton Data
A. Gasparian PAC33, January 15, 2008 32
Compton Cross section: TheoryPure QED process: Calculable at the percent level
Leading Order: Klein-Nishina
Corrections to LO:
Rad. correction (virtual/soft)
Double Compton (hard emiss.)
Klein-Nishina + full rad. Corr. (Monte Carlo Method)
Klein-Nishina + full rad. Corr. (Numerical Integration Method)
A. Gasparian PAC33, January 15, 2008 33
Trigger Improvement
A. Gasparian PAC33, January 15, 2008 34
Impact of Giant Excitation of Nucleus on 0 Primakoff production
• With nuclear collective excitation, the longitudinal momentum transfer in 0 photo-production is Δin= Δ+Eav, where the average excitation energy Eav for 12C is ~20-25 MeV.
• The ratio of the cross section of the 0 photo-production in the Coulomb field with nuclear excitation to “elastic” electromagnetic production can be estimated as:
Cqq
ZAEmN
dddd
inavpel
in
12722
222
for )(qpeak Primakoffat 10)(
)(2
4.1
• Nuclear Giant Excitation effect for lead is small as well.