an updated high precision measurement of the neutral pion lifetime via the primakoff effect

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%


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→


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→


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.


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


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


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.


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


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


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


Differential Cross section

Experimental Yield per

GEANT: acceptances; efficiencies; resolutions;

Diff. cross section


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


Fit to Extracted 0 Decay Width:208Pb Target

Theoretical angular distributions smeared with experimental resolutions are fit to the data


PrimEx Current Result

() = 7.93eV2.1%2.0%

0

D

ecay

wid

th (e

V)

±1.%


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%


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)


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%


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


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 (%)


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


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


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.


The End


0 Event selection (cont.)

Three groups analyzed the data independently


Model dependence of Γ(0) Extraction

Model error in Γ(0) Extraction can be controlled at < 0.25%


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


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%


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


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


Uncertainties:Statistical

E = 5.220 GeV

2% Band

T Counter: 3

Run Number

TheoryPrimEx Compton Data

σ (m

b)


Primakoff Method

22..4

43

3

2Pr

3

sin)(8 QFQE

mZ

dd

me

ρ, ω

Challenge: Extract the Primakoff amplitude


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


Uncertainties:StatisticalSystematic

Compton Total Cross SectionTheoryPrimEx Compton Data


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)


Trigger Improvement


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.

an updated high precision measurement of the neutral pion lifetime via the primakoff effect

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