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

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

eVF

mNc 725.7576 23

3220

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

Corrections to 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 percent level will provide a stringent test of fundamental predictions of QCD.

0→

Recent calculations in QCD sum rule: (B.L. Ioffe, at al. Phys. Lett. B647, p. 389, 2007)

Γ() is only input parameter0- mixing includedΓ(0) = 7.93eV ± 1.5%


Decay Length Measurements (Direct Method)

1x10-16 sec too small to measuresolution: Create energetic 0 ‘s,

L = vE/m

But, for E= 1000 GeV, Lmean 100 μm very challenging experiment

Measure 0 decay length

An experiment had been done at CERN, in 1984, P=450 GeV proton beam2 variable (5-250m) foilsResult:(0) = 7.34eV3.1%(total) Dominant systematic error:Uncertainty in P0 (1.5%)

Limitations of method unknown P0 spectrum foil position dependent exptl. bgnd.


e+e- Collider Experiment e+e-e+e-**e+e-0e+e- e+, e- scattered at small angles (not detected)

only detected

experiment: DORIS II @ DESY

Results: Γ(0) = (7.7 ± 0.5 ± 0.5 eV ( ± 10.0%)

dominant systematic errors: luminosity (~6%)

beam-residual gas interaction

Not included in PDG average

Limitations of method luminosity unknown q2 for **


Primakoff Method

22

..4

43

3

2Pr

3

sin)(8

QFQ

E

m

Z

d

dme

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) bresstrahlung beam

E=4 and 6 GeV targets: Be, Al, Cu, Ag, UResult: (0)=(7.920.42) eV

5.3% dominant systematic errors: N (4%) and quantameter (2%)

All previous experiments used: Bremsstrahlung (untagged) 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 to 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 August-November 2004 run.

First publication is expected in spring, 2008.

Preliminary results had been released at APS April, 2007 meeting with AIP press conference.


Luminosity Control: Pair Spectrometer

Dipole

Precision cross section measurement:

photon flux at 1% level required

e-

e+

HYCAL

Photon beam

Scint. Det.

absolute tagging ratios: TAC measurements at low intensities

Checked by cross sections of known EM processes at the 1% level:

Compton scattering e+e- pair production

relative tagging ratios: pair spectrometer at low and high intensities


Electromagnetic Calorimeter: HYCAL

1152 PbWO4 crystal detectors 576 Pb-glass Cherenkov detectors

Energy resolution Position resolution Good photon detection efficiency @ 0.1 – 5 GeV; Large geometrical acceptance

PbWO4 crystals resolutionPb-glass budget

Design concept hybrid calorimeter


- Invariant Mass Resolution


0 Event selection

We measure:

initial photon energy: E and time energies of decayed photons: E1, E2 and time X,Y positions of decayed photons

Kinematical constrains:

Conservation of energy; Conservation of momentum; m invariant mass

Three groups analyzed the data independently


Differential Cross section

Experimental Yield per

GEANT: acceptances; efficiencies; resolutions;

Diff. cross section


0 Forward Photoproduction off Complex Nuclei(theoretical models)

Coherent Production A→0A

Primakoff Nuclear coherent

0 rescattering Photon shadowing

Leading order processes:

Next-to-leading order:


0 Forward Photoproduction off Complex Nuclei(theoretical models)

Incoherent Production A→0A´

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

Deviation in Γ(0) Extraction:

less than <0.2%


Fit to Extract 0 Decay Width

Combined average from three groups:

Γ(0) 7.93 eV 2.10%(stat.) 2.0% (syst)

Theoretical angular

distributions smeared with

experimental resolutions

are fit to the data


PrimEx Current Result

() = 7.93eV2.1%2.0%

0

D

eca

y w

idth

(eV

)

±1.%


Estimated Systematic Errors Type of Systematic

ErrorsEstimated

contributions in first run

Estimated contributions for current 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%


Compton Cross section: Theory

Pure QED process: Should be calculable on 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)


Compton Cross section: Experiment

4.9 5.0 5.1 5.2 5.3 5.4 5.5

Energy (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 Section

Klein-NishinaPrimex Compton Data

4.9 5.0 5.1 5.2 5.3 5.4 5.5

Energy (GeV)

-10

-5

0

5

10

Devi

atio

n (%

)


Uncertainties:Statistical

Experiment To Theory Comparison

No DeviationExperiment / Theory

Average stat. error: 0.6% Average syst. error: 1.2%

Total: 1.3% Δ

σ/Δ

Ω (

mb

/6.9

msra

d)


Summary

A state-of-the-art high resolution experimental setup including a high precision 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 2.10% (stat.) 2.0% (syst.)

The 0 lifetime is one of the few parameter-free predictions in QCD reflecting effects of fundamental symmetry and axial anomaly.

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 1.3% level.

The experimental setup is capable for a percent level cross section measurement.

Availability of high resolution and high intensity tagging facility together with recent developments in calorimetry made the Primakoff method the viable way to reach the projected percent level 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


Stability of relative tagging ratios

Monitored by PS during production data taking.

PS+tagger

Tagger


0 Event selection (cont.)

Three groups analyzed the data independently


Theoretical Study of 0 Forward Photoproduction off Complex Nuclei

Coherent Production A→0A:

PrimakoffNuclear coherent

0 rescattering Photon shadowing

Absorption of 0


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


Incoherent Production A→0A´

Two independent approaches: • Glauber theory• Cascade Model

Deviation in Γ(0) Extraction is <0.2%


Differential Cross section

Experimental Yield per

GEANT: acceptances; efficiencies; resolutions;

Diff. cross section


New from Ilya, 011208(animation)

Combined average from three groups:

Γ(0) 7.93 eV 2.10%(stat.)

Theoretical angular

distributions smeared with

experimental resolutions

are fit to the data


Control of Systematic Errors: Compton

e e

Events Selection Energy conservation

3-momentume conservation

(including co-planarity)

We measure:

Initial photon energy: E and time Energies of scattered particles: E, Ee and time X,Y positions on HYCAL


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 19

0.24

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

(mb

)


Primakoff Method

22

..4

43

3

2Pr

3

sin)(8

QFQ

E

m

Z

d

dme

ρ, ω

Challenge: Extract the Primakoff amplitude


Compton Cross section

4.9 5.0 5.1 5.2 5.3 5.4 5.5

Energy (GeV)

0.23

0.24

0.25

0.26

0.27

0.28

0.29

0.30

0.31

0.32


Uncertainties:StatisticalSystematic

Compton Total Cross Section

TheoryPrimEx Compton Data


Trigger Improvement


e+e- Pair Production in PrimEx

Agreement with theory at the level of 2.5%

Work in progress to reduce the systematic errors to 1-2% level


An Example: Precision Measurement of → decay width

All decay widths are calculated from decay width and experimental Branching Ratios (B.R.):

ΓΓ((η→η→ decay) = decay) = ΓΓ((→→) × B.R.) × B.R.

Any improvement in ΓΓ((→→))

will change the whole will change the whole - sector in PDB- sector in PDB


Compton Cross section

4.9 5.0 5.1 5.2 5.3 5.4 5.5

Energy (GeV)

8.0

8.5

9.0

9.5

10.0

10.5

11.0

11.5

12.0


Forward Compton Scattering Cross Section

Klein-NishinaPrimex Compton Data


PbWO4 Development:Optical Properties

Optical transparency


PbWO4 Development

Specified

Size: 20.5x20.5x180 mm3

Tolerances: +0.0-0.1 in trans. +0.3-0.0 in long.

Collaboration managed to double the number of crystals: to from 650 to 1250 Critical for the experiment


0 decay width: recent theoretical advances

QCD sum rule approach: f0 - f+ caused by strong interaction

shown to be small 0 - mixing included

Γ(0) = 7.93eV ± 1.5%

error is dominated by Γ() decay width

Precision measurements of (0→) at percent level will provide ultimate test of fundamental predictions of QCD.

0→


(0→) World Data (do not need)

0 is lightest quark-antiquark hadron

The lifetime:

= B.R.( 0 →γγ)/(0 →γγ) 0.8 x 10-16 second

Branching ratio: B.R. ( 0→γγ)= (98.8±0.032)% 0

→

±1%


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:

Cq

q

ZAEm

N

d

dd

d

inavpel

in

12722

222

for )(qpeak Primakoffat 10)(

)(

2

4.1

• Nuclear Giant Excitation effect for lead is small as well.

Outline

Physics Motivation Different methods of lifetime measurements The PrimEx experiment and our first results Control of systematic errors Summary

Documents

An Updated High Precision Measurement of the Neutral Pion Lifetime via the Primakoff Effect