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Status of the PrimEx Experiment: A Precision Measurement of the Neutral Pion Lifetime. 7 th European Conference on “Electromagnetic Interactions with Nucleons and Nuclei” Milos Island, Greece September 12-15, 2007 Dan Dale Idaho State University. Participating Institutions.

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status of the primex experiment a precision measurement of the neutral pion lifetime
Status of the PrimEx Experiment:A Precision Measurement of the Neutral Pion Lifetime

7th European Conference on

“Electromagnetic Interactions with Nucleons and Nuclei”

Milos Island, Greece

September 12-15, 2007

Dan Dale

Idaho State University

participating institutions
Participating Institutions

North Carolina A&T State University University of Massachusetts

University of Kentucky Idaho State University

Jefferson Lab Hampton University

MIT University of North Carolina at Wilmington

Catholic University of America Arizona State University

University of Virginia ITEP, Russia

IHEP, Russia Norfolk State University

University of Illinois Kharkov Inst. of Physics and Technology

Chinese Institute of Atomic EnergyIHEP, Chinese Academy of Sciences

George Washington University Yerevan Physics Institute, Armenia

Southern University at New Orleans Tomsk Polytechnical University, Russia

University of Sao Paulo

a high precision measurement of p o g g
A High Precision Measurement of p o -> g g
  • Physics motivation
  • The Primakoff effect
  • Experimental setup
  • Towards high precision
  • Current analysis

event selection

radiative width extraction

  • Summary
the axial anomaly
The axial anomaly

Leading order corrections (Donoghue, 1989)

Corrections O(p4)

current experimental and theoretical status
Current experimental and theoretical status

po -> g g decay width (eV)

Next to Leading

Order, +- 1%

Expected PrimEx error bar,

arbitrarily projected to the

leading order chiral anomaly

recent theoretical advances b l ioffe and a g oganesian phys lett b647 p 389 2007
Recent Theoretical Advances(B.L. Ioffe and A.G. Oganesian, Phys. Lett. B647, p. 389, 2007)
  • Sum rule approach to axial-vector-vector (AVV) form factor.
  • fpo – fp+ caused by strong interaction shown to be small.
  • Eta width only input parameter.
  • po - h mixing included.

G(pogg) = 7.93 eV

the primakoff effect
The Primakoff Effect

Radiative width

Known, measured quantities

slide10

Angular distribution

enables separation of

amplitudes

for high precision
For high precision:
  • Measure angular distribution well to separate various amplitudes

 new state of the art po detector

  • Measure on a variety of targets to test validity of background extraction (sPrimakoff a Z 2).
  • Tight photon energy (speak a E4 ) and flux control

 photon tagging

  • Measure QED processes (Compton and pair production) to

validate setup and analysis techniques.

slide12

incident electrons

Tagged photon beam

bremsstrahlung photon tagging
Bremsstrahlung photon tagging

Post-brems electrons

Electron beam

photon calorimetry the hycal calorimeter
Photon Calorimetry – the HYCAL calorimeter

1152 PbWO4 and

576 lead glass modules

1.6% energy resolution

1-2 mm position resolution

slide18

Effect of periodicity

of detector modules.

luminosity analysis efforts
Luminosity analysis efforts
  • Electron counting (absolute flux)
  • Pair spectrometer (relative tagging ratios)
  • TAC (absolute tagging ratios)
  • Target thickness
  • Known cross sections (pair production,

Compton)

online flux monitoring
Online Flux Monitoring

plastic

scintillators

dipole

magnet

e+ - e - pair spectrometer

stability of relative tagging ratios
Stability of relative tagging ratios

Monitored during production data taking.

pair production cross section
Pair production cross section

(1)Bethe-Heitler (modified by nuclear form factor).

(2)Virtual Compton scattering.

(3)Radiative effects.

(4)Atomic screening.

(5)Electron field pair production.

umass analysis
UMass Analysis

Event selection:

Normalized probability

distributions for

  • HYCAL – Tagger timing
  • Invariant Mass
  • Elasticity

Total Probability = Timing x Mass x Elasticity

slide30

Elasticity versus

invariant mass

Hybrid mass

umass analysis radiative width for carbon
UMass Analysis: Radiative Width for Carbon

Primakoff peak

Nuclear coherent and

Primakoff – NC interference

slide32

MIT/JLab Analysis: Event Selection

Invariant mass

HYCAL – Tagger timing

Elasticity

mit jlab analysis radiative widths for carbon and lead
MIT/JLab Analysis: Radiative Widths for Carbon and Lead

Carbon:G = 8.20 ±0.15 eV ± 0.20 eV Lead: G = 8.11 ± 0.16 eV± 0.20 eV

itep nca t analysis radiative width for carbon
ITEP/NCA&T Analysis: Radiative Width for Carbon

G = 7.98 ± 0.18eV

  • All possible cluster combinations.
  • Elasticity imposed.
  • HYCAL – tagger timing < 4 nsec.
summary i
Summary I

UMass Analysis

Carbon G = 8.41 eV± 0.15 eV ± 1.8%

MIT/JLab Analysis:

Carbon: G = 8.20 eV ± 0.15 eV ± 0.20 eV

Lead: G = 8.11 eV± 0.16 eV ± 0.20 eV

ITEP/NCA&T Analysis:

Carbon: G = 7.98 eV± 0.18 eV

Lead: G = 7.94eV± 0.18 eV

Combined fit,

Carbon and Lead: G = 7.95eV± 0.13 eV

summary
Summary
  • The neutral pion lifetime is a stringent test of confinement scale QCD.
  • High precision of the experiment has pushed the limits of photon calorimetery, luminosity monitoring, and QED calculations.
  • Experimental systematic errors controlled by pair production and Compton scattering.
  • High quality data taken in late 2004. Three quasi-independent analyses.
  • Our preliminary result is:
  • G = 7.93eV± 2.1% (stat) ± 2.0% (stat)
  • In good agreement with ChPT predictions.
  • Further studies of backgrounds underway.
slide45

Previous Experiment: The Direct Method

(Phys. Lett., vol 158B, no. 1, 81, 1985)

po`s produced by 450 GeV protons in tungsten foils.

p o decays observed by detection of 150 GeV/c positrons

produced by decay g rays converting in foils.

Y(d) = N[A + B(1-e-d/l)]

Dalitz decay, g conversion in first foil