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, GreeceSeptember 12-15, 2007

Dan DaleIdaho State University

Participating Institutions

North Carolina A&T State University University of MassachusettsUniversity of Kentucky Idaho State UniversityJefferson Lab Hampton UniversityMIT University of North Carolina at WilmingtonCatholic University of America Arizona State UniversityUniversity of Virginia ITEP, RussiaIHEP, Russia Norfolk State UniversityUniversity of Illinois Kharkov Inst. of Physics and TechnologyChinese Institute of Atomic Energy IHEP, Chinese Academy of Sciences George Washington University Yerevan Physics Institute, ArmeniaSouthern University at New Orleans Tomsk Polytechnical University, RussiaUniversity of Sao Paulo

A High Precision Measurement of

• Physics motivation• The Primakoff effect• Experimental setup• Towards high precision• Current analysis event selection radiative width extraction • Summary

The axial anomaly

Leading order corrections (Donoghue, 1989)

Corrections O(p4)

Current experimental and theoretical status

Expected PrimEx error bar,arbitrarily projected to theleading order chiral anomaly

decay

wid

th (

eV

)

Next to LeadingOrder, +- 1%

Sum rule approach to axial-vector-vector (AVV) form factor.

f – f caused by strong interaction shown to be small.

Eta width only input parameter. o mixing included.

o eV

Recent Theoretical Advances(B.L. Ioffe and A.G. Oganesian, Phys. Lett. B647, p. 389, 2007)

The Primakoff Effect

Radiative width Known, measured quantities

Angular distributionenables separation ofamplitudes

For high precision:

• Measure angular distribution well to separate various amplitudes

new state of the art o detector

• Measure on a variety of targets to test validity of background extraction (Primakoff

• Tight photon energy (peak4and flux control

photon tagging

• Measure QED processes (Compton and pair production) to validate setup and analysis techniques.

incident electrons

Tagged photon beam

Bremsstrahlung photon tagging

Electron beam

Post-brems electrons

Tagging Ratios

Photon Calorimetry – the HYCAL calorimeter

1152 PbWO4 and 576 lead glass modules

1.6% energy resolution

1-2 mm position resolution

In-beam calibration

Effect of periodicityof detector modules.

Energy resolution – lead tungstate

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

e+ - e - pair spectrometer

dipolemagnet

plastic scintillators

Stability of relative tagging ratios

Monitored during production data taking.

Stability of relative tagging ratios

Stable to 1%.

Relative tagging ratios versus rate

Independent of rate!

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.

Compton Scattering--Stability of Setup

Angular distribution for CarbonPrimakoff Peak

UMass Analysis

HYCAL – Tagger timing

Invariant Mass Elasticity

Total Probability = Timing x Mass x Elasticity

Event selection:

Normalized probabilitydistributions for

Elasticity versus invariant mass

Hybrid mass

UMass Analysis: Radiative Width for Carbon

Primakoff peak

Nuclear coherent andPrimakoff – NC interference

MIT/JLab Analysis: Event Selection

Invariant mass

HYCAL – Tagger timing

Elasticity

Angular distributions and background

Carbon Lead

MIT/JLab Analysis: Radiative Widths for Carbon and Lead

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

ITEP/NCA&T Analysis: Radiative Width for Carbon

= 7.98 ± 0.18eV

All possible cluster combinations.Elasticity imposed.HYCAL – tagger timing < 4 nsec.

ITEP/NCA&T Analysis: Radiative Width for Lead

= 7.94 ± 0.18eV

ITEP/NCA&T Analysis: Radiative Width Combined Fit for Carbon and Lead

= 7.95 ± 0.13eV

Carbon

Lead

Summary IUMass AnalysisCarbon 8.41 eV ± 0.15 eV ± 1.8%

MIT/JLab Analysis: Carbon: = 8.20 eV ± 0.15 eV ± 0.20 eVLead: = 8.11 eV ± 0.16 eV ± 0.20 eV

ITEP/NCA&T Analysis:Carbon: = 7.98 eV ± 0.18 eVLead: = 7.94 eV ± 0.18 eVCombined fit, Carbon and Lead: = 7.95 eV ± 0.13 eV

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:

= 7.93 eV ± 2.1% (stat) ± 2.0% (stat)

• In good agreement with ChPT predictions.• Further studies of backgrounds underway.

Extra Slides

PDF1: HYCAL – Tagger timing

PDF 3: Invariant mass

PDF 2: Elasticity

Previous Experiment: The Direct Method(Phys. Lett., vol 158B, no. 1, 81, 1985)

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

o decays observed by detection of 150 GeV/c positrons produced by decay rays converting in foils.

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

Dalitz decay, conversion in first foil