A Precision Measurement of the Neutral Pion Lifetime: New Results from the PRIMEX

ExperimentRory Miskimen

University of Massachusetts, Amherstfor the

PRIMEX collaboration

• A short review of 0→• The PRIMEX experiment at JLab

• Electromagnetic calibration reactions

i. Pair production

ii. Double arm Compton scattering

• Preliminary results for the 0 lifetime

• PRIMEX at 12 GeV

A short review of 0→ : pre-QCD

• 0 discovered by Bjorklund, Crandall, Moyer and York at Berkeley Cyclotron in 1950.

• The soft-pion limit of PCAC predicts A→= 0

• Adler, Bell, Jackiw and Bardeen discover triangle diagrams that alter PCAC predictions for o decay

p

k1

k2

eV725.7F576

mN23

32c

20

• Wess, Zumino and Witten construct anomalous O(p4) lagrangian that permits transitions between even and odd numbers of pseudo-scalar mesons

• The chiral anomaly has special status in QCD: there are no low energy constants in lagrangian

A short review of 0→ : post-QCD

• Corrections due to

(i) u- d- quark masses

(ii) isospin breaking (, , and ´ mixing) proportional to quark mass differences

• Recent calculations in NLO ChPT gives

eV10.80

≈ 5% higher than LO, with uncertainty of less than 1%

A short review of 0→ : era of ChPT

→

D

eca

y w

idth

(eV

)

NLO, 1% error

Primakoff ReactionPrimakoff

Coherent

Interference

Incoherent

Arizona State University, Tempe, AZ, Catholic University of America, Washington, DC, Chinese Institute of Atomic Energy, Beijing, China, Eastern Kentucky

University, Richmond, KY, George Washington University, Washington, DC, Hampton University, Hampton, VA, Institute for High Energy Physics, Chinese

Academy of Sciences, Beijing, China, Institute for High Energy Physics, Protvino, Moscow region, Russia, Institute for Theoretical and Experimental Physics,

Moscow, Russia, Kharkov Institute of Physics and Technology, Kharkov, Ukraine, Massachusetts Institute of Technology, Cambridge, MA, Norfolk State University,

Norfolk, VA, North Carolina A&T State University, Greensboro, NC, North Carolina Central University, Durham, NC, Thomas Jefferson National Accelerator

Facility, Newport News, VA, Tomsk Polytechnical University, Tomsk, Russia, Idaho State University, Pocatello, ID, University of Illinois, Urbana, IL, University of Kentucky, Lexington, KY, University of Massachusetts, Amherst, MA, University

of North Carolina at Wilmington, Wilmington, NC, University of Virginia, Charlottesville, VA, Yerevan Physics Institute, Yerevan, Armenia

PRIMEX Collaboration

EXPERIMENTAL SETUPEXPERIMENTAL SETUP

DATA TAKING FALL 2004

C, PbC, Pb TARGETS TARGETS

JLAB HALL B JLAB HALL B PHOTONPHOTON TAGGING FACILITYTAGGING FACILITY – – HIGH INTENSITY,HIGH INTENSITY, HIGH RESOLUTION DEVICEHIGH RESOLUTION DEVICE STATE-OF-THE-ARTSTATE-OF-THE-ART CALORIMETRY – CALORIMETRY – HyCalHyCal

PAIR SPECTROMETERPAIR SPECTROMETER

/T=0.3%

PRIMEX Targets

The effective number of carbon atoms/cm2 in the carbon target is known to precision of ±0.05%

Lead target

Pair Spectrometer

PrimEx Hybrid Calorimeter - HyCal

A highly segmented hybrid calorimeter

– 576 lead-glass detectors (4.00×4.00×40 cm3)

– 1152 lead-tungstate detectors (2.125×2.125×21.5 cm3)

– energy resolution

– position resolution

E/%3.1

E/mm5.1

Electromagnetic calibration reactions:

1.Pair production cross section on 12C

2.Double arm Compton scattering on 12C

Pair Production in PrimEx*

*analysis results from A. Teymurazyan

Pair Production Events

Experimental results: differential cross sections

Theory by Alexandr Korchin

Systematic Error Estimation

Photon Flux 2.5 %

Target Thickness 0.05%

Background Estimation 0.5%

Calibrations 2.8 %

Total 3.8 %

Summary of pair analysis•Agreement with theory at the level of ~3.8 %•Work in progress to reduce the systematic errors to 1 – 2 % level

COMPTON SCATTERING IN PRIMEX*

* analysis results from by P. Ambrozewicz, L. Gan, Y. Prok

COMPTON SCATTERINGCOMPTON SCATTERINGEVENTSEVENTS

MINIMAL OPENING ANGLE/MINIMAL OPENING ANGLE/ MINIMAL SEPARATION CUTMINIMAL SEPARATION CUT

ADDITIONAL CUTS SERVEADDITIONAL CUTS SERVE TO MINIMIZE TO MINIMIZE

SIGNAL-TO-NOISE RATIOSIGNAL-TO-NOISE RATIO

RADIATIVE EFFECTS IN THE RADIATIVE EFFECTS IN THE DATADATA

Radiative corrections: I. virtual loops, and soft double Compton scattering, Brown and Feynman; II. hard double Compton scattering Mork, and Mandl and Skyrme

SYSTEMATIC ERROR ESTIMATION

PHOTON FLUX 2.0 %PHOTON FLUX 2.0 % TARGET TARGET 0.05 % 0.05 % SIGNAL/BACKGROUND SEPARATION < 1.5 %SIGNAL/BACKGROUND SEPARATION < 1.5 % RADIATIVE EFFECTS IN ACCEPTANCE CALC. 2.5 %RADIATIVE EFFECTS IN ACCEPTANCE CALC. 2.5 %

TOTAL TOTAL 3.6 % 3.6 %

Summary of Compton analysis• Agreement with theory at the level of few %• Work in progress to reduce the systematic errors to 1 – 2 % level

New results for the 0 lifetime*

* analysis results from E. Clinton, I. Larin, and D. McNulty

E

EE 21

M

Extracting Elastic Pion Yields versus

Analysis 1*

• Find pion yield in bin centered on Elasticity = 1• Subtract inelastic pions: find pion yield as a function of elasticity* D. McNulty

Analysis 2*

• Kinematic fitting to the condition Elasticity = 1

• Fit peak in mass distribution

* I. Larin

Analysis 3*

• Project events onto axis perpendicular to the kinematic correlation between Elasticity and M for elastic events

• Fit peak in mass distribution

* E. Clinton

yield (analysis 1, PbWO only)

Fitting the radiative width

Analysis 1 and 2:

• Data are corrected using (1) geometric acceptance, or (2) full GEANT simulation.

• Angular resolution smeared theoretical distributions are fit to the data

Analysis 3:

• Theoretical distributions are thrown in full GEANT simulation

• Resulting angular distributions include beam energy, resolution, acceptance, and reconstruction effects, and are fitted to the uncorrected experimental angular distributions

Fitted distribution (analysis 2, PbWO only)

Primakoff Coherent

Interference

Incoherent

Fitted distribution (analysis 3, PbWO and lead glass)

Average = 7.93 eV ± 2.1%(stat) ± 2.5%(sys)

PRELIMINARY RESULTS

Systematic Errors (major contributors)

Photon Flux 1.1 %

Lineshape uncertainty 1.0 %

Integration cutoffs 1.1 %

Cluster position finding 0.33 %

Veto counter conversion 0.25 %

Incoherent background shape*

1.5 %

Total 2.4 %

Estimated systematic errors from analysis 3

* Analysis I estimate. Calculations by T. Rodrigues in progress

7.93 eV ± 2.1%(stat) ± 2.5%(sys)

PRELIMINARY RESULTS

→

D

eca

y w

idth

(eV

)

NLO, 1% error

PrimEx Program at 12 GeV JLAB

We propose to measure:

– Two-Photon Decay Widths: Γ(η―>γγ), Γ(η׳―>γγ)

– Transition Form Factor Fγγ* P of π0, η and η׳ at low Q2 (0.001--0.5 GeV2/c2)

via the Primakoff effect.

PrimEx Program at 12 GeV JLAB

11 GeV photoproduction on proton

Fundamental input to physics:

Determination of quark mass ratioDetermination of quark mass ratio

--´́ mixing mixing Interaction radius of Interaction radius of 00, , and and ´́ Is theIs the ´́ an approximate an approximate

Goldstone boson?Goldstone boson?

s

ud

m

mm

Summary and Conclusions Pair production and Compton cross sections have

been extracted with 4 % total errors. The goal is to obtain errors at the level of 1-2%

We are in the process of finalizing the πo lifetime measurement, and the systematic errors. Our preliminary result is:

%5.2%1.2eV93.70

Our result is in agreement with LO and NLO ChPT

Look forward to additional running to reduce our statistical and systematic errors

A Primakoff program at 12 GeV holds great promise for studies of the pseudo-scalar mesons