A Precision Measurement A Precision Measurement of the Neutral Pion of the Neutral Pion

Lifetime via the Primakoff Lifetime via the Primakoff EffectEffect

Jefferson Lab Experiment E99-014Jefferson Lab Experiment E99-014PrimEx CollaborationPrimEx Collaboration

Eric R. I. ClintonEric R. I. ClintonRory Miskimen, PhD., AdvisorRory Miskimen, PhD., Advisor

Given at NNPSS, June 15, Given at NNPSS, June 15, 20052005

The goal of the PrimEx The goal of the PrimEx CollaborationCollaboration

Make a high precision measurement of the Make a high precision measurement of the oo to to decay rate dominated by the axial anomaly. decay rate dominated by the axial anomaly.

This will be done using the Primakoff Effect, This will be done using the Primakoff Effect, photopion production in the Coulomb field of photopion production in the Coulomb field of the nucleus.the nucleus.

Using the tagged photon facility in Hall B, a Using the tagged photon facility in Hall B, a state of the art Hybrid Calorimeter, and well state of the art Hybrid Calorimeter, and well understood nuclear targets, the collaboration understood nuclear targets, the collaboration hopes to measure the Primakoff cross section to hopes to measure the Primakoff cross section to 1.5% or better.1.5% or better.

The Chiral The Chiral AnomalyAnomaly

What is the physics What is the physics motivation?motivation?

In the Chiral limit where the u and d quark In the Chiral limit where the u and d quark masses vanish, we can exactly compute the masses vanish, we can exactly compute the decay amplitude. decay amplitude.

AA = = NNc c / ( 3/ ( 3ff))

And we know the decay width of the And we know the decay width of the o o to to

CLCL= m= m33|A|A||22 / ( 64 / ( 64 ) = 7.725 +- 0.044eV ) = 7.725 +- 0.044eV

More physics motivationMore physics motivation

Real world quark masses are not 0 MeVReal world quark masses are not 0 MeV Mass of u,d quarks on order of 5-7 MeV.Mass of u,d quarks on order of 5-7 MeV.

Adler and BardeenAdler and Bardeen Non-renormalization theorem gives quark mass Non-renormalization theorem gives quark mass

correction.correction. The The oo mixing with mixing with and and ’ -- 2-3% increase in the decay ’ -- 2-3% increase in the decay

width. width.

Smaller corrections (~ 0.001)Smaller corrections (~ 0.001) Saturation of the chiral anomaly by heavier mesons. Saturation of the chiral anomaly by heavier mesons.

Thus, Thus, ((oo to to ) is the most accurate prediction in ) is the most accurate prediction in QCD, depending only on the number of colors.QCD, depending only on the number of colors.

Recent Theory Recent Theory DevelopmentsDevelopments

Joity, Bernstein, Donoghue, and HolsteinJoity, Bernstein, Donoghue, and Holstein Calculated NLO corrections to Calculated NLO corrections to 00 width. width.

Result: Result: NLONLO = 8.10 ± 0.081 eV = 8.10 ± 0.081 eVCLCL = 7.725 +- 0.044eV = 7.725 +- 0.044eV

PDGPDG = 7.84 ± 0.556eV = 7.84 ± 0.556eV

Mousaallam et. al. have a very similar Mousaallam et. al. have a very similar result. result.

Other measurements of Other measurements of the the oo decay. decay.

-There are three reactions that allow -There are three reactions that allow us to conveniently measure the us to conveniently measure the oo

decay width.decay width.

““Direct measurement” through time of Direct measurement” through time of flight dataflight data

collisionscollisions

The Primakoff Effect.The Primakoff Effect.

The Primakoff EffectThe Primakoff Effect

The Primakoff Effect is photopion production in the Coulomb field of a high Z The Primakoff Effect is photopion production in the Coulomb field of a high Z nucleus.nucleus.

Previous Primakoff Previous Primakoff ExperimentsExperiments

Browman et al.Browman et al. = 8.02 ± 0.42eV = 8.02 ± 0.42eV

(no luminousity or detector efficiency uncertainties (no luminousity or detector efficiency uncertainties included).included).

decay widths measured using a similar set-up and decay widths measured using a similar set-up and analysis yielded questionable results.analysis yielded questionable results.

DESY DESY

1.0 GeV beam---- 1.0 GeV beam---- = 9.02 ± .95 eV = 9.02 ± .95 eV 1.5-2.0 GeV beam----1.5-2.0 GeV beam---- = 11.7 ± 1.2 eV. = 11.7 ± 1.2 eV.

TomskTomsk 1.1 GeV beam---- 1.1 GeV beam---- = 7.32 ± 0.5 eV. = 7.32 ± 0.5 eV.

PrimEx at Jefferson PrimEx at Jefferson LabLab

Tagged Photon Facility at JLabTagged Photon Facility at JLab Increase precision and get cleaner Increase precision and get cleaner

kinematics.kinematics.

The invariant pion mass will be The invariant pion mass will be deduced by detecting the neutral pion deduced by detecting the neutral pion decay decay ’s.’s.

Good neutral pion detector Good neutral pion detector Identify nuclear coherent and multi-photon Identify nuclear coherent and multi-photon

background contributionsbackground contributions Provide good energy and angular resolution.Provide good energy and angular resolution.

PrimEx, continuedPrimEx, continued

The Primakoff cross section for unpolarized photons The Primakoff cross section for unpolarized photons isis

dd33/d/d = =

((88ZZ2233EE44|F|Femem(Q)|(Q)|22/m/m33QQ44)sin)sin22

Strong maximum in the Primakoff CS at Strong maximum in the Primakoff CS at

~ m~ m22/(2E/(2E

22)) Falls off rapidly at higher angles.Falls off rapidly at higher angles.

That’s not the whole storyThat’s not the whole story Nuclear coherent and incoherent processes and a cross Nuclear coherent and incoherent processes and a cross

term between NC and Primakoff effect contribute to the term between NC and Primakoff effect contribute to the cross section.cross section.

Error budgetError budget

StatisticalStatistical 0.4%0.4% Target ThicknessTarget Thickness 0.7%0.7% Photon Flux Photon Flux 1.0%1.0% oo detector acceptance and misalignment detector acceptance and misalignment

0.4%0.4% Background subtractionBackground subtraction 0.2%0.2% Beam energyBeam energy 0.2%0.2% Distorted form factor calculation errorsDistorted form factor calculation errors

0.3%0.3%

Total ErrorTotal Error 1.4%1.4%

Conceptual PrimEx set-upConceptual PrimEx set-up

Photon TaggerPhoton Tagger

The Hybrid CalorimeterThe Hybrid Calorimeter HyCal is a highly segmented array of lead HyCal is a highly segmented array of lead

tungstate and lead glass crystalstungstate and lead glass crystals 7.5 meters downstream of the targets.7.5 meters downstream of the targets. The interior array of crystalsThe interior array of crystals

1152 lead tungstate modules, each 2.05x2.05.18 1152 lead tungstate modules, each 2.05x2.05.18 cmcm33, 20 Xo, and 2.0 cm Moliere radius., 20 Xo, and 2.0 cm Moliere radius.

The outer arrayThe outer array 576 lead glass modules, each 3.84x3.84.45 cm576 lead glass modules, each 3.84x3.84.45 cm33

and 17 Xo.and 17 Xo. The small size of the interior detectors means The small size of the interior detectors means

very fine angular resolution (better than 0.02very fine angular resolution (better than 0.02oo) ) and energy resolution (~3.5 Mev).and energy resolution (~3.5 Mev).

Final ThoughtsFinal Thoughts

The PrimEx took date from September The PrimEx took date from September 2004 to November 2004.2004 to November 2004. We see lots of pi-zeros.We see lots of pi-zeros.

Good statistics on Carbon, fair statistics on Good statistics on Carbon, fair statistics on Lead.Lead.

Preliminary analysis, calibration, and Preliminary analysis, calibration, and book-keeping stage.book-keeping stage.

Then, about 1.5 years from now, I Then, about 1.5 years from now, I graduate.graduate.

I’m crossing my fingers…I’m crossing my fingers…