Status of the PrimEx Experiment at JLab:A Precision Measurement of the Neutral Pion Lifetime

Yelena ProkMIT

For the PrimEx Collaboration

The 2006 Photonuclear Gordon Research Conference August 1, 2006

Outline

Physics MotivationThe Primakoff EffectExperimental SetupCompton Scattering on Electrons Preliminary 0 AnalysisSummary and Outlook

Chiral AnomalyThe 0 decay rate is a fundamental prediction of confinement scale QCD.

Chiral Anomaly – the closed-loop trianglediagram results in axial vector current non-conservation, even in the limit ofvanishing quark masses.

The anomaly leads to the reduced decayamplitude, in leading order (chiral limit)

where F= 92.42+/-0.25 MeV is the pion decay constant.

1-GeV0251304

.πF

αA

π

emγγ

Decay WidthThe 0 decay width is related to the amplitude:

is presently known to 10%

The goal of the Primex Experimentis to measure the decay width toan accuracy of 1.5%.

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0

Am

Test axial anomaly and next to leading order PT

Primakoff Effect

The 0 photoproduction from Coulomb field of the nucleus.

Production (*!0) and decay (0!) mechanisms imply the Primakoff cross section is proportional to the 0 lifetime.

22

4

43

3

2

sin8

0 QFQ

E

m

Z

d

dem

emP

Cross Section Components)cos(2

d

d

d

d

d

d

d

d

d

d

d

d CPICP

Primakoff NuclearCoherent

NuclearIncoherent

Interference

Primakoff photoproduction from a Coulomb field of a nucleus ~ Z2, peak at =m

2/2E2

Nuclear Coherent nucleus left in the ground state, peak at~2/ER, ~ A2

Nuclear Incoherent nucleus left in excited state, which may be either bound or particle unstable , ~ A

Interference between the Primakoff process and the coherent non-spin-flip nuclear photoproduction

The Primex Experiment was conducted at Jefferson Lab. The measurement was taken in Hall B using the photon tagging system. The CLAS detector was not employed. Instead, a new detector was designed and installed specificallyfor the Primex Experiment.

Jefferson Lab

PrimEx Setup in Hall B

Tagged photons of energy 4.9-5.5 GeVwere used to measurethe absolute cross sectionof small angle 0 photoproduction from 12C and 208Pb targets

HYbrid CALorimeter HYCAL

•Hybrid calorimeter was designed and installed to detect the 2 decay photons• Central Part: PbWO4 x 1152 ch E/E ~ 1.2 % at 2.5 GeV x ~ 1.55 mm • Outer part: Lead Glass x 576 chE/E ~ 5 % at 2.5 GeV x ~ 5 mm• Light Monitoring System : 10-20 Hz sampling of the PMT gains

4 cm x 4 cm beam hole

Compton Scattering off Atomic Electrons

10/09 11/22

1st calibration

Production runs throughout

2nd calibration

Periodic Compton Runs

Well known process, can be used for systematic studies of experimental setupData is used for: •Detector alignment•Monitoring of HYCAL gain drifts with time•Calculation of absolute cross section

Compton Scattering off Atomic Electrons e! e

PW

OLG

LG

incident

scattered ’

scattered e-

•Compton scattering at small angles•Targets: 5 % r.l 12C, 0.5 % r.l.9Be•What we measure:

•Incident photon energy•Energy and position of the scattered particles

•Double Arm Compton Data: BPS=0, I=5 nA detect both particles, dedicated runs •Single Arm Compton Data: by-product of the 0 production runs only scattered photons are detected

Double Arm Compton Event

Compton event in “primsim”

2 clusters are detected on the calorimeter

target

Helium Bag

Veto countersHYCAL

Monitoring Beam Position: DA Compton data

Looking for an intersection point between lines connecting Compton e- and by minimizing the sum of squares of distances between a point (x,y) and each line (1-2)/2 (deg)

(1+

2)/

2

Monitoring Energy Gains: DA Compton Data

Module 1498Using photons

Gain_Corr = 1.038» 3.8 %

(e1+e2)/E

before

after

Energy Resolution

e/ec

“Compton” energy of the photons/electrons can be calculated from the Compton kinematics:ec=E/[1+2E/me*sin2((el)/2)]Can obtain gain correction factor by looking at the ratios of the measured and expected energies: e/ec

DA Compton: Event Selection (Be target)

Reconstruct the vertex of Compton reactionZ=(x2+y2)0.5[/(E/e-1)]0.5

Z (cm)

Apply kinematic constraints:energy and momentum conservationReconstruct Z again

2<100 removes mostof the background

He bag

DA Compton: Extracting the Cross Section

very preliminary

very preliminary

error bars are statistical onlyradiative corrections are not applied

DA Compton: Integrated Cross Section

0 production on 12C

Eve

nts/

0.04

deg

0 production on 208Pb

Eve

nts/

0.04

deg

Summary and Outlook

The PrimEx experiment collected 0 production data on 12C and 208Pb

Analysis of Compton scattering provides a useful tool for monitoring of experimental conditions

Preliminary analysis of Double Arm Compton Cross Section is in good agreement with theory

More systematic checks and radiative corrections are needed for the Compton scattering analysis

Items on the Primakoff 0 production to be completed: Finalize 0 yields and normalize to photon flux Extract 0! by fitting latest theoretical models to dN/d

distribution. The models will be corrected for acceptance and efficiencies of the experiment

Many thanks to people who provided plots and advice

Additional slides

Monitoring Beam Position: SA Compton Data

Xcompton=(2/*z2(E/e-1)-y2)0.5

Ycompton=(2/*z2(E/e-1)-x2)0.5

E/me

Relative beam-HYCAL positioncan be checked every ~ 10 min of data taking

DA Compton: Counting Events

9Be, MC 12C, MC

9Be data

12Cdata

Eve

nts/

2

Vertex Z1 (cm) Vertex Z1 (cm)

Eve

nts/

2

Monitoring Energy Gains: SA Compton Data

carbonlead

Using single arm compton data in the production runs allows for run-by run gain monitoringCompton gains are consistent with the Light Monitoring System

22

4

43

3

2

sin8

0 QFQ

E

m

Z

d

dem

emP

222 sinQFCAd

dN

C

d

dQGA

d

d HI 1

is the pion decay width.em is fine structure constant.Z is the number of protons in the nucleus.mare the pion mass, velocity, and production angle.E is the incoming photon energy.Q is the momentum transfer to the nucleus.Fem(Q) is the electromagnetic form factor of the nucleus.

•A is the number of nucleons.•Csin2() is the spin independent part of the 0 photoproduction amplitude from a single nucleon.•FN(Q) is the nuclear form factor of the nucleus mass distribution.

• is an absorption factor.• 1-G(Q) is a factor that reduces the cross section at small Q.• dH/d is the 0 photoproduction cross section on a single nucleon.

Primakoff Cross Section

Coherent Cross Section Incoherent Cross Section

Proposed Error Budget

Statistical 0.4%

Target thickness 0.7%

Photon flux 1.0%

HYCAL acceptance and misalignment 0.4%

Background subtraction 0.2%

Beam energy 0.2%

Form factor calculation errors 0.3%

Total 1.4%

DA Compton: Extracting the Cross Section

Total Cross Section (per electron)

T=N/(L*F*A* N=nevents L=luminosity=*t*NA/ t=target thickness =target density NA=Avogadro’s Number =atomic mass =efficiency (from MC) A=atomic number F=photon flux

Error bars are statistical only Radiative corrections are not applied

very preliminary

Decay WidthThe 0 decay width is related to the amplitude:

The Particle Data Group value is 7.84+/-0.56 eV.The Next to Leading order predictionis 8.10+/- 1% which is 4% higher thanthe present experimental value.

The goal of the Primex Experimentis to measure the decay width toan accuracy of 1.5%.

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