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

1. Status of the PrimEx Experiment at JLab:A Precision Measurement of the Neutral Pion Lifetime Yelena Prok MIT For the PrimEx Collaboration The 2006 Photonuclear Gordon Research Conference August 1, 2006

2. Outline • Physics Motivation • The Primakoff Effect • Experimental Setup • Compton Scattering on Electrons • Preliminary 0 Analysis • Summary and Outlook

3. Chiral Anomaly The p0 decay rate is a fundamental prediction of confinement scale QCD. Chiral Anomaly – the closed-loop triangle diagram results in axial vector current non-conservation, even in the limit of vanishing quark masses. The anomaly leads to the reduced decay amplitude, in leading order (chiral limit) where Fp = 92.42+/-0.25 MeV is the pion decay constant.

4. Decay Width The p0ggg decay width is related to the amplitude: is presently known to 10% The goal of the Primex Experiment is to measure the decay width to an accuracy of 1.5%. Test axial anomaly and next to leading order  PT

5. 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.

6. Cross Section Components Primakoff Nuclear Coherent Nuclear Incoherent Interference Primakoff photoproduction from a Coulomb field of a nucleus ~ Z2, peak at =m2/2E2 Nuclear Coherentnucleus left in the ground state, peak at ~2/ER, ~ A2 Nuclear Incoherentnucleus left in excited state, which may be either bound or particle unstable , ~ A Interferencebetween the Primakoff process and the coherent non-spin-flip nuclear photoproduction

7. Jefferson Lab 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 specifically for the Primex Experiment.

8. PrimEx Setup in Hall B Tagged photons of energy 4.9-5.5 GeV were used to measure the absolute cross section of small angle 0 photoproduction from 12C and 208Pb targets

9. 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 ch • E/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

10. Compton Scattering off Atomic Electrons • Well known process, can be used for systematic studies of experimental setup • Data is used for: • Detector alignment • Monitoring of HYCAL gain drifts with time • Calculation of absolute cross section Periodic Compton Runs 1st calibration 2nd calibration 10/09 11/22 Production runs throughout

11. Compton Scattering off Atomic Electrons e! 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 LG scattered e- incident  PWO  scattered ’ LG

12. Double Arm Compton Event Compton event in “primsim” target Helium Bag Veto counters HYCAL 2 clusters are detected on the calorimeter

13. Monitoring Beam Position: DA Compton data (1+2)/2 Looking for an intersection point between lines connecting Compton e- and  by minimizing the sum of squares of distances between a point (x0,y0) and each line (1-2)/2 (deg)

14. Monitoring Energy Gains: DA Compton Data Module 1498 Using photons Gain_Corr = 1.038 » 3.8 % e/ec Energy Resolution after “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 before (e1+e2)/E

15. DA Compton: Event Selection (Be target) Reconstruct the vertex of Compton reaction Z=(x2+y2)0.5[/(E/e-1)]0.5 Apply kinematic constraints: energy and momentum conservation Reconstruct Z again Z (cm) 2<100 removes most of the background He bag

16. DA Compton: Extracting the Cross Section very preliminary very preliminary error bars are statistical only radiative corrections are not applied

17. DA Compton: Integrated Cross Section

18. 0 production on 12C Events/0.04 deg

19. 0 production on 208Pb Events/0.04 deg

20. 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

21. Additional slides

22. 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 position can be checked every ~ 10 min of data taking

23. DA Compton: Counting Events 9Be, MC 12C, MC 9Be data 12C data Events/2 Events/2 Vertex Z1 (cm) Vertex Z1 (cm)

24. Monitoring Energy Gains: SA Compton Data carbon lead Using single arm compton data in the production runs allows for run-by run gain monitoring Compton gains are consistent with the Light Monitoring System

25. Primakoff Cross Section • Gpgggis the pion decay width. • aem is fine structure constant. • Z is the number of protons in the nucleus. • mp,b,qpare 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. Coherent Cross Section Incoherent Cross Section • A is the number of nucleons. • Csin2(qp) is the spin independent part • of the p0 photoproduction amplitude from • a single nucleon. • FN(Q) is the nuclear form factor of the • nucleus mass distribution. • x is an absorption factor. • 1-G(Q) is a factor that reduces the • cross section at small Q. • dsH/dWis the 0 photoproduction • cross section on a single nucleon.

26. Proposed Error Budget

27. 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

28. Decay Width The p0ggg decay width is related to the amplitude: The Particle Data Group value is 7.84+/-0.56 eV. The Next to Leading order prediction is 8.10+/- 1% which is 4% higher than the present experimental value. The goal of the Primex Experiment is to measure the decay width to an accuracy of 1.5%.