1.  0 life time analysis updates, preliminary results from Primex experiment 08/13/2007 I.Larin, Hall-B meeting

2. What do we measure 0 →  decay width: QCD prediction for massless quarks: Corrections: recent calculations in NLO give:

3. How do we measure:  0 Primakoff production cross-section gives the possibility to calculate  0 decay width

4. PrimEx Experiment  Hall-B high resolution high intensity photon tagging facility  New pair spectrometer for photon flux control at high intensities  High resolution hybrid multi- channel calorimeter (HYCAL)

5. Primex Production Run Primex data have been collected during October-November 2004 –  0 production runs (with full magnetic field in PS-magnet). Working energy range 4.9…5.5GeV (first 11 T-counters). –e + e - production runs (intermediate magnetic field in PS allowed pairs to hit calorimeter) –Compton runs (zero field in PS-magnet allows compton pairs to reach Hycal)

6. Event Display

7. Trigger MOR (first 11 T-counters) plus Hycal (energy deposition greater than 2GeV) Time difference between MOR and Hycal, [ns] Resolution for time difference between MOR and Hycal signals ~1.0…1.2 ns Selected events

8. Hycal Calibration Hycal calibration has been performed during “snake scans” with Hycal placed on transporter and exposed to tagged beam Additionally physical calibration with  0 s has been performed using all (elastic/inelastic/accidental) collected  0 s

9.  0 Reconstruction: Invariant mass spectrum Elastic and inelastic  0 s, using energies reconstructed by Hycal Elastic  0 s, Beam energy (well known from Tagger) has been used to correct cluster energies (Resolution dominated by coordinate reconstruction). Crystal part of Hycal

10. Inv. Mass distribution had been constructed for each elasticity bin  1.7% Event selection: Elasticity spectrum  0 mass  0 elasticity

11. Event selection: Elasticity spectrum Cluster energies were corrected so they would give exact  0 mass (constraint)  1.0% 1.7%  0 elasticity

12.  0 event distribution (C 12 target,  0 s from all processes) Elastic  0 s Vector mesons decays Primakoff  0 s Production angle Energy Tagged range

13. Angular spectrum CarbonLead

14. Physical background  AA            A  A      n            n       nn ±  ±  ±   n ±  ±  ±   Production of  and  mesons was taken into account in analysis

15.  and  decays background Data, Carbon target Simulated contribution from  and  decays, scaled according to known cross-section values to Photon beam flux from the data Elasticity

16. Effect of  and  subtraction:  0.9%  0 elasticity  1.0% Before subtractionAfter subtraction

17. Syst. error from  and  background subtraction: Carbon target: contribution from  and  was varied with its cross-section uncertainty (which is 20%) Estimated error budget contribution by this variation is 0.24%

18.  0 production terms: Coulomb Nuclear Coherent Their interference Incoherent All together C Pb Fit to Extract  0  Decay Width  Theoretical distributions of these processes were smeared with experimental resolution

19. Formfactors Distorted formfactors have been calculated with most up-to-date charge density distribution (E.Offermann, L.Cardman et al, Phys Rev C 91 Vol.44) 12 C charge density 12 C strong formfactor

20. Target number of atoms0.05 Photon beam flux1.0  Branching Ratio 0.03 Beam energy uncertainty0.13 Beam position and slope uncertainty0.1 Beam width uncertainty0.3 Production angle resolution0.25 Hycal response function0.5 Hycal z-position uncertainty0.4 Trigger efficiency0.1 Error budget

21. Error budget ( continuation ) Hycal efficiency0.3 target absorption0.1 ADC status during the runnegligible Beam selection efficiency0.3 Energy cut on single  0.2 Timing cut (“tdif”)1.0  invariant mass fit (signal / background separation) 1.0 Coherent cross-section dependence on energy0.1  and background subtraction 0.24 Total (quadratic) sum2.0

22. PrimEx Preliminary Result  (  ) = 7.93eV  2.1%  2.0%  0  Decay width (eV) ±1.%

23. Summary The first stage of  0 data analysis has been done. All “good” runs (in terms of beam stability and hardware functioning) for C and Pb targets have been analyzed The physical background has been studied and simulated The formfactors have been revisited by theorist and fully implemented in the fitting procedure The major systematic error items have been investigated and finalized Current experimental data set has been analyzed. Preliminary  0 radiation width Γ( 0 )  7.93 eV3.0% (total error) has been extracted

24. Spare slides

25. Use of “Leakage detector”: (small, roughly segmented calorimeter ~30…50 channels behind Hycal) MC of snake scan, event examples: beam is perpendicular to Hycal, i.e. parallel to modules wrapping: leakage is possible Normal eventBeam is close to wrapping: leakage Future plans: ways to eliminate uncertainties

26. Hycal response, reconstructed beam: Hycal VS Tagger Snake scan, data: beam is perpendicular to Hycal, i.e. parallel to modules wrapping: leakage is visible (MC reproduces this effect) Beam direction Hycal modules Leakage through wrapping Ratio of rec-d beam Hycal VS Tagger Beam position, [cm]

27. He Bag is a source of Compton coming in parallel with  0 s which could be used for additional normalization testing Omega’s produced in He Bag are seen, cross-section could be extracted Pressure meters are needed to be set up and included in slow control system Future plans: ways to improve experiment      inv. mass, resolution for PWO crystal part of Hycal ~ 23MeV (photoproduction at He, ~1-2m from Hycal)

28. Primex targets Carbon target: –thickness ~1 cm; –Number of atoms is known to precision 0.04% Lead-208 (monoisotopic) target: –thickness ~30  m (foil); –Number of atoms is known to precision 0.32%