High Precision Measurement Of The Neutral Pion Lifetime Via The Primakoff Effect

As the lightest particle in the hadron spectrum, π02 plays an important role in understanding the fundamental symmetries of QCD. The π0→rr decay is a key test of the QCD predictions based on the chiral anomaly and spontaneous chiral symmetry breaking. Theoretical activities over the last decade have resulted in a high precision (1% level) in the calculations of the decay amplitude of the π0 into two photons. The experimental measurement of this parameter with a comparable precision will be critical to test these important predictions. The PrimEx collaboration at Jefferson Lab has developed and performed experiments to measure the π0 radiative decay width via the Primakoff effect. The published result from the first experimental data (PrimEx-I) has a 2.8% total uncertainty in the π0 radiative decay width. The second experiment (PrimEx-II) was carried out in fall 2010 with the final goal of 1.4% precision. This thesis has concentrated on data analysis of PrimEx-II to get the. π0 radiative decay width.PrimEx-II experiment has used roughly 4.4 GeV-5.3GeV real photons to hit the nuclear targets. A π0 was produced through the interaction between a real incoming photon and a virtual photon in the coulomb field of the nucleus of target atoms, i.e.π0 photon production via Primakoff effect.π0 has a very short life time(10-17 second) which immediately decays into two real photons. The energy and timing of the incident photons were determined by Jefferson Lab Hall-B tagging facility. The energy and position of two decay photons from the neutral pion were measured in a very high resolution, state-of-the-art hybrid type of electromagnetic calorimeter (HyCal)。The knowledge of the timing and the energy of incident photon and the energy and the position of two decay photons is allowing to reconstruct the invariant mass and the angular distribution of the π0 . The elastic pion yield over small production angles is extracted by analyzing the reconstructed π0 invariant mass distribution in the small angular bin. Finnaly, the pion decay width is obtainned by fitting the differential cross section which is from elastic pion yield.PrimEx-II has two types of target C and Si, both of 10% radiation length. The preliminary pion decay width from this two targets is:Γπ0 →rr=7.69±0.12 (stat.)eV (12C);Γπ0 →vv=7.76±0.065 (stat.)eV (28Si). The most of systematic uncertainty items of pion decay width are studied and the present results are 0.50% for 12C target and 0.58% 28Si target. Some major items such as bem flux and yield exctraction have not been studied yet and will be finished in the future work.In order to verify the target measurement, the total photon interaction cross section has been extracted for carbon and silicon target separately, and the precision of finally result is about 2%, which needs to be compared with theoretical calculation and other experimental results.