Photon flux determination for a precision measurement of the neutral pion lifetime

The Jefferson Lab Hall B PrimEx Collaboration is using tagged photons to perform a 1.4% level measurement of the absolute cross section for the photo-production of neutral pions in the Coulomb field of a nucleus as a test of Chiral Perturbation Theory. Such a high precision pushes the limits of the photon tagging technique in regards to the determination of the absolute photon flux. A multifaceted approach to this problem has included measuring the absolute tagging ratios with a Total Absorption Counter (TAC) as well as relative tagging ratios with a Pair Spectrometer (PS), and determining the rate of the tagging counters using multi-hit TDC's and a clock trigger. This enables the determination of the absolute tagged photon flux for the PrimEx experiment with uncertainty of ∼1.0%, which is unprecedented.; In view of the stringent constraints on the required precision of the photon flux for this experiment, periodic measurements of the pair production cross section were performed through-out the run. In these measurements, both the photon energy and flux were determined by the Jefferson Lab Hall B tagger, and the electron-positron pairs were swept by a magnetic field and detected in the new 1728 channel hybrid calorimeter (HyCal). The pair production cross-section was extracted with an uncertainty of ∼2%, producing an agreement with theoretical calculations at the level of ∼2%. This measurement provided a unique opportunity to verify the photon flux determination procedure for the PrimEx experiment.; Finally, the photon flux determination procedure was used by the PrimEx collaboration to obtain the absolute normalization of the pion photo-production cross section, which allowed to extract neutral pion radiative width: Gamma pi0→gammagamma = 7.93 eV +/- 2.1%(stat.) +/- 2.0%(sys. ) (PRELIMINARY).; Keywords: PrimEx, Neutral Pion Lifetime, Photon Tagging, Photon Flux, Lepton Pair Production.