The $\pi^0$ Lifetime: Experimental Probe of the QCD Axial Anomaly

The $\pi^0 \rightarrow \gamma\gamma$ decay rate is a fundamental prediction of QCD which gives insight into one of its most profound symmetry issues---namely, the Axial or Chiral Anomaly. It is this anomalous symmetry-breaking mechanism by which the $\pi^0 \rightarrow \gamma\gamma$ decay channel primarily proceeds, and thus a measure of its rate or partial width, $\Gamma_{\gamma\gamma}$, represents a direct probe of the anomaly plus chiral corrections. The PrimEx Collaboration at Jefferson Lab has extracted $\Gamma_{\gamma\gamma}$ from precision measurements of $\pi^0$ photo-production cross sections using their Primakoff components. Measurements were made using 5\% X$_{\mathrm{o}}$ nuclear targets of $^{12}$C and $^{208}$Pb with incident photons between 4.9 and 5.5 GeV tagged by the Hall B tagger facility. The $\pi^0$ decay photons were detected by a specially constructed high resolution hybrid calorimeter (HYCAL). In this presentation, recent preliminary results from the PrimEx measurement will be given with emphasis on the cross section analysis and lifetime extraction.