The $\pi^0$ Lifetime: Experimental Probe of the QCD Chiral 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 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 $^{12}\mathrm{C}$ and $^{208}\mathrm{Pb}$ with incident photons between $4.9$ and $5.5\mathrm{GeV}$ tagged by the Hall B tagger facility. The $\pi^0$ decay photons were detected at very forward angles by a specially constructed high resolution hybrid calorimeter (HyCal). In this presentation, the final result from the PrimEx measurement will be given along with a general overview of the physics, experimental setup, and data analysis strategies and techniques.