Measuring the Charged Pion Polarizability in the $\gamma\gamma\rightarrow\pi^{+{}}\pi^{-{}}$ Reaction

A new measurement of the charged pion polarizability $\alpha_{\pi}-\beta_{\pi}$ has been recently approved to run at Jefferson Lab using the GlueX detector in experimental Hall-D. This will be done via the $\gamma\gamma\rightarrow\pi^{+{}}\pi^{-{}}$ cross section accessed via the Primakoff mechanism on a $Sn^{116}$ target. The linearly polarized photon source in Hall-D will be used to separate the Primakoff cross-section from coherent $\rho^{\circ}$. An additional detector will also need to be constructed to help distinguish pion and muon events. The charged pion polarizability ranks among the most important tests of low-energy QCD presently unresolved by experiment. Analogous to precision measurements of $\pi^{\circ}\rightarrow\gamma\gamma$ that test the intrinsic odd-parity (anomalous) sector of QCD, the pion polarizability tests the intrinsic even-parity sector of QCD.