Measurement of the Pion Polarisability at COMPASS

The COMPASS experiment at CERN has investigated Primakoff reactions of 190 GeV/c pions with the Coulomb field of various target nuclei, predominatly lead, in a pilot run in the year 2004. The photoproduction process $\pi^{−}Z\rightarrow\pi^{-}Z\gamma$ is related to Compton scattering on the pion, and gives access to the electric and magnetic polarisabilities, $\alpha_{\pi}$ and $\beta_{\pi}$. The unique feature of the COMPASS setup to measure the equivalent reaction with identified muons during the same beam time allows for a reliable treatment of systematic apparative effects. In a first analysis, the value $\alpha_{\pi} =$ 2.5 $\pm$ 1.7$_{stat}$ $\pm$ 0.6$_{sys}$ ·10$^{−4}$ fm$^{3}$ was obtained, assuming $\alpha_{\pi} = −\beta_{\pi}$ on theoretical reasons.