Measurement of the Beam Asymmetry $\Sigma$ in $\pi^+$ $\pi^-$ Photoproduction using the CLAS Spectrometer at Jefferson Laboratory

For an unambiguous extraction of resonance parameters via partial-wave analysis, not only unpolarized differential cross sections are needed, but also polarization observables. The beam asymmetry in double-pion production will provide important model constraints for the determination of resonance contributions to this reaction. The CLAS g8b experiment, as part of the $N^*$ spectroscopy program at Jefferson Lab in Newport News, VA, has accumulated photoproduction data using a linearly-polarized tagged-photon beam incident on an unpolarized hydrogen target. The excellent degree of polarization reached values exceeding 90\% at different positions of the coherent bremsstrahlung peak between 1.3 and 2.1 GeV. The g8b $\pi^+$$\pi^-$ data sample comprises of roughly 100M events and thus provides sufficient statistics to study the 5-dimensional observable $\Sigma$ as a function of various masses and angles. In addition to $\Sigma$, preliminary results on the extraction of the additional observable $I^s$ will be discussed, which arises for two pseudoscalar mesons in the final state.