Backward spin polarizability $γ_π$ of the proton

Using the Mainz $48 cm Ø\times 64 cm$ NaI(Tl) detector and the segmented Göttingen recoil detector SENECA in coincidence, Compton scattering by the proton at $θ^{lab}_γ= 136^\circ$ has been measured at MAMI (Mainz) in the energy range from 200 to 470 MeV. The new data confirm the previous observation that there is a systematic discrepancy between MAMI and LEGS (Brookhaven) data leading to different spin polarizabilities $γ_π= - 38.7 \pm 1.8$ and $-27.2 \pm 3.1$ $(\times 10^{-4}fm^4)$, respectively.

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