First L/T separation of $\pi^0$ electroproduction from Jefferson Laboratory Hall A

29 Apr 2015, 09:45


WG6 Spin Physics WG6 Spin Physics




The deeply virtual $\pi^0$ production cross section can be decomposed according to the polarization of the virtual photon. \begin{equation} \frac{d\sigma}{dt}=\frac{d\sigma_T}{dt}+\epsilon \frac{d\sigma_L}{dt}+\sqrt{2\epsilon (\epsilon+1)}\frac{d\sigma_{TL}}{dt} \cos(\phi)+\epsilon \frac{d\sigma_{TT}}{dt} \cos(2\phi) \end{equation} where $\epsilon$ represents the degree of polarization of the virtual photon and $\phi$ the angle between the leptonic and the hadronic plane. The unseparated $\pi^0$ electroproduction cross sections have been extracted in the Halls A and B of Jefferson Laboratory. Using the $\phi$-dependence, both experiments measured a large $\sigma=\sigma_T+\epsilon \sigma_L$. However generalized parton distributions-based models predict a small contribution $\sigma_L$ in a theoretical framework where factorization has only been proven for longitudinal polarized photons. Assuming the validity of factorization for transverse photons, Kroll and Goloskokov developed a model where the transversity GPDs would couple to the twist-3 distribution amplitudes of the pion, amplifying the $\sigma_T$ signal. In order to verify this prediction, the E07-007 experiment measured the $\pi^0$ electroproduction cross section on proton in the Hall A of Jefferson Laboratory. Using two beam energies to measure the cross section at same $Q^2$ and $x_B$, we change the $\epsilon$ value and are able to separate the transverse from the longitudinal contribution. For the first time, I will present the fully separated $\pi^0$ electroproduction cross section and compare it to the existing models.

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