Diffractive Slope Extraction of Exclusive $\rho^{0}_{L}$ and $\rho^{0}_{T}$ at HERMES

Hard exclusive $\rho^0$ electroproduction from a $^1$H target has been studied at the HERMES experiment. In particular, the meson helicity dependence of the cross-section slope parameter $b$ as a function of the squared four-momentum of the virtual photon $Q^{2}$ is presented. The parameter $b$ is related to the meson transverse size, and at higher $Q^{2}$ accesses color transparency. While $b$ is well studied for the helicity averaged case, there exists no previous experimental data regarding the $\rho^{0}$ helicity dependence. The HERMES data suggests a sizeable difference in the values of $b$ (and thus the transverse size of the $\rho^{0}$) for different helicities and a sizeable decrease of $b$ with increasing $Q^{2}$, an effect known as shrinkage.