Medium modifications of the bound nucleon GPDs and the quark contribution to the spin sum rule

Abstract

We estimate the nuclear medium modifications of the quark contribution to the bound nucleon spin sum rule, J^{q*}, as well as the separate helicity, \Delta \Sigma*, and the angular momentum, L^{q*}, contributions to J^{q*}. For the calculation of the bound nucleon generalized parton distributions (GPDs), we use as input the bound nucleon elastic form factors predicted in the quark-meson coupling model. Our model for the bound nucleon GPDs is relevant for incoherent deeply virtual Compton scattering (DVCS) with nuclear targets. We find that the m5Aedium modifications increase J^{q*} and L^{q*} and decrease \Delta \Sigma* compared to the free nucleon case. The effect is large and increases with increasing nuclear density \rho. For instance, at \rho=\rho_0=0.15 fm^{-3}, J^{q*} increases by 7%, L^{q*} increases by 20%, and \Delta \Sigma* decreases by 17%. These in-medium modifications of the bound nucleon spin properties may be understood qualitatively in terms of the enhancement of the lower component of the quark Dirac spinor in the nuclear medium.