Gluon Generalized Parton Distributions and the Angular Momentum Sum Rule*

Understanding how the proton spin is carried by its constituents, the quarks and gluons, represents one of the main challenges in particle physics. In particular, the generalized parton distributions, $H_{q,g},$, E_{q,g}$, which are found in the matrix elements for the deeply virtual Compton scattering process, allow us to measure the total angular momentum carried by the quarks and gluons, $J_q$ and $J_g$, respectively. The latter are obtained as the second moment of the generalized parton distributions in the $x$ variable. The nucleon helicity-flip distributions $E_q$ and $E_g$, are lesser known than their counterparts $H_q$ and $H_g$, because their forward limit values cannot be obtained from inclusive processes. Here we present a model calculation of $E_g$ in the reggeized diquark model, using recent lattice QCD calculations to take into account its normalization. Based on our model calculation, we study the impact of $E_g$ on the proton angular momentum sum rule.