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.