Violation of Positivity Bounds in Models of Generalized Parton Distributions

Understand

As with parton distributions, flexible phenomenological parameterizations of generalized parton distributions (GPDs) are essential for their extraction from data.

• The large number of constraints imposed on GPDs make simple Lorentz covariant models viable; but, such models are often incomplete in that they employ the impulse approximation.
• Using the GPD of the pion as a test case, we show that the impulse approximation can lead to violation of the positivity bound required of GPDs.
• We focus on a particular model of the pion bound-state vertex that was recently proposed and demonstrate that satisfying the bound is not guaranteed by Lorentz covariance.