PoS - Proceedings of Science
Volume 408 - XV International Workshop on Hadron Physics (XVHadronPhysics) - Section Posters
Low-$Q^2$ parametrizations of the $\gamma^\ast N \to N^\ast$ transition amplitudes
G. Ramalho
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Pre-published on: August 01, 2022
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The electromagnetic structure of the nucleon resonances $N^\ast$ are usually parametrized by $\gamma^\ast N \to N^\ast$ helicity amplitudes at the resonance rest frame.
Those amplitudes are, however, constrained by kinematic conditions in the limit where the photon three-momentum vanishes (pseudothreshold limit).
Although the pseudothreshold limit is below the photon point ($Q^2=0$) it has an impact on the structure of the helicity amplitudes at low $Q^2$. Most of the empirical parametrizations of the data ignore those constraints.
In this work we study the effect of the pseudothreshold constraints on some analytic parametrizations of the data, performing analytic continuations of the parametrizations to the $Q^2 < 0$ region.
We conclude that the pseudothreshold constraints are fundamental for some resonances, particularly for the $\Delta(1232)$ and the $N(1535)$.
DOI: https://doi.org/10.22323/1.408.0059
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