$\gamma^\ast N \to N^\ast(1520)$ form factors in the timelike regime

TL;DR

This paper models the $ o N^ ext{*}(1520)$ transition form factors in the timelike regime using a covariant spectator quark approach, predicting decay widths and aiding interpretation of dielectron production in proton collisions.

Contribution

It extends the covariant spectator quark model to the timelike region for the $ o N^ ext{*}(1520)$ transition, providing new predictions for form factors and decay widths.

Findings

QED approximation underestimates electromagnetic coupling by 1-2 orders of magnitude.

Predicted decay widths are comparable to those of $ o ext{Delta}(1232)$ Dalitz decay.

Form factor behavior in the timelike region is successfully modeled.

Abstract

The covariant spectator quark model, tested before in a variety of electromagnetic baryon excitations, is applied here to the $\gamma^\ast N \to N^\ast(1520)$ reaction in the timelike regime. The transition form factors are first parametrized in the spacelike region in terms of a valence quark core model together with a parametrization of the meson cloud contribution. The form factor behavior in the timelike region is then predicted, as well as the $N^\ast(1520) \to \gamma N$ decay width and the $N^\ast (1520)$ Dalitz decay, $N^\ast (1520) \to e^+ e^- N$. Our results may help in the interpretation of dielectron production from elementary $pp$ collisions and from the new generation of HADES results using a pion beam. In the $q^2=0$--1 GeV$^2$ range we conclude that the {\it QED approximation} (a $q^2$ independent form factor model) underestimates the electromagnetic coupling of the $N^\ast(1520)$ from 1 up to 2 orders of magnitude. We conclude also that the $N^\ast (1520)$ and the $\Delta(1232)$ Dalitz decay widths are comparable.