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

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.

Comments: Published version. 12 pages, 10 figures