Electromagnetic isovector form factors of the transition from the $N^*(1520)$ to the nucleon

Abstract

Dispersion theory is used to provide a model-independent low-energy representation of the three electromagnetic isovector transition form factors $N^{*}(1520)\to N$. At low energies the virtual photon couples dominantly to a pion pair. Taking the very well understood pion vector form factor and pion re-scattering into consideration, the determination of the transition form factors is traced back to the determination of pion-baryon scattering amplitudes. Their low-energy aspects are parametrized by baryon exchange, accounting for the main decay channels of the $N^*(1520)$. Short-distance physics is encoded in subtraction constants that are fitted to data on space-like form factors and hadronic decays. It is shown that a limitation in the determination of the subtraction constants lies in the fact that isovector form factors require sufficient information about the differences between protons and neutrons. In particular, this calls for improvements in the form factor extraction from the electroproduction of the $N^*(1520)$ on the neutron. Via the dispersion relations, space- and time-like regions are naturally connected from first principles. This allows to predict the time-like form factors that enter the Dalitz decays $N^*(1520)\to N e^- e^+$ and $N^*(1520)\to N \mu^- \mu^+$. Under the assumption of the dominance of the isovector over the isoscalar channel, the Dalitz decay distributions are predicted.