First Observation of the $\hat{I}›(1405)$ Line Shape in Electroproduction

Abstract

We report the first observation of the line shape of the $\hat{I}›(1405)$ from electroproduction, and show that it is not a simple Breit-Wigner resonance. Electroproduction of $K^{+}\hat{I}›(1405)$ off the proton was studied by using data from CLAS at Jefferson Lab in the range $1.0<Q^2<3.0$ (GeV/c)${}^2$. The analysis utilized the decay channels $\hat{I}{\pounds }^{+}\ddot{I}{€}^{-}$ of the $\hat{I}›(1405)$ and $p\ddot{I}{€}^0$ of the $\hat{I}{\pounds }^{+}$. Neither the standard (PDG) resonance parameters, nor free parameters fitting to a single Breit-Wigner resonance represent the line shape. In our fits, the line shape corresponds approximately to predictions of a two-pole meson-baryon picture of the $\hat{I}›(1405)$, with a lower mass pole near 1368 MeV/c${}^2$ and a higher mass pole near 1423 MeV/c${}^2$. Furthermore, with increasing photon virtuality the mass distribution shifts toward the higher mass pole.

8 pages, 8 figures. Version 2 has some added references; submitted to Phys. Rev. C