First Observation of the Λ(1405) Line Shape in Electroproduction

We report the first observation of the line shape of the $\Lambda(1405)$ from electroproduction, and show that it is not a simple Breit-Wigner resonance. Electroproduction of $K^+ \Lambda(1405)$ 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 $\Sigma^+ \pi^-$ of the $\Lambda(1405)$ and $p \pi^0$ of the $\Sigma^+$. 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 $\Lambda(1405)$, 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.