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 Particle Data Group 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.