Line-shape and poles of the ψ (3770)

We study the non-Breit-Wigner line-shape of the psi(3770) resonance, predominantly a 1 D-3(1) (c) over barc state, using an unitarized effective Lagrangian approach, including the one-loop effects of the nearby thresholds D+ D- and D-0(D) over bar (0). A fit of the theoretical result to the total cross-section e(+) e(-) -> D (D) over bar is performed, leading to a good description of data (chi(2)/d.o.f. similar to 1.03). The partial cross sections e(+) e(-) -> D-0(D) over bar (0) and e(+)e(-) -> D+ D- turn out to be separately in good agreement with the experiment. We find a pole at 3777 - i12 MeV, that is within the Particle Data Group (PDG) mass and width estimation for this state. Quite remarkably, we find an additional, dynamically generated, companion pole at 3741 - i19 MeV, which is responsible for the deformation on the lower energy side of the line-shape. The width for the leptonic decay psi (3770) -> e(+) e(-) is 111 eV, hence smaller than the PDG fit of 262 +/- 18 eV, yet in agreement with a recent experimental study.