Mass and width of the lowest resonance in QCD
  We demonstrate that near the threshold, the pi pi scattering amplitude contains a pole with the quantum numbers of the vacuum - commonly referred to as the sigma - and determine its mass and width within small uncertainties. Our derivation does not involve models or parametrizations, but relies on a straightforward calculation based on the Roy equation for the isoscalar S-wave. 
Irinel Caprini, Gilberto Colangelo and Heinrich Leutwyler