title: Mass and width of the lowest resonance in QCD.
abstract: 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.
authors.affiliation: National Institute of Physics and Nuclear Engineering, Bucharest, R-077125 Romania.
authors.forename: I
authors.initials: I
authors.lastname: Caprini
authors.forename: G
authors.initials: G
authors.lastname: Colangelo
authors.forename: H
authors.initials: H
authors.lastname: Leutwyler