title: η→3π: Study of the Dalitz Plot and Extraction of the Quark Mass Ratio Q.
abstract: The η→3π amplitude is sensitive to the quark mass difference m_{u}-m_{d} and offers a unique way to determine the quark mass ratio Q^{2}≡(m_{s}^{2}-m_{ud}^{2})/(m_{d}^{2}-m_{u}^{2}) from experiment. We calculate the amplitude dispersively and fit the KLOE Collaboration data on the charged mode, varying the subtraction constants in the range allowed by chiral perturbation theory. The parameter-free predictions obtained for the neutral Dalitz plot and the neutral-to-charged branching ratio are in excellent agreement with experiment. Our representation of the transition amplitude implies Q=22.0±0.7.
authors.affiliation: Albert Einstein Center for Fundamental Physics, Institute for Theoretical Physics, University of Bern, Sidlerstrasse 5, CH-3012 Bern, Switzerland.
authors.forename: Gilberto
authors.initials: G
authors.lastname: Colangelo
authors.affiliation: Albert Einstein Center for Fundamental Physics, Institute for Theoretical Physics, University of Bern, Sidlerstrasse 5, CH-3012 Bern, Switzerland.
authors.forename: Stefan
authors.initials: S
authors.lastname: Lanz
authors.affiliation: Albert Einstein Center for Fundamental Physics, Institute for Theoretical Physics, University of Bern, Sidlerstrasse 5, CH-3012 Bern, Switzerland.
authors.forename: Heinrich
authors.initials: H
authors.lastname: Leutwyler
authors.affiliation: Department of Physics, Indiana University, Bloomington, Indiana 47405, USA; Center for Exploration of Energy and Matter, Indiana University, Bloomington, Indiana 47403, USA; and Theory Center, Thomas Jefferson National Accelerator Facility, Newport News, Virginia 23606, USA.
authors.forename: Emilie
authors.initials: E
authors.lastname: Passemar