New high statistics measurement of Ke4 decay form factors and ππ scattering phase shifts

Abstract

We report results from a new measurement of the Ke4 decay K ±→π+π-e ±ν by the NA48/2 collaboration at the CERN SPS, based on a partial sample of more than 670∈000 Ke4 decays in both charged modes collected in 2003. The form factors of the hadronic current (F,G,H) and ππ phase difference (δ=δs-δp) have been measured in ten independent bins of the ππ mass spectrum to investigate their variation. A sizeable acceptance at large ππ mass, a low background and a very good resolution contribute to an improved experimental accuracy, a factor two better than in the previous measurement, when extracting the ππ scattering lengths a00 and a02. Under the assumption of isospin symmetry and using numerical solutions of the Roy equations, the following values are obtained in the plane (a00,a02): a00=0.233±0.016stat±0.007syst,a02=-0.0471±0.011stat±0.004syst. The presence of potentially large isospin effects is also considered and will allow comparison with precise predictions from Chiral Perturbation Theory. © 2008 Springer-Verlag / Società Italiana di Fisica.