Detailed Study of the Ke4 Decay Mode Properties

The NA48/2 collaboration at CERN has accumulated an unprecedented statistics of the semileptonic Ke4 charged kaon decay in the "charged pions" (Ke4(+-)) mode K-+/- -> pi(+)pi(-) ev and in the "neutral pions" (Ke4(00)) mode K-+/- -> pi(0)pi(0) ev with respectively <1% and similar to 1% background contamination, allowing a detailed study of the decay properties.More than one million Ke4(+-) decays have been analyzed leading to the improved determination of the branching ratio and detailed studies of the form factors. The resulting BR(K(+/-)e4(+-)) = (4.257 +/- 0.004(stat) +/- 0.016(syst) +/- 0.031(ext)) x 10(-5) is 3 times more precise than the corresponding PDG value and the error is dominated by the external uncertainty from the branching ratio of the normalization decay mode. It has been possible to obtain the absolute value of the form factor f(s) = 5.705 +/- 0.017(exp) +/- 0.017(syst) +/- 0.031(ext) and we can now translate into absolute form all the relative form factors previously published by NA48/2. This result complements the study of the S and P wave hadronic form factors obtained by NA48/2 in a simultaneous analysis of the pi pi scattering length obtained from the same data sample.Concurrently, 65,210 K-+/- decays into pi(0)pi(0)e(+/-)v have been collected, increasing the world available statistics by several orders of magnitude. A study of the differential rate provides the first measurement of the form factors. The achieved precision makes possible also the observation of small effects such as the evidence for a cusp -like structure in the distribution of the pi(0)pi(0) invariant mass squared around the threshold 4m(pi+)(2)Using a model independent description of the form factors we have obtained the final result, inclusive of radiative decays, BR(K(+/-)e4(00)) = (2.552 +/- 0.010(stat) +/- 0.010(syst) +/- 0.032(ext)) x 10(-5), also dominated by the external uncertainty from the branching ratio of the normalization decay mode, and improving the world average precision by an order of magnitude.The detailed study of form factors is sensitive to small isospin symmetry breaking effects and brings new inputs to low energy QCD description and crucial tests of predictions from Chiral Perturbation Theory and lattice QCD calculations.