Low energy QCD measurements and ChPT tests with Kaons at CERN

The NA48/2 experiment at CERN SPS collected data in 2003-2004 allowing several precise measurements in the charged kaon decay sector. This work presents preleminary measurements of the form factors of charged kaon semileptonic decays, based on 4.0 million K± e3 (K±→π0e±ν) and 2.5 million K± μ3 (K±→π0μ±ν) decays, both with negligible background: the K± e3 and K± μ3 results are in good agreement allowing for a combined result which matches the precision of the current world average. Furthermore, the collaboration has collected large samples of Ke4 +− (K±→π+π−e±ν) and Ke4 00 (K±→π0π0e±ν) decays. Using 1.1 million Ke4 +− and 66000 Ke4 00 decays an improved determination of the branching fraction and detailed form factor studies are performed. In the charged mode the measurement of the phase difference between the S- and Pwave states of the ππ system allows a precise determination of 𝑎0 𝐼 , the isospin I=0 and I=2 Swave ππ scattering lengths. A combination of this result with the NA48/2 measurement obtained in K3π (K±→π0π0π±) decays leads to a further improved determination of 𝑎0 0 and the first precise experimental measurement of 𝑎0 2 .