Observation of a cusp-like structure in the π0π0 invariant mass distribution from K±→π±π0π0 decay and determination of the ππ scattering lengths
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Abstract
We report the results from a study of a partial sample of 2.3 10 7 K 0 0 decays recorded by the NA48/2 experiment at the CERN SPS, showing an anomaly in the 0 0 invariant mass (M 00 ) distribution in the region around M 00 = 2m + , where m + is the charged pion mass. This anomaly, never observed in previous experiments, can be interpreted as an effect due mainly to the final state charge exchange scattering process Cabibbo, Phys. Rev. Lett. 93 (2004) 121801]. It provides a precise determination of a 0a 2 , the difference between the scattering lengths in the isospin I = 0 and I = 2 states. A best fit to a rescattering model [N. Cabibbo, G. Isidori, JHEP 0503 (2005) 21] corrected for isospin symmetry breaking gives (a 0a 2 )m + = 0.268 0.010(stat) 0.004(syst), with additional external uncertainties of 0.013 from branching ratio and theoretical uncertainties. If the correlation between a 0 and a 2 predicted by chiral symmetry is taken into account, this result becomes (a 0a 2 )m + = 0.264 0.006(stat) 0.004(syst) 0.013(ext).
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