Abstract
We report the results from a study of a partial sample of ∼2.3×107K±→π±π0π0 decays recorded by the NA48/2 experiment at the CERN SPS, showing an anomaly in the π0π0 invariant mass (M00) distribution in the region around M00=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 π+π−→π0π0 in K±→π±π+π− decay [N. Cabibbo, Phys. Rev. Lett. 93 (2004) 121801]. It provides a precise determination of a0−a2, the difference between the ππ 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 (a0−a2)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 a0 and a2 predicted by chiral symmetry is taken into account, this result becomes (a0−a2)m+=0.264±0.006(stat)±0.004(syst)±0.013(ext).
Highlights
The NA48/2 experiment at the CERN SPS is searching for direct CP violation in K± decay to three pions
We report the results from a study of a partial sample of ∼ 2.3 × 107 K± → π±π0π0 decays recorded in 2003, showing an anomaly in the π0π0 invariant mass (M00) distribution in the region around M00 = 2m+, where m+ is the charged pion mass
It has been shown that analyticity and chiral symmetry provide a constraint between a0 and a2 [12]: a2m+ = (−0.0444±0.0008)+0.236(a0m+ −0.22)−0.61(a0m+ −0.22)2 −9.9(a0m+ −0.22)3
Summary
The NA48/2 experiment at the CERN SPS is searching for direct CP violation in K± decay to three pions. We report the results from a study of a partial sample of ∼ 2.3 × 107 K± → π±π0π0 decays recorded in 2003, showing an anomaly in the π0π0 invariant mass (M00) distribution in the region around M00 = 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 π+π− → π0π0 in K± → π±π+π− decay [1].
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