Observation of Significant Flavor-SU(3) Breaking in the Kaon Wave Function at 12<Q2<25 GeV2 and Discovery of the Charmless Decay ψ (3770) → KS0 KL0

We present cross sections for the reaction e+e-→KS0KL0 at center-of-mass energies ranging from 3.51 to 4.95 GeV using data samples collected in the BESIII experiment, corresponding to a total integrated luminosity of 26.5 fb-1. The ratio of neutral-to-charged kaon form factors at large momentum transfers (12<Q2<25 GeV2) is determined to be 0.21±0.01, which indicates a small but significant effect of flavor-SU(3) breaking in the kaon wave function, and, consequently, excludes the possibility that flavor-SU(3) breaking is the primary reason for the strong experimental violation of the pQCD prediction |F(π±)|/|F(K±)|=fπ2/fK2, where F(π±) and F(K±) are the form factors, and fπ and fK are the decay constants of charged pions and kaons, respectively. We also observe a significant signal for the charmless decay ψ(3770)→KS0KL0 for the first time. Within a 1σ contour of the likelihood value, the branching fraction for ψ(3770)→KS0KL0 is determined to be B=(2.63-1.59+1.40)×10-5, and the relative phase between the continuum and ψ(3770) amplitudes is φ=(-0.39-0.10+0.05)π. The branching fraction is in good agreement with the S- and D-wave charmonia mixing scheme proposed in the interpretation of the "ρπ puzzle"between J/ψ and ψ(3686) decays.