First observation and dalitz analysis of the D0 → K S0ηπ0 Decay

Using $9.0\\text{ }\\text{ }{\\mathrm{f}\\mathrm{b}}^{\\ensuremath{-}1}$ of integrated luminosity in ${e}^{+}{e}^{\\ensuremath{-}}$ collisions near the $\\ensuremath{\\Upsilon}(4\\mathrm{S})$ mass collected with the CLEO II.V detector we report the first observation of the decay ${D}^{0}\\ensuremath{\\rightarrow}{K}_{S}^{0}\\ensuremath{\\eta}{\\ensuremath{\\pi}}^{0}$. We measure the ratio of branching fractions, $\\frac{BR({D}^{0}\\ensuremath{\\rightarrow}{K}_{S}^{0}\\ensuremath{\\eta}{\\ensuremath{\\pi}}^{0})}{BR({D}^{0}\\ensuremath{\\rightarrow}{K}_{S}^{0}{\\ensuremath{\\pi}}^{0})}=0.46\\ifmmode\\pm\\else\\textpm\\fi{}0.07\\ifmmode\\pm\\else\\textpm\\fi{}0.06$. We perform a Dalitz analysis of 155 selected ${D}^{0}\\ensuremath{\\rightarrow}{K}_{S}^{0}\\ensuremath{\\eta}{\\ensuremath{\\pi}}^{0}$ candidates and find leading contributions from ${a}_{0}(980){K}_{S}^{0}$ and ${K}^{*}(892)\\ensuremath{\\eta}$ intermediate states.