Search for Production of Invisible Final States in Single-Photon Decays of<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi mathvariant="italic">Υ</mml:mi><mml:mo stretchy="false">(</mml:mo><mml:mn>1</mml:mn><mml:mi>S</mml:mi><mml:mo stretchy="false">)</mml:mo></mml:math>

We search for single-photon decays of the Υ(1S) resonance, Υ → γ + invisible, where the invisible state is either a particle of definite mass, such as a light Higgs boson A⁰, or a pair of dark matter particles, χχ. Both A⁰ and χ are assumed to have zero spin. We tag Υ(1S) decays with a dipion transition Υ(2S) → π⁺π⁻Υ(1S) and look for events with a single energetic photon and significant missing energy. We find no evidence for such processes in the mass range m(A⁰) ≤ 9.2 GeV and m(χ) ≤ 4.5 GeV in the sample of 98 × 10⁶ Υ(2S) decays collected with the BABAR detector and set stringent limits on new physics models that contain light dark matter states.