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 A^0, or a pair of dark matter particles, χχ̅ . Both A^0 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_(A^0 ≤ 9.2 GeV and m_χ ≤ 4.5 GeV in the sample of 98×10^6 Υ(2S) decays collected with the BABAR detector and set stringent limits on new physics models that contain light dark matter states
