Search for a Low-Mass Scalar Higgs Boson Decaying to a Tau Pair in Single-Photon Decays of ϒ(1s)

We search for a low-mass scalar CP-odd Higgs boson, A⁰, produced in the radiative decay of the upsilon resonance and decaying into a τ⁺τ⁻ pair: ϒ(1S)→γA⁰. The production of ϒ(1S) mesons is tagged by ϒ(2S) π⁺π⁻ ϒ(1S) transitions, using a sample of (98.3 ± 0.9) x 10⁶ ϒ(2S) mesons collected by the BABAR detector. We find no evidence for a Higgs boson in the mass range 3.5 ≤ m_{A⁰} ≤ 9.2 GeV, and combine these results with our previous search for the tau decays of the light Higgs in radiative ϒ(3S) decays, setting limits on the coupling of A⁰ to the bb̅ quarks in the range 0.09-1.9. Our measurements improve the constraints on the parameters of the next-to-minimal-supersymmetric Standard Model and similar theories with low-mass scalar degrees of freedom.