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, A^0, produced in the radiative decay of the upsilon resonance and decaying into a τ^+τ^- pair: Υ(1S) → γA^0. The production of Υ(1S) mesons is tagged by Υ(2S) → π^+π^-Υ(1S) transitions, using a sample of (98.3 ± 0.9) × 10^6 Υ(2S) mesons collected by the BABAR detector. We find no evidence for a Higgs boson in the mass range 3.5 ≤ m_(A^0) ≤ 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 A^0 to the bb 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.