Search for a Low-Mass Scalar Higgs Boson Decaying to a Tau Pair in Single-Photon Decays of Upsilon(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 tau+tau- pair: Upsilon(1S)->gamma A0. The production of Upsilon(1S) mesons is tagged by Upsilon(2S)->pi+pi-Upsilon(1S) transitions, using a sample of (98.3+/-0.9) million Upsilon(2S) mesons collected by the BaBar detector. We find no evidence for a Higgs boson in the mass range 3.5 GeV<= m(A0)<=9.2 GeV, and combine these results with our previous search for the tau decays of the light Higgs in radiative Upsilon(3S) decays, setting limits on the coupling of A0 to the b/bbar 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.